Vector Transmission Factor
Anderson, J.F., T.G. Andreadis, A.J. Main, F.J. Ferrandino, and C.R. Vossbrinck (2006). West Nile virus from female and male mosquitoes (Diptera: Culicidae) in subterranean, ground, and canopy habitats in Connecticut. Journal of Medical Entomology 43(5): 1010-1019.
Abstract: In total, 93,532 female mosquitoes (Diptera: Culicidae) were captured in traps placed in subterranean (catch basin), ground (approximately 1.5 m above ground), and canopy (approximately 7.0 m above ground) habitats in Stamford and Stratford, CT, during 2003-2005. Culex pipiens L. was the most abundant (64.8%) of the 31 species identified. Significantly greater numbers of Cx. pipiens were captured in canopy-placed mosquito magnet experimental traps, and significantly greater numbers were collected in catch basin-placed (Centers for Disease Control) CDC traps than in CDC traps placed elsewhere. Culex restuans Theobald was captured in significantly greater numbers in traps placed in catch basins. Aedes vexans (Meigen), Aedes cinereus Meigen, and Aedes cantator (Coquillett) were significantly more abundant in ground traps. In total, 429 isolations of West Nile virus (WNV) were made from seven species of mosquitoes from late June through the end of October during 2003 through 2005. Three hundred ninety-eight (92.8%) isolates were from Cx. pipiens. Others were from Cx. restuans (n = 16), Culex salinarius Coquillett (n = 5), Ae. vexans (n = 4), Ae. cantator (n = 3), Aedes triseriatus (Say) (n = 2), and Ae. cinereus (n = 1). Multiple isolates from Cx. pipiens were made each week, primarily during the later part of July through the end of September. Weekly minimum infection rates (MIRs) were lower in 2004 (highest weekly MIR = 7.1) when no human cases were reported in Connecticut in comparison with 2003 and 2005 (highest weekly MIR = 83.9) when human cases were documented. Frequencies of infected pools were significantly higher in Cx. pipiens captured in traps in the canopy and significantly higher in catch basin placed traps than in traps at ground level. The physiological age structure of Cx. pipiens captured in the canopy was significantly different from that of Cx. pipiens collected in catch basins. Invariably, Cx. pipiens captured in the canopy were nulliparous or parous with ovaries in Christophers' stage 2, whereas 58.7% of the females captured in catch basins possessed ovaries filled with mature oocytes in Christophers' stage 5. Our results suggest that females in the canopy are seeking hosts, and after digestion of the bloodmeal and development of mature oocytes, they descend to catch basins for shelter and deposition of eggs. WNV was isolated from three, one, and two pools of male Cx. pipiens captured in catch basin-, ground-, and canopy-placed traps, respectively, and from six nulliparous Cx. pipiens females collected in the canopy. Weekly MIR ranged from 1.2 to 31.1 per 1,000 male specimens. These data show that mosquitoes become infected by means other than by blood feeding, possibly by transovarial transmission. The placement of traps in tree canopies and in catch basins can be used to augment current practices of placement of traps near the ground for surveillance of mosquitoes infected with WNV and for studies of the ecology of WNV.
Descriptors: West Nile virus, mosquitoes, culex, aedes, traps, subterranean, above ground, CDC, male, female, surveillance.
Anderson, J.F. and A.J. Main (2006). Importance of vertical and horizontal transmission of West Nile virus by Culex pipiens in the Northeastern United States. Journal of Infectious Diseases 194(11): 1577-1579.
Abstract: West Nile virus (WNV) has become established in the northeastern United States, where mosquitoes are inactive during winter. There have been no documented studies to explain how this virus survives winter and reinitiates infection in spring. We report that WNV was vertically transmitted to 2 F(1) female Culex pipiens from a naturally infected female collected in Stratford, Connecticut. One vertically infected F(1) female, which was 168 days old, fed on a hamster that died 8 days later of West Nile disease. This suggests that WNV survives winter in unfed, vertically infected C. pipiens with amplification initiated in spring by horizontal transmission.
Descriptors: West Nile virus, Culex pipens, winter, infection, vertrical transmission, horozontil transmission, mosquitoes, spring.
Balenghien, T., F. Fouque, P. Sabatier, and D.J. Bicout (2006). Horse-, bird-, and human-seeking behavior and seasonal abundance of mosquitoes in a West Nile virus focus of southern France. Journal of Medical Entomology 43(5): 936-946.
Abstract: After 35 yr of disease absence, West Nile virus (family Flaviviridae, genus Flavivirus, WNV) circulation has been regularly detected in the Camargue region (southern France) since 2000. WNV was isolated from Culex modestus Ficalbi, which was considered the main vector in southern France after horse outbreaks in the 1960s. Recent WNV transmissions outside of the Cx. modestus distribution suggested the existence of other vectors. To study potential WNV vectors, horse- and bird-baited traps and human landing collections of mosquitoes were carried out weekly from May to October 2004 at two Camargue sites: one site in a wet area and the other site in a dry area, both chosen for their past history of WNV transmission. At the wet site, the most abundant species in bird-baited traps were Culex pipiens L. and Cx. modestus; both species also were found in lower proportions on horses and humans. The most abundant species in horse-baited traps and human landing collections were Aedes caspius (Pallas), Aedes vexans (Meigen), and Anopheles hyrcanus (Pallas) sensu lato; some of these species were occasionally collected with avian blood at the end of the summer. Anopheles maculipennis Meigen sensu lato was an abundant horse feeder, but it was rarely collected landing on human bait and never contained avian blood. At the dry site, Cx. pipiens was the most abundant species in bird- and horse-baited traps. The seasonal and circadian dynamics of these species are analyzed, and their potential in WNV transmission in Camargue discussed.
Descriptors: West Nile virus, bird, human, horse, mosquitoes, seeking behavior, seasonal abundance, southern France, transmission.
Bell, J.A., N.J. Mickelson, and J.A. Vaughan (2005). West Nile virus in host-seeking mosquitoes within a residential neighborhood in Grand Forks, North Dakota. Vector Borne and Zoonotic Diseases 5(4): 373-382.
Abstract: West Nile virus (WNV) was first recovered in North Dakota near the city of Grand Forks in June 2002. During 2002, 2003, and 2004, we collected mosquitoes from Grand Forks using Mosquito Magnet traps and tested them for WNV. The seasonal abundance, species composition, and reproductive status of female mosquitoes were correlated with local environmental temperature and state surveillance data on WNV to determine the factors affecting local transmission of WNV. Over 90% of the mosquitoes collected were Aedes vexans, Ochlerotatus dorsalis, and Culex tarsalis, but WNV was detected only in Cx. tarsalis. Average summertime temperatures and relative abundance of mosquitoes were highest in 2002 but no WNV-positive mosquitoes were detected until the following summer. In 2003, nulliparous Cx. tarsalis appeared in mid-June (first summer brood), and parous Cx. tarsalis appeared in mid-July. The first WNV-positive pool occurred 21 July, and minimum daily infections rates increased thereafter until 27 August. The minimum infection rate (MIR) for Cx. tarsalis during the season was 5.7 infected mosquitoes per 1,000 tested, with the highest infection rates occurring at the end of the season as Cx. tarsalis populations started to decline. Mid-to-late August was identified as the period of highest risk for being bitten by a WNV-infected mosquito in Grand Forks during 2003. In 2004, viral activity in Grand Forks was low, due to very cool temperatures throughout the summer. To examine the genetic diversity of the 2003 WNV isolates from Grand Forks, we sequenced a 366-nucleotide region of the capsid and premembrane gene. Thirteen (46%) of the 28 WNV isolates contained at least one nucleotide substitution when compared to the homologous region of the progenitor WN NY-99 strain, and seven of these 13 substitutions coded for amino acid changes. Thus, WNV is established in North Dakota, it appears to be evolving and it is vectored primarily by Cx. tarsalis.
Descriptors: West Nile virus, host seeking mosquitoes, seasonal abundance, traps, Culex tarsalis, North Dakota, vectors.
Colton, L., B.J. Biggerstaff, A. Johnson, and R.S. Nasci (2005). Quantification of West Nile virus in vector mosquito saliva. Journal of the American Mosquito Control Association 21(1): 49-53.
Abstract: Saliva was collected from 4 species of mosquitoes intrathoracically inoculated with West Nile virus (WNV). The amount of infectious virus in the saliva was quantified by plaque assay and the number of WNV genomic equivalents (GE) was measured by reverse transcriptase-polymerase chain reaction. Ochlerotatus triseriatus had the greatest mean amount of infectious virus per saliva collection, followed by Aedes albopictus, Culex pipiens, and Cx. quinquefasciatus. The mean GE/saliva collection was also greatest in Oc. triseriatus, followed by Cx. quinquefasciatus, Cx. pipiens, and Ae. albopictus. The variance of log GE/saliva collection for Ae. albopictus was significantly lower than the variance for the other 3 species. This study provides a basis for comparing this component of vector competence and for determining the amounts of virus inoculated into vertebrates in experimental host competence studies.
Descriptors: culicidae virology, insect vectors virology, saliva virology, West Nile virus, aedes virology, culex virology, ochlerotatus virology, polymerase chain reaction.
Colton, L. and R.S. Nasci (2006). Quantification of West Nile virus in the saliva of Culex species collected from the southern United States. Journal of the American Mosquito Control Association 22(1): 57-63.
Abstract: Culex quinquefasciatus. Cx. restuans, Cx. pipiens complex, and Cx. nigripalpus were collected as larvae or egg rafts from the southern USA. Adult female mosquitoes were intrathoracically inoculated with approximately 1,000 plaque-forming units of West Nile virus (WNV) and saliva was collected from them 5 days later. The amount of infectious WNV in the saliva samples was quantified by plaque assay and WNV RNA was detected by reverse transcriptase polymerase chain reaction (RT-PCR). More than 90% of the mosquitoes had either infectious virus or viral RNA in their saliva. The RT-PCR assay detected a greater percent of samples with WNV RNA than the plaque assay detected infectious virus. Pairwise comparisons revealed 6 significant differences between the 7 groups surveyed. The Cx. nigripalpus secreted lower mean amounts of WNV than 3 other groups, and a difference was found between early- and late-season Cx. quinquefasciatus collected in Louisiana.
Descriptors: culex virology, West Nile virus, Florida, insect vectors virology, Louisiana, plaque assay, reverse transcriptase polymerase chain reaction, saliva virology, Tennessee, West Nile fever transmission.
Cooke, W.H.3., K. Grala, and R.C. Wallis (2006). Avian GIS models signal human risk for West Nile virus in Mississippi. International Journal of Health Geographics 5: 36.
Abstract: BACKGROUND: West Nile virus (WNV) poses a significant health risk for residents of Mississippi. Physicians and state health officials are interested in new and efficient methods for monitoring disease spread and predicting future outbreaks. Geographic Information Systems (GIS) models have the potential to support these efforts. Environmental conditions favorable for mosquito habitat were modeled using GIS to derive WNV risk maps for Mississippi. Variables important to WNV dissemination were selected and classified as static and dynamic. The static variables included road density, stream density, slope, and vegetation. The dynamic variable represented seasonal water budget and was calculated using precipitation and evaporation estimates. Significance tests provided deterministic evidence of variable importance to the models. RESULTS: Several models were developed to estimate WNV risk including a landscape-base model and seasonal climatic sub-models. P-values from t-tests guided variable importance ranking. Variables were ranked and weights assigned as follows: road density (0.4), stream density (0.3), slope (0.2) and vegetation (0.1). This landscape-base model was modified by climatic conditions to assess the importance of climate to WNV risk. Human case data at the zip code level were used to validate modeling results. All models were summarized by zip codes for interpretation and model validation. For all models, estimated risk was higher for zip codes with at least one human case than for zip codes where no human cases were recorded. Overall median measure of risk by zip code indicated that 67% of human cases occurred in the high-risk category. CONCLUSION: Modeling results indicated that dead bird occurrences are correlated with human WNV risk and can facilitate the assessment of environmental variables that contribute to that risk. Each variable's importance in GIS-based risk predictions was assigned deterministically. Our models indicated non-uniform distribution of risk across the state and showed elevated risk in urban and as well as rural areas. Model limitations include resolution of human data, zip code aggregation issues, and quality/availability of vegetation and stream density layers. Our approach verified that WNV risk can be modeled at the state level and can be modified for risk predictions of other vector-borne diseases in varied ecological regions.
Descriptors: disease outbreaks, environment, geographic information systems, models, theoretical, West Nile fever epidemiology, West Nile virus isolation and purification, birds, mississippi epidemiology, risk factors, West Nile virus.
Davis, A., M. Bunning, P. Gordy, N. Panella, B. Blitvich, and R. Bowen (2005). Experimental and natural infection of North American bats with West Nile virus. American Journal of Tropical Medicine and Hygiene 73(2): 467-469. ISSN: 0002-9637.
Abstract: Big brown (Eptesicus fuscus) and Mexican free-tailed (Tadarida brasiliensis) bats were inoculated with the New York 99 strain of West Nile virus to assess their potential to serve as amplifying hosts and determine the clinical effect of infection. Groups of three or four bats were bled at daily intervals between 1 and 6 days after inoculation to determine the pattern of viremia. Beginning 2 days after inoculation, virus was isolated each day from one or more E. fuscus bats, in titers ranging from 10 to 180 plaque-forming units per milliliter of serum. Virus was not isolated from any of the sera collected from T brasiliensis bats. None of the bats from either species showed clinical signs associated with exposure to virus. Sera from an additional 149 bats collected in Louisiana in 2002 during an epizootic of West Nile fever were tested for antibodies to virus, and two were found to be positive. These data suggest that bats from these two widely distributed species are unlikely to serve as amplifying hosts for West Nile virus.
Descriptors: Eptesicus fuscus, Tadarida brasiliensis, transmission of viruses, West Nile virus, experimental and natural infection, USA.
Diuk Wasser, M.A., H.E. Brown, T.G. Andreadis, and D. Fish (2006). Modeling the spatial distribution of mosquito vectors for west Nile virus in Connecticut, USA. Vector Borne and Zoonotic Diseases 6(3): 283-295. ISSN: 1530-3667.
Abstract: The risk of transmission of West Nile virus (WNV) to humans is associated with the density of infected vector mosquitoes in a given area. Current technology for estimating vector distribution and abundance is primarily based on Centers for Disease Control and Prevention (CDC) light trap collections, which provide only point data. In order to estimate mosquito abundance in areas not sampled by traps, we developed logistic regression models for five mosquito species implicated as the most likely vectors of WNV in Connecticut. Using data from 32 traps in Fairfield County from 2001 to 2003, the models were developed to predict high and low abundance for every 30 X 30 m pixel in the County. They were then tested with an independent dataset from 16 traps in adjacent New Haven County. Environmental predictors of abundance were extracted from remotely sensed data. The best predictive models included non-forested areas for Culex pipiens, surface water and distance to estuaries for Cx. salinarius, surface water and grasslands/agriculture for Aedes vexans and seasonal difference in the normalized difference vegetation index and distance to palustrine habitats for Culiseta melanura. No significant predictors were found for Cx. restuans. The sensitivity of the models ranged from 75% to 87.5% and the specificity from 75% to 93.8%. In New Haven County, the models correctly classified 81.3% of the traps for Cx. pipiens, 75.0% for Cx. salinarius, 62.5% for Ae. vexans, and 75.0% for Cs. melanura. Continuous surface maps of habitat suitability were generated for each species for both counties, which could contribute to future surveillance and intervention activities.
Descriptors: culicidae, ecological techniques, transmission of viruses, flavivirus, modelling vector spatial distribution and abundance, population density, Connecticut.
Ebel, G., I. Rochlin, J. Longacker, and L. Kramer (2005). Culex restuans (Diptera: Culicidae) relative abundance and vector competence for West Nile virus. Journal of Medical Entomology. 42(5): 838-843. ISSN: 0022-2585.
Abstract: The abundance and vector competence of Culex restuans Theobald and Culex pipiens L. were compared to determine the relative importance of these species as West Nile virus (WNV) vectors in the northeastern United States. Abundance was estimated from egg raft collections at 12 sites in Albany, Suffolk, and Richmond counties, New York, during July, August, and September 2002 and 2003. Cx. restuans was more abundant than Cx. pipiens in both urban and rural areas, comprising 86% of 1,623 egg rafts collected. Vector competence for WNV was estimated after feeding on an artificial bloodmeal and in vitro transmission assays. The vector competence of the two species for WNV was similar, but the dynamics of infection seems to be mosquito species dependent. These findings suggest an important role for Cx. restuans in WNV transmission cycles in New York.
Descriptors: Culex restuans, Culex pipiens, population density, insect vectors, West Nile virus, vector competence, hematophagy, infection, virus transmission, virus replication, vector potential, New York.
Erickson, S.M., K.B. Platt, B.J. Tucker, R. Evans, S. Tiawsirisup, and W.A. Rowley (2006). The potential of Aedes triseriatus (Diptera: Culicidae) as an enzootic vector of West Nile virus. Journal of Medical Entomology 43(5): 966-970. ISSN: 0022-2585.
Abstract: The susceptibility of Aedes triseriatus (Say) (Diptera: Culicidae) to low levels of West Nile virus (family Flaviviridae, genus Flavivirus, WNV) was determined and compared with that of Culex pipiens L. to assess the likelihood of its participation in an enzootic cycle involving mammals. Ae. triseriatus and Cx. pipiens were exposed to WNV by feeding on baby chickens with WNV serum titers ranging from 10(4.1 +/- 0.1) to 10(8.6 +/- 0.1) plaque-forming units (PFU)/ml and from 10(4.1 +/- 0.1) to 10(7.0) PFU/ml, respectively. Infection rates and 95% confidence intervals (CIs) of 8% (4, 14) and 25% (15, 38) occurred in Ae. triseriatus and Cx. pipiens after feeding on chickens with WNV titers of 10(4.1 +/- 0.1) PFU/ml and increased to 65% (49, 79) and 100% (72, 100) in Ae. triseriatus and Cx. pipiens after feeding on chickens with titers of 10(7.1 +/- 0.1) PFU/ml. The mean infection rate of Ae. triseriatus ranged from 97% (84, 100) to 100% (79, 100) after feeding on chickens with WNV titers of > or = 10(8.2) PFU/ml. The infectious dose (ID)50 values for Ae. triseriatus and Cx. pipiens were 10(6.5) (6.4, 6.7) and 10(4.9) (4.6, 5.1) PFU/ml, respectively. The combined estimated transmission rate of Ae. triseriatus at 14 and 18 d after feeding on chickens with a mean WNV titer of 10(8.6 +/- 0.1) PFU/ml was 55%. Although Ae. triseriatus is significantly less susceptible to WNV than Cx. pipiens, the susceptibility of Ae. triseriatus to WNV titers < 10(5.0) PFU/ml and its ability to transmit WNV suggest that Ae. triseriatus has the potential to be an enzootic vector among mammalian populations.
Descriptors: West Nile virus, Aedes triseriatus, Culex pipiens, susceptibility, feeding, baby chickens, infection rate, enzootic vector.
Formosinho, P. and M.M. Santos Silva (2006). Experimental infection of Hyalomma marginatum ticks with West Nile virus. Acta Virologica 50(3): 175-180.
Abstract: To define the possible role of Hyalomma marginatum ticks in the transmission of West Nile virus (WNV) in Portugal an experimental infection was established. Ticks were fed on viremic rabbits previously infected with WNV. In different developmental stage of H. marginatum virus isolation and detection of viral antigen and viral RNA were attempted. The oral infection rates were 3%, 33% and 75% for engorged larvae, nymphs and females after oviposition, respectively. Transstadial transmission rates for nymphs exposed to virus as larvae, for adults exposed as larvae, and for adults exposed as nymphs were 33%, 11% and 46%, respectively. No evidence of transovarial transmission was obtained. Ticks in the stages of nymphs and adults were able to transmit the infection to uninfected hosts. This study demonstrated that H. marginatum could be involved in the natural circulation of WNV in Portugal.
Descriptors: West Nile virus, ticks, experimental infection, vector, Hyalomma marginatum, viremic rabbits, infection rates, transviral transmission.
Furumizo, R., W. Warashina, and H. Savage (2005). First collection of Anopheles punctipennis (Say) on Oahu, Hawaii: Implications for the potential introduction of West Nile virus. Journal of the American Mosquito Control Association. 21(2): 225-226. ISSN: 8756-971X.
Descriptors: Anopheles punctipennis, mosquitoes, geographical distribution, new geographic records, introduced species, insect vectors, West Nile virus, pest monitoring, disease surveillance, quarantine, Hawaii, pest-introduction, vector-surveillance.
Fyodorova, M.V., H.M. Savage, J.V. Lopatina, T.A. Bulgakova, A.V. Ivanitsky, O.V. Platonova, and A.E. Platonov (2006). Evaluation of potential West Nile virus vectors in Volgograd region, Russia, 2003 (Diptera: Culicidae): Species composition, bloodmeal host utilization, and virus infection rates of mosquitoes. Journal of Medical Entomology 43(3): 552-563. ISSN: 0022-2585.
Abstract: Potential West Nile virus (family Flaviviridae, genus Flavivirus, WNV) vectors were assessed during 2003 at indoor and outdoor collection sites in urban Volgograd, Russia, and in three nearby towns and surrounding rural areas. In total, 9,182 female mosquitoes comprising 13 species in six genera were collected. Relative abundance and bloodmeal host utilization differed temporarily and spatially. During June and July in Volgograd, Aedes vexans (Meigen) (85.4%) and Culex p. pipiens L. (7.6%) were the two most abundant species collected indoors, whereas during August, Cx. p. pipiens was the dominant species, accounting for 87.9% of specimens collected. Two WNV-positive mosquito pools were detected in August: one pool was composed of Cx. p. pipiens and the other pool of Culex modestus Ficalbi. Anopheles messeae Falleroni, Aedes caspius (Pallas), Ae. vexans, Cx. modestus, and Cx. p. pipiens used both humans and birds as bloodmeal sources. In urban areas, 20.4% of the Cx. p. pipiens fed on humans, 58.1% fed on chickens, and six specimens were positive for both chicken and human blood. Culex p. pipiens collected from flooded basements were predominantly autogenous (91.7%), whereas adult females resting in buildings with dry basements were composed of 67.5% anautogenous and 32.5% autogenous specimens. Our data suggest that the primary WNV vectors in the Volgograd region were Cx. p. pipiens and Cx. modestus and that intense transmission of WNV to humans in urban areas during the epidemic of 1999 may have been facilitated by the abundance and concentration of anautogenous Cx. p. pipiens in multistory buildings. The role of autogenous Cx. p. pipiens in urban transmission remains unresolved.
Descriptors: insect vectors, West Nile virus, climate, environment, feeding behavior, host parasite relations, Russia, urban population, West Nile fever epidemiology.
Gibbs, S.E., M.C. Wimberly, M. Madden, J. Masour, M.J. Yabsley, and D.E. Stallknecht (2006). Factors affecting the geographic distribution of West Nile virus in Georgia, USA: 2002-2004. Vector Borne and Zoonotic Diseases 6(1): 73-82. ISSN: 1530-3667.
Abstract: The distribution of West Nile virus (WNV) is dependent on the occurrence of both susceptible avian reservoir hosts and competent mosquito vectors. Both factors can be influenced by geographic variables such as land use/landcover, elevation, human population density, physiographic region, and temperature. The current study uses geographic information systems (GIS) and logistic regression analyses to model the distribution of WNV in the state of Georgia based on a wild bird indicator system, and to identify human and environmental predictor variables that are important in the determination of WNV distribution. A database for Georgia was constructed that included (1) location points of all the avian samples tested for WNV, (2) local land use classifications, including temperature, physiographic divisions, land use/landcover, and elevation, (3) human demographic data from the U.S. Census, and (4) statistics summarizing land cover, elevation, and climate within a 1-km-radius landscape around each sample point. Logistic regression analysis was carried out using the serostatus of avian collection sites as the dependent variable. Temperature, housing density, urban/suburban land use, and mountain physiographic region were important variables in predicting the distribution of WNV in the state of Georgia. While weak, the positive correlation between WNV-antibody positive sites and the urban/suburban environment was consistent throughout the study period. The risks associated with WNV endemicity appear to be increased in urban/ suburban areas and decreased in the mountainous region of the state. This information may be used in addressing regional public health needs and mosquito control programs.
Descriptors: West Nile virus distribution, avian reservoir hosts, vectors, geographic variables, control programs.
Gingrich, J.B., R.D. Anderson, G.M. Williams, L. O'Connor, and K. Harkins (2006). Stormwater ponds, constructed wetlands, and other best management practices as potential breeding sites for West Nile virus vectors in Delaware during 2004. Journal of the American Mosquito Control Association 22(2): 282-291.
Abstract: We performed longitudinal surveys of mosquito larval abundance (mean mosquito larvae per dip) in 87 stormwater ponds and constructed wetland in Delaware from June to September 2004. We analyzed selected water quality factors, water depth, types of vegetation, degree of shade, and level of insect predation in relation to mosquito abundance. The 2004 season was atypical, with most ponds remaining wet for the entire summer. In terms of West Nile virus (WNV) vectors, wetlands predominantly produce Aedes vexans, Culex pipiens pipiens, and Culex restuans. Retention ponds generally produced the same species as wetlands, except that Cx. p. pipiens was more abundant than Cx. restuans in retention ponds. Aedes vexans and Culex salinarius were the most abundant species to Conservation Restoration Enhancement Program ponds. Sand filters uniquely produced high numbers of Cx. restuans, Cx. p. pipiens, and Aedes japonicus japonicus, a newly invasive vector species. Site that alternately dried and flooded, mostly detention ponds, forebays of retention ponds, and some wetlands often produced Ae. vexans, an occasional WNV bridge vector species. Overall, seasonal distribution of vectors was bimodal, with peaks occurring during early and late summer. Ponds with shallow sides and heavy shade generally produced an abundance of mosquitoes, unless insect predators were abundant. Bright, sunny ponds with steep sides and little vegetation generally produced the fewest mosquitoes. The associations among mosquito species and selected vegetation types are discussed.
Descriptors: Culicidae virology, insect vectors, West Nile virus, Aedes, Anopheles, Culex virology, Delaware, fresh water ecology, population density, seasons, waste disposal.
Gingrich, J.B. and G.M. Williams (2005). Host feeding patterns of suspected West Nile virus mosquito vectors in Delaware, 2001-2002. Journal of the American Mosquito Control Association 21(2): 194-200. ISSN: 8756-971X.
Abstract: Paucity of data on host-feeding patterns and behavior of 43 mosquito species that are reported as suspected West Nile virus (WN) vectors has limited full evaluation of their vectorial capacity. Recent studies addressing this issue need additional confirmation and should also be expanded to include collections of species or subpopulations attracted to humans. We used 4 types of collection methods to collect mosquitoes, including omnidirectional Fay-Prince traps, Centers for Disease Control-type light traps, gravid traps, and human-landing collections. Mosquitoes were collected during 2 full WN transmission seasons in 2001 and 2002, at 9 different sites across Delaware. We collected from various habitats, including salt marshes, brackish water areas, woodlands, a tire dump, a racetrack, and a mushroom farm. Blood-meal analyses were performed on parous mosquitoes by using a sandwich-type enzyme-linked immunosorbent assay. We tested primarily for 5 common host species, including rabbits, dogs, deer, horses, and chickens. We obtained substantial host-feeding data from 8 mosquito species. The most mammalophilic species were Anopheles quadrimaculatus, Coquillettidia perturbans, and Aedes albopictus, whereas the most ornithophilic species was Culex pipiens. Aedes albopictus was the most anthropophilic species, whereas Ae. vexans and Cq. perturbans exhibited relatively low attraction to humans. Culex salinarius was the species with the most diverse host-feeding activity. Based on feeding behavior, Cx. pipiens and Cx. salinarius appear to be the most likely bridge vectors. Other species may have opportunities to be bridge vectors under special circumstances, as discussed in the paper.
Descriptors: culicidae, feeding behaviour, avian hosts, human hosts, mammalian hosts, transmission of viruses, West Nile virus, avian, mammalian and human host preferences and feeding behaviour, suspected vectors, Delaware.
Girard, Y., V. Popov, J. Wen, V. Han, and S. Higgs (2005). Ultrastructural study of West Nile virus pathogenesis in Culex pipiens quinquefasciatus (Diptera: Culicidae). Journal of Medical Entomology. 42(3): 429-444. ISSN: 0022-2585.
Abstract: The ultrastructural features of West Nile virus (WNV) replication and dissemination in orally infected Culex pipiens quinquefasciatus Say were analyzed over a 25-d infection period. To investigate the effects of virus replication on membrane induction, cellular organization, and cell viability in midgut and salivary gland tissues, midguts were dissected on days 3, 7, 14, and 21, and salivary glands were collected on days 7, 14, 21, and 25 postinfection (d.p.i.) for examination by transmission electron microscopy (TEM). Whole mosquito heads were embedded for TEM analysis 14 d.p.i. to localize WNV particles and to investigate the effects of replication on nervous tissues of the brain. Membrane proliferation was induced by WNV in the midgut epithelium, midgut muscles, and salivary glands, although extensive endoplasmic reticulum swelling was a unique feature of salivary gland infection. TEM revealed WNV-induced pathology in salivary glands at 14, 21, and 25 d.p.i., and we hypothesize that long-term virus infection of this tissue results in severe cellular degeneration and apoptotic-like cell death. This finding indicates that the efficiency of WNV transmission may decrease with mosquito age postinfection.
Descriptors: Culex pipiens, insect vectors, West Nile virus, infection, pathogenesis, tissue distribution, histopathology, midgut, salivary glands, brain, ultrastructure, apoptosis, virus replication, virus-dissemination.
Godsey, M.S.J., R. Nasci, H.M. Savage, S. Aspen, R. King, A.M. Powers, K. Burkhalter, L. Colton, D. Charnetzky, S. Lasater, V. Taylor, and C.T. Palmisano (2005). West Nile virus-infected mosquitoes, Louisiana, 2002. Emerging Infectious Diseases 11(9): 1399-1404. ISSN: 1080-6040.
Abstract: Human cases of West Nile virus (WNV) disease appeared in St. Tammany and Tangipahoa Parishes in southeastern Louisiana in June 2002. Cases peaked during July, then rapidly declined. We conducted mosquito collections from August 3 to August 15 at residences of patients with confirmed and suspected WNV disease to estimate species composition, relative abundance, and WNV infection rates. A total of 31,215 mosquitoes repre senting 25 species were collected by using primarily gravid traps and CO2-baited light traps. Mosquitoes containing WNV RNA were obtained from 5 of 11 confirmed case sites and from 1 of 3 sites with non-WNV disease. WNV RNA was detected in 9 mosquito pools, including 7 Culex quinquefasciatus, 1 Cx salinarius, and 1 Coquillettidia perturbans. Mosquito infection rates among sites ranged from 0.8/1,000 to 10.9/1,000. Results suggest that Cx. quinquefasciatus was the primary epizootic/epidemic vector, with other species possibly playing a secondary role.
Descriptors: culicidae, human hosts, transmission of viruses, West Nile virus, vector community structure and human disease prevalence relationships, community structure, population density, Louisiana, St Tammany and Tangipahoa Parishes.
Godsey, M.J., M. Blackmore, N. Panella, K. Burkhalter, K. Gottfried, L. Halsey, R. Rutledge, S. Langevin, R. Gates, and K. Lamonte (2005). West Nile virus epizootiology in the southeastern United States, 2001. Vector Borne and Zoonotic Diseases. 5(1): 82-89. ISSN: 1530-3667.
Descriptors: West Nile virus, virus transmission, insect vectors, Culex, Culiseta melanura, mosquitoes, disease reservoirs, birds, disease detection, antibody detection, seroprevalence, chickens, sentinel animals, Southeastern United States, Florida, Georgia.
Gu, W., R. Lampman, N. Krasavin, R. Berry, and R. Novak (2006). Spatio-temporal analyses of West Nile virus transmission in culex mosquitoes in northern Illinois, USA, 2004. Vector Borne and Zoonotic Diseases 6(1): 91-98. ISSN: 1530-3667.
Abstract: After a severe outbreak of West Nile virus (WNV) in Cook County, Illinois, in 2002, detections of WNV in mosquitoes were frequent across the state in the following years despite small numbers of human cases. We conducted a spatio-temporal analysis of Culex (subgenus Culex) mosquitoes collected in 2004 in three mosquito abatement districts (MAD) in Cook County by calculating monthly estimates of mosquito density, prevalence of infected mosquitoes, and exposure intensity, which in turn is a product of mosquito density and infection rates. Mosquito infections were detected early at three sites in late May and were widely detected throughout the three MADs in the summer with infection rates as high as 13 per 1000 in August. Exposure intensities were higher at sites adjacent to the Des Plaines River, especially in August and September. The aggregated pattern of WNV transmission along the river might be related to the existence of substantial forest preserves and wetlands that might produce ecological conditions favorable for mosquito proliferation and interactions between mosquitoes and birds.
Descriptors: West Nile virus, culex mosquitoes, infection rate, wetlands, birds, prevalence, ecological conditions.
Hahn, D.C., N.M. Nemeth, E. Edwards, P.R. Bright, and N. Komar (2006). Passive West Nile virus antibody transfer from maternal Eastern screech-owls (Megascops asio) to progeny. Avian Diseases 50(3): 454-455. ISSN: 0005-2086.
Abstract: Transovarial antibody transfer in owls has not been demonstrated for West Nile virus (WNV). We sampled chicks from captive adult WNV-antibody-positive Eastern Screech-Owls (Megascops asio) to evaluate the prevalence of transovarial maternal antibody transfer, as well as titers and duration of maternal antibodies. Twenty-four owlets aged 1 to 27 days old circulated detectable antibodies with neutralizing antibody titers ranging from 20 to 1600 (median 1:40). Demonstrating that WNV antibodies are passively transferred transovarially is important for accurate interpretation of serologic data from young birds.
Descriptors: viral blood antibodies, bird diseases, maternally acquired immunity, West Nile virus, screech owls.
Hayes, E.B., N. Komar, R.S. Nasci, S.P. Montgomery, D.R. O'Leary, and G.L. Campbell (2005). Epidemiology and transmission dynamics of West Nile virus disease. Emerging Infectious Diseases 11(8): 1167-1173. ISSN: 1080-6040.
Abstract: From 1937 until 1999, West Nile virus (WNV) garnered scant medical attention as the cause of febrile illness and sporadic encephalitis in parts of Africa, Asia, and Europe. After the surprising detection of WNV in New York City in 1999, the virus has spread dramatically westward across the United States, southward into Central America and the Caribbean, and northward into Canada, resulting in the largest epidemics of neuroinvasive WNV disease ever reported. From 1999 to 2004, >7,000 neuroinvasive WNV disease cases were reported in the United States. In 2002, WNV transmission through blood transfusion and organ transplantation was described for the first time, intrauterine transmission was first documented, and possible transmission through breastfeeding was reported. This review highlights new information regarding the epidemiology and dynamics of WNV transmission, providing a new platform for further research into preventing and controlling WNV disease.
Descriptors: disease outbreaks, insect vectors, West Nile fever epidemiology, West Nile virus growth and development, humans, United States.
Higgs, S., B. Schneider, D. Vanlandingham, K. Klingler, and E. Gould (2005). Nonviremic transmission of West Nile virus. Proceedings of the National Academy of Sciences of the United States of America. 102(25): 8871-8874. ISSN: 0027-8424.
Abstract: West Nile virus (WNV) is now the predominant circulating arthropod-borne virus in the United States with >15,000 human cases and >600 fatalities since 1999. Conventionally, mosquitoes become infected when feeding on viremic birds and subsequently transmit the virus to susceptible hosts. Here, we demonstrate nonviremic transmission of WNV between cofeeding mosquitoes. Donor, Culex pipiens quinquefasciatus mosquitoes infected with WNV were fed simultaneously with uninfected "recipient" mosquitoes on naive mice. At all times, donor and recipient mosquitoes were housed in separate sealed containers, precluding the possibility of mixing. Recipients became infected in all five trials, with infection rates as high as 5.8% and no detectable viremia in the hosts. Remarkably, a 2.3% infection rate was observed when 87 uninfected mosquitoes fed adjacent to a single infected mosquito. This phenomenon could potentially enhance virus survival, transmission, and dispersion and obviate the requirement for viremia. All vertebrates, including immune and insusceptible animals, might therefore facilitate mosquito infection. Our findings question the status of dead-end hosts in the WNV transmission cycle and may partly explain the success with which WNV established and rapidly dispersed throughout North America.
Descriptors: West Nile virus, disease transmission, Culex quinquefasciatus, Culex pipiens, mosquitoes, insect vectors, hematophagy, mice, Culex-pipiens-quinquefasciatus.
Hutcheson, H.J., C.H. Gorham, C. Machain Williams, M.A. Lorono Pino, A.M. James, N.L. Marlenee, B. Winn, B.J. Beaty, and C.D. Blair (2005). Experimental transmission of West Nile virus (Flaviviridae: Flavivirus) by Carios capensis ticks from North America. Vector Borne and Zoonotic Diseases 5(3): 293-295. ISSN: 1530-3667.
Abstract: Seabird soft ticks, Carios capensis (Ixodida: Argasidae), originally collected from coastal Georgia, USA, were allowed to ingest a blood meal from pekin ducklings (Anas domesticus) infected with WNV. After 35 days of extrinsic incubation, the ticks transmitted virus to naive ducklings. WNV was detected via plaque assay and RTPCR in ticks and in tissues and serum of ducklings 7 days post infestation.
Descriptors: arachnid vectors, bird diseases, ducks, ticks, West Nile fever, viral RNA analysis, reverse transcriptase polymerase chain reaction, viremia.
Kilpatrick, A.M., P. Daszak, M.J. Jones, P.P. Marra, and L.D. Kramer (2006). Host heterogeneity dominates West Nile virus transmission. Proceedings. Biological Sciences The Royal Society 273(1599): 2327-2333.
Abstract: Heterogeneity in host populations and communities can have large effects on the transmission and control of a pathogen. In extreme cases, a few individuals give rise to the majority of secondary infections, which have been termed super spreading events. Here, we show that transmission of West Nile virus (WNV) is dominated by extreme heterogeneity in the host community, resulting in highly inflated reproductive ratios. A single relatively uncommon avian species, American robin (Turdus migratorius), appeared to be responsible for the majority of WNV-infectious mosquitoes and acted as the species equivalent of a super spreader for this multi-host pathogen. Crows were also highly preferred by mosquitoes at some sites, while house sparrows were significantly avoided. Nonetheless, due to their relative rarity, corvids (crows and jays) were relatively unimportant in WNV amplification. These results challenge current beliefs about the role of certain avian species in WNV amplification and demonstrate the importance of determining contact rates between vectors and host species to understand pathogen transmission dynamics.
Descriptors: birds, Culicidae physiology, feeding behavior, West Nile fever transmission, West Nile virus isolation and purification.
Kilpatrick, A.M., L.D. Kramer, S.R. Campbell, E.O. Alleyne, A.P. Dobson, and P. Daszak (2005). West Nile virus risk assessment and the bridge vector paradigm. Emerging Infectious Diseases 11(3): 425-429. ISSN: 1080-6040.
Abstract: In the northeast United States, control of West Nile virus (WNV) vectors has been unfocused because of a lack of accurate knowledge about the roles different mosquitoes play in WNV transmission. We analyzed the risk posed by 10 species of mosquitoes for transmitting WNV to humans by using a novel risk-assessment measure that combines information on the abundance, infection prevalence, vector competence, and biting behavior of vectors. This analysis suggests that 2 species (Culex pipiens L. and Cx. restuans Theobald [Diptera: Cilicidae]) not previously considered important in transmitting WNV to humans may be responsible for up to 80% of human WNV infections in this region. This finding suggests that control efforts should be focused on these species which may reduce effects on nontarget wetland organisms. Our risk measure has broad applicability to other regions and diseases and can be adapted for use as a predictive tool of future human WNV infections.
Descriptors: culex mosquitoes physiology, insect vectors, West Nile fever transmission, New England, risk, West Nile virus.
Kilpatrick, A.M., L.D. Kramer, M.J. Jones, P.P. Marra, and P. Daszak (2006). West Nile virus epidemics in North America are driven by shifts in mosquito feeding behavior. PLoS Biology 4(4): E82.
Abstract: West Nile virus (WNV) has caused repeated large-scale human epidemics in North America since it was first detected in 1999 and is now the dominant vector-borne disease in this continent. Understanding the factors that determine the intensity of the spillover of this zoonotic pathogen from birds to humans (via mosquitoes) is a prerequisite for predicting and preventing human epidemics. We integrated mosquito feeding behavior with data on the population dynamics and WNV epidemiology of mosquitoes, birds, and humans. We show that Culex pipiens, the dominant enzootic (bird-to-bird) and bridge (bird-to-human) vector of WNV in urbanized areas in the northeast and north-central United States, shifted its feeding preferences from birds to humans by 7-fold during late summer and early fall, coinciding with the dispersal of its preferred host (American robins, Turdus migratorius) and the rise in human WNV infections. We also show that feeding shifts in Cx. tarsalis amplify human WNV epidemics in Colorado and California and occur during periods of robin dispersal and migration. Our results provide a direct explanation for the timing and intensity of human WNV epidemics. Shifts in feeding from competent avian hosts early in an epidemic to incompetent humans after mosquito infection prevalences are high result in synergistic effects that greatly amplify the number of human infections of this and other pathogens. Our results underscore the dramatic effects of vector behavior in driving the transmission of zoonotic pathogens to humans.
Descriptors: West Nile virus, vector-borne diseases, mosquito hosts, Culex pipens, feeding preferences of mosquitos, birds to human shift in preference, seasonal shift, WNV epidemics.
Kipp, A.M., J.A. Lehman, R.A. Bowen, P.E. Fox, M.R. Stephens, K. Klenk, N. Komar, and M.L. Bunning (2006). West Nile virus quantification in feces of experimentally infected American and fish crows. American Journal of Tropical Medicine and Hygiene 75(4): 688-690. ISSN: 0002-9637.
Abstract: To better understand the potential environmental health risk presented by West Nile virus (WNV)-contaminated feces, we quantified the amount of WNV present in the feces of experimentally infected American crows (Corvus brachyrhynchos) and fish crows (Corvus ossifragus). Peak fecal titers ranged from 10(3.5) to 10(8.8) plaque-forming units (PFU)/g for 10 American crows and from 10(2.3) to 10(6.4) PFU/g for 10 fish crows. The presence of infectious WNV in bird feces indicates a potential for direct transmission of WNV. Thus, handlers of sick or dead birds should take appropriate precautions to avoid exposure to fecal material.
Descriptors: bird diseases, crows, feces, virus shedding, West Nile fever, West Nile virus isolation and purification.
Komar, N., N.A. Panella, S.A. Langevin, A.C. Brault, M. Amador, E. Edwards, and J.C. Owen (2005). Avian hosts for West Nile Virus in St. Tammany Parish, Louisiana, 2002. American Journal of Tropical Medicine and Hygiene 73(6): 1031-1037. ISSN: 0002-9637.
Abstract: West Nile virus (WNV) infections in free-ranging birds were studied in Slidell, St. Tammany Parish, Louisiana, after a human encephalitis outbreak peaked there in July 2002. Seroprevalence in resident, free-ranging wild birds in one suburban site was 25% and 24% in August and October, respectively, indicating that most transmission had ceased by early August. Mortality rates, seroprevalence rates, host competence, and crude population estimates were used in mathematical models to predict actual infection rates, population impacts, and importance as amplifying hosts for several common passerine birds. Northern cardinal (Cardinalis cardinalis) and house sparrow (Passer domesticus) were the principal amplifying hosts, but blue jay (Cyanocitta cristata) and northern mockingbird (Mimus polyglottos) also contributed. The blue jay population was reduced by an estimated 47%. A variety of passerine bird species combined to play an important role as amplifying hosts in the WNV transmission cycle.
Descriptors: aves, viral diseases, West Nile virus, prevalence survey and role in transmission cycle, transmission of viruses, hosts role in transmission cycle, Luisiana, St Tammany parish, viral disease prevalence survey and role in transmission cycle.
Kunkel, K.E., R.J. Novak, R.L. Lampman, and W. Gu (2006). Modeling the impact of variable climatic factors on the crossover of Culex restauns [restuans] and Culex pipiens (Diptera: Culicidae), vectors of West Nile virus in Illinois. American Journal of Tropical Medicine and Hygiene 74(1): 168-173. ISSN: 0002-9637.
Abstract: The aim of this study was to model the impact of temperature on the timing of the seasonal shift in relative proportion of Culex restuans Theobald and Culex pipiens L. in Illinois. The temporal pattern of West Nile virus (WNV) and St. Louis encephalitis virus transmission in the midwest exhibits a late summer to early fall peak in activity, which parallels the temporal increase in the abundance of Cx. pipiens. The daily number of egg rafts oviposited by each species has been monitored at multiple surveillance sites in Urbana-Champaign in central Illinois for more than 13 years. The time when the two Culex species are in equal abundance (crossover) varies considerably from year to year. Our investigation of several thermal measures indicated that this variation was related in large part to climatic conditions with warmer (cooler) temperatures correlated to earlier (later) crossover dates. Models based on degree days and the number of days in which the daily maximum temperature exceeded an upper temperature threshold explained more than 60% of the variance in crossover dates. In contrast, models based on the number of days in which the daily minimum temperature exceeded a lower temperature threshold explained no more than 52% of the variance. An evaluation of these models demonstrated that they provide relatively simple and accurate estimates of crossover date from daily temperature data, a necessary component for developing an overall climatic index for the risk of WNV transmission in Illinois.
Descriptors: Culex pipiens, Culex restuans, viral diseases, West Nile virus, transmission of viruses, temperature effect on seasonal change in relative abundance relations, relative abundance, climate and weather, temperature, Illinois, Urbana Champaign region.
Lampman, R., M. Slamecka, N. Krasavin, K. Kunkel, and R. Novak (2006). Culex population dynamics and West Nile virus transmission in east-central Illinois. Journal of the American Mosquito Control Association 22(3): 390-400.
Abstract: Temporal changes in the abundance Culex restuans and Culex pipiens were monitored in east-central Illinois for over a decade using infusion-baited oviposition traps. The 2 species typically exhibited a seasonal shift in relative abundance with a mean crossover date (when the proportion of egg rafts from both species is equal) of August 10 or 11, depending on leap year, with a 95% confidence interval of +/- 10.7 days. The date of crossover was linearly related to the date of last spring frost and occurred on average about 123 days after the last spring frost. Despite the predictability of crossover, the weekly pattern in the proportion of Cx. pipiens before and after crossover varied considerably, even between years with similar crossover dates. After West Nile virus became established in our area, we found that transmission based on Culex from gravid traps did not increase until Cx. pipiens abundance increased in oviposition traps. Infection rates peaked within the half-month period after crossover. The peak in Cx. pipiens abundance in oviposition traps during this 3-year period was between the 2nd half of August and the end of September. A higher magnitude of transmission in 2002 coincided with warmer temperatures during July and August and an extended period in which the 2 Culex species were in relatively equal abundance.
Descriptors: Culex sp., West Nile fever transmission, West Nile virus, Illinois, epidemiology, insect vectors virology, oviposition, population dynamics, seasons, time factors.
Medlock, J., K. Snow, and S. Leach (2005). Potential transmission of West Nile virus in the British Isles: an ecological review of candidate mosquito bridge vectors. Medical and Veterinary Entomology. 19(1): 2-21. ISSN: 0269-283X.
Descriptors: West Nile virus, arboviruses, insect vectors, Culicidae, mosquitoes, virus transmission, vector potential, insect ecology, zoonoses, literature reviews, British Isles, Ireland, United Kingdom, vector-ecology.
Miramontes, R.J., W.E. Lafferty, B.K. Lind, and M.W. Oberle (2006). Is agricultural activity linked to the incidence of human West Nile virus? American Journal of Preventive Medicine 30(2): 160-163.
Abstract: BACKGROUND: West Nile virus (WNV) has spread throughout the contiguous United States. During the 2002-2003 period, there were 14,023 laboratory-confirmed human cases of WNV in 45 states and 541 associated deaths. Factors that affect case distribution are poorly understood. This study assessed the relationship of environmental factors and agricultural activity with the presence of human WNV cases. METHODS: County-level data were collected that included 2002 and 2003 WNV surveillance data, temperature, dairy revenue, precipitation, total irrigated acres, and total crop revenue. Logistic regression models were used to determine which risk factors were significantly associated with WNV human cases. RESULTS: Significant independent predictors for counties with human WNV cases were population (odds ratio [OR]=1.20, p<0.0001); higher average daily temperature in April through October (OR=1.19 for each additional degree Fahrenheit, p<0.0001); and total crop sales (OR=1.14 (p<0.001). The ORs for these predictors increased in an analysis of counties with ten or more cases. CONCLUSIONS: Higher temperature and farming activity may be strongly associated with the incidence of human WNV infection. Larger studies of more agricultural centers are warranted to determine which environmental factors increase the risk of human infection and how these infections can be prevented.
Descriptors: agriculture, Culex mosquitoes, environmental microbiology, insect vectors, West Nile fever epidemiology, logistic models, risk factors, United States.
Molaei, G. and T.G. Andreadis (2006). Identification of avian- and mammalian-derived bloodmeals in Aedes vexans and Culiseta melanura (Diptera: Culicidae) and its implication for West Nile virus transmission in Connecticut, U.S.A. Journal of Medical Entomology 43(5): 1088-1093. ISSN: 0022-2585.
Abstract: To evaluate the host-feeding patterns of Aedes vexans (Meigen) and Culiseta melanura (Coquillett) as secondary vectors of West Nile virus (family Flaviviridae, genus Flavivirus, WNV) in Northeastern United States, we identified the source of vertebrate bloodmeals by sequencing portions of the cytochrome b gene of mitochondrial DNA. Analysis of polymerase chain reaction products from a total of 119 Ae. vexans revealed that 92.4% of individuals acquired blood solely from mammalian and 2.5% from avian hosts. Mixed bloodmeals from both avian and mammalian hosts were detected in 5% of individuals of this species. Ae. vexans obtained vertebrate bloodmeals most frequently from white-tailed deer (80%) followed by domestic horse, American robin, eastern cottontail, and domestic cat. In contrast, Cs. melanura fed predominantly on avian species (89.6%) but exhibited some inclination for mammalian blood (4.2%). Individual mosquitoes containing mixed bloodmeals were also identified in 6% of Cs. melanura. American robin was the most common source of vertebrate blood for Cs. melanura (23%), followed by wood thrush and gray catbird. American crow represented only 2% of the bloodmeals identified in Cs. melanura, as was similarly found with other recognized Culex vectors of WNV in the northeast. These findings support the view that Ae. vexans is likely to be a relatively important "bridge vector" to large mammals, including deer and horse, whereas Cs. melanura likely plays a secondary role in enzootic transmission of WNV among free-ranging birds in more rural environs.
Descriptors: Aedes vexans, Culiseta melanura, vectors, bloodmeals, mitochondrial DNA, polymerase chain reaction, horse, American robin, Eastern cottontail rabbit.
Molaei, G., T.G. Andreadis, P.M. Armstrong, J.F. Anderson, and C.R. Vossbrinck (2006). Host feeding patterns of Culex mosquitoes and West Nile virus transmission, northeastern United States. Emerging Infectious Diseases 12(3): 468-474. ISSN: 1080-6040.
Abstract: To evaluate the role of Culex mosquitoes as enzootic and epidemic vectors for WNV, we identified the source of vertebrate blood by polymerase chain reaction amplification and sequencing portions of the cytochrome b gene of mitochondrial DNA. All Cx. restuans and 93% of Cx. pipiens acquired blood from avian hosts; Cx. salinarius fed frequently on both mammals (53%) and birds (36%). Mixed-blood meals were detected in 11% and 4% of Cx. salinarius and Cx. pipiens, respectively. American robin was the most common source of vertebrate blood for Cx. pipiens (38%) and Cx. restuans (37%). American crow represented 1% of the blood meals in Cx. pipiens. and none in Cx. restuans. Human-derived blood meals were identified from 2 Cx. salinarius and 1 Cx. pipiens. Results suggest that Cx. salinarius is an important bridge vector to humans, while Cx. pipiens and Cx. restuans are more efficient enzootic vectors in the northeastern United States.
Descriptors: aves, mammalia, dipteran parasites, culex, host feeding patterns and role in viral disease transmission, viral diseases, West Nile virus, dipteran vector role inferred from host feeding patterns.
Mumcuoglu, K.Y., C. Banet Noach, M. Malkinson, U. Shalom, and R. Galun (2005). Argasid ticks as possible vectors of West Nile virus in Israel. Vector Borne and Zoonotic Diseases 5(1): 65-71. ISSN: 1530-3667.
Abstract: Mites and soft ticks collected directly from wild and domestic birds and their nests were tested for the presence of West Nile virus (WNV). The cattle egret argas, Argas arboreus, was collected from the nests of seven cattle egret colonies. Out of 1,000 A. arboreus pools examined, 16 were positive for WNV based on RT-PCR technique. The positive pools were from four nesting colonies of birds. Out of 37 cattle egret squabs examined, 37.8% had serum-neutralizing antibodies to WNV. WNV RNA was also detected in one out of 15 pools of R. turanicus, in one out of 21 pools of O. sylviarum, and in one out of 18 pools of D. gallinae, while 63 pools of A. reflexus, 11 of R. sanguineus, and 30 of Hyalomma spec. were negative. The role of mites and ticks in maintaining the endemic state of WNV in Israel is discussed.
Descriptors: arachnid vectors, argasidae, bird diseases, West Nile fever transmission, West Nile virus isolation and purification, birds as disease vectors, Israel, reverse transcriptase polymerase chain reaction, tick infestations, ticks, West Nile fever, zoonoses.
Ozer, N. (2006). Bati Nil virusu ve vektorleri. [West Nile virus and its vectors]. Mikrobiyoloji Bulteni 40(1-2): 121-128.
Abstract: There are more than five hundred known arthropod-borne viruses (arboviruses) all around the world and approximately hundred of them may cause disease in humans. During the past 20 years there has been a dramatic resurgence or emergence of epidemic arboviral diseases affecting both humans and domestic animals. Many factors play important roles in the emergence of arboviral diseases like Yellow Fever, Dengue, West Nile encephalitis, and of other diseases such as malaria and leishmaniasis in countries where they have not been previously encountered and in the increase in incidences where they have been under control. Some of these are demographic factors such as global population increase and uncontrolled urbanization; social changes such as modern transportation, human encroachment on natural disease hotspots; changes in agricultural activities such as the use of new irrigation techniques; deforestation; genetic changes in the pathogens; preventive measures and probably global climate changes. Mosquitoes are among the most important vectors carrying viruses belonging to Alphavirus, Flavivirus, Bunyavirus and Phlebovirus genera. All of the above factors have contributed to the increase in mosquito populations and closer contact between humans and mosquito vectors. West Nile virus notable after the epidemic of 1996 in Romania in Europe is one of the latest examples indicating that viruses can jump continents and produce epidemics. In this review article, the distribution of West Nile virus and its principal vectors and also its importance by means of public health, have been discussed.
Descriptors: West Nile virus, arthropod-borne arboviruses, mosquito populations, literature review.
Language of Text: Turkish.
Paz, S. (2006). The West Nile Virus outbreak in Israel (2000) from a new perspective: The regional impact of climate change. International Journal of Environmental Health Research 16(1): 1-13.
Abstract: The West Nile Virus (WNV) outbreak in Israel in 2000 appeared after medical and climatic warning signs. Re-analysis of the epidemic from a new viewpoint, the regional impact of global warming, especially the worsening in the summers' heat conditions, is presented. The disease appeared averagely at a lag of 3-9 weeks (strongest correlation = lag of 7 weeks). The minimum temperature was found as the most important climatic factor that encourages the disease earlier appearance. Extreme heat is more significant than high air humidity for increasing WNV cases. An early extreme rise in the summer temperature could be a good indicator of increased vector populations. While 93.5% of cases were in the metropolitan areas, the disease was not reported in the sub-arid regions. The outbreak development was comparable to the cases from Romania (1996) and NYC (1999). Each of those epidemics appeared after a long heatwave.
Descriptors: disease outbreaks, disease vectors, West Nile fever epidemiology, West Nile virus, climate, humidity, Israel epidemiology, seasons, temperature, time factors, urban population.
Phillips, R.A. and K. Christensen (2006). Field-caught Culex erythrothorax larvae found naturally infected with West Nile virus in Grand County, Utah. Journal of the American Mosquito Control Association 22(3): 561-562.
Abstract: Culex erythrothorax larvae were collected from a bulrush-cattail marsh near Moab, Grand County, Utah, October 28, 2004, and found positive for West Nile virus (WNV) RNA by real-time reverse-transcriptase polymerase chain reaction (RT-PCR). This demonstrates WNV vertical transmission in this species in the wild and suggests that vertical transmission in overwintering Cx. erythrothorax larvae may contribute to WNV overwintering.
Descriptors: Culex sp., West Nile virus, larva virology, reverse transcriptase polymerase chain reaction, Utah.
Reisen, W.K., C.M. Barker, R. Carney, H.D. Lothrop, S.S. Wheeler, J.L. Wilson, M.B. Madon, R. Takahashi, B. Carroll, S. Garcia, Y. Fang, M. Shafii, N. Kahl, S. Ashtari, V. Kramer, C. Glaser, and C. Jean (2006). Role of corvids in epidemiology of west Nile virus in southern California. Journal of Medical Entomology 43(2): 356-367. ISSN: 0022-2585.
Abstract: The invasion of different southern California landscapes by West Nile virus (WNV) and its subsequent amplification to epidemic levels during 2004 enabled us to study the impact of differing corvid populations in three biomes: the hot Colorado desert with few corvids (Coachella Valley), the southern San Joaquin Valley (Kern County) with large western scrub-jay but small American crow populations, and the cool maritime coast (Los Angeles) with a large clustered American crow population. Similar surveillance programs in all three areas monitored infection rates in mosquitoes, seroconversion rates in sentinel chickens, seroprevalence in wild birds, numbers of dead birds reported by the public, and the occurrence of human cases. Infection rates in Culex tarsalis Coquillett and sentinel chicken seroconversion rates were statistically similar among all three areas, indicating that highly competent mosquito hosts were capable of maintaining enzootic WNV transmission among less competent and widely distributed avian hosts, most likely house sparrows and house finches. In contrast, infection rates in Culex pipiens quinquefasciatus Say were statistically higher in Kern and Los Angeles counties with elevated corvid populations than in Coachella Valley with few corvids. Spatial analyses of dead corvids showed significant clusters near known American crow roosts in Los Angeles that were congruent with clusters of human cases. In this area, the incidence of human and Cx. p. quinquefasciatus infection was significantly greater within corvid clusters than without, indicating their importance in virus amplification and as a risk factor for human infection. In contrast the uniform dispersion by territorial western scrub-jays resulted in a high, but evenly distributed, incidence of human disease in Kern County.
Descriptors: crows, culex mosquitoes, insect vectors, West Nile fever epidemiology, West Nile virus, California, chickens, incidence, population density, population surveillance, sentinel surveillance, zoonoses transmission.
Reisen, W.K., S.S. Wheeler, S. Yamamoto, Y. Fang, and S. Garcia (2005). Nesting Ardeid colonies are not a focus of elevated West Nile virus activity in southern California. Vector Borne and Zoonotic Diseases 5(3): 258-266. ISSN: 1530-3667.
Abstract: A large nesting colony of Ardeid birds at the Finney-Ramer Wildlife Refuge in Imperial County, California, did not appear to be a focus of West Nile virus (WNV) amplification during the summer of 2004. Blood samples taken during June and July from 155 nestlings of four species of Ardeid birds (cattle egrets, black-crowned night herons, great egrets, and snowy egrets) and five nestling double-crested cormorants yielded a single WNV isolation from a 3-week-old cattle egret. Antibody was detected by enzyme immunoassay from 20 nestlings (13%), 14 (70%) of which were confirmed as positive by plaque reduction neutralization test (PRNT). However, titration end points against WNV and St. Louis encephalitis virus (SLEV) were similar precluding viral identification. The grouping of positives within few nests, highest PRNT titers in youngest birds (<1 weeks of age), the decline of titer with nestling age, and the lack of antibody specificity indicated that antibody may have been acquired maternally and did not represent new infections. Infection rates in Culex tarsalis mosquitoes collected near the Ardeid colony at Ramer Lake (3.1 per 1,000) were statistically similar to rates estimated at the nearby Wister Unit wetlands (5.3 per 1,000) that lacked an Ardeid nesting colony. Black-crowned night heron nestlings experimentally infected with the NY99 strain of WNV produced viremias >5 log10 plaque forming units (PFU)/mL and were considered moderately competent hosts, whereas cattle egret nestlings had viremias that remained <5 log10 PFU/mL and were incompetent hosts.
Descriptors: viral blood antibodies, bird diseases, culex mosquitoes, insect vectors, West Nile fever, West Nile virus, wild animals, birds, California, disease reservoirs, St. Louis encephalitis virus, neutralization tests, species specificity, zoonoses.
Reisen, W.K., Y. Fang, H.D. Lothrop, V.M. Martinez, J. Wilson, P. O'Connor, R. Carney, B. Cahoon Young, M. Shafii, and A.C. Brault (2006). Overwintering of West Nile virus in southern California. Journal of Medical Entomology 43(2): 344-355. ISSN: 0022-2585.
Abstract: West Nile virus (family Flaviviridae, genus Flavivirus, WNV) invaded southern California dining 2003, successfully overwintered, amplified to epidemic levels, and then dispersed to every county in the state. Although surveillance programs successfully tracked and measured these events, mechanisms that allowed the efficient overwintering and subsequent amplification of WNV have not been elucidated. Our current research provided evidence for three mechanisms whereby WNV may have persisted in southern California during the winters of 2003-2004 and 2004-2005: 1) continued enzootic transmission, 2) vertical transmission by Culex mosquitoes, and 3) chronic infection in birds. WNV was detected in 140 dead birds comprising 32 species, including 60 dead American crows, thereby verifying transmission during the November-March winter period. Dead American crows provide evidence of recent transmission because this species always succumbs rapidly after infection. However, WNV RNA was not detected concurrently in 43,043 reproductively active female mosquitoes comprising 11 species and tested in 1,258 pools or antibody in sera from 190 sentinel chickens maintained in 19 flocks. Although efficient vertical transmission by WNV was demonstrated experimentally for Culex tarsalis Coquillett infected per os, 369 females collected diapausing in Kern County and tested in 32 pools were negative for WNV. Vertical transmission was detected in Culex pipiens quinquefinciatus Say adults reared from field-collected immatures collected from Kern County and Los Angeles during the summer transmission period. Chronic infection was detected by finding WNV RNA in 34 of 82 birds that were inoculated with WNV experimentally, held for >6 wk after infection, and then necropsied. Frequent detection of WNV RNA in kidney tissue in experimentally infected birds >6 wk postinfection may explain, in part, the repeated detection of WNV RNA in dead birds recovered during winter, especially in species such as mourning doves that typically do not die after experimental infection. In summary, our study provides limited evidence to support multiple modes of WNV persistence in southern California. Continued transmission and vertical transmission by Culex p. quinquefasciatus Say seem likely candidates for further study.
Descriptors: aves, viral diseases, West Nile virus, overwintering, transmission of viruses, California, south, viral disease overwintering.
Reisen, W.K., Y. Fang, and V.M. Martinez (2006). Effects of temperature on the transmission of West Nile virus by Culex tarsalis (Diptera: Culicidae). Journal of Medical Entomology 43(2): 309-317. ISSN: 0022-2585.
Abstract: Culex tarsalis Coquillett females were infected with the NY99 strain of West Nile virus (family Flaviviridae, genus Flavivirus, WNV) and then incubated under constant temperatures of 10-30[degree]C. At selected time intervals, transmission was attempted using an in vitro capillary tube assay. The median time from imbibing an infectious bloodmeal until infected females transmitted WNV (median extrinsic incubation period, EIP50) was estimated by probit analysis. By regressing the EIP rate (inverse of EIP50) as a function of temperature from 14 to 30[degree]C, the EIP was estimated to require 109 degree-days (DD) and the point of zero virus development (x-intercept) was estimated to be 14.3[degree]C. The resulting degree-day model showed that the NY99 WNV strain responded to temperature differently than a lineage II strain of WNV from South Africa and approximated our previous estimates for St. Louis encephalitis virus (family Flaviviridae, genus Flavivirus, SLEV). The invading NY99 WNV strain therefore required warm temperatures for efficient transmission. The time for completion of the EIP was estimated monthly from temperatures recorded at Coachella Valley, Los Angeles, and Kern County, California, during the 2004 epidemic year and related to the duration of the Cx. tarsalis gonotrophic cycle and measures of WNV activity. Enzootic WNV activity commenced after temperatures increased, the duration of the EIP decreased, and virus potentially was transmitted in two or less gonotrophic cycles. Temperatures in the United States during the epidemic summers of 2002-2004 indicated that WNV dispersal and resulting epicenters were linked closely to above-average summer temperatures.
Descriptors: Culex tarsalis, viral diseases, West Nile virus, temperature effect on transmission, transmission of viruses, temperature.
Root, J.J., P.T. Oesterle, N.M. Nemeth, K. Klenk, D.H. Gould, R.G. McLean, L. Clark, and J.S. Hall (2006). Experimental infection of fox squirrels (Sciurus niger) with West Nile virus. American Journal of Tropical Medicine and Hygiene 75(4): 697-701. ISSN: 0002-9637.
Abstract: Tree squirrels (Sciurus spp.) have exhibited high seroprevalence rates, suggesting that they are commonly exposed to West Nile virus (WNV). Many characteristics of WNV infections in tree squirrels, such as the durations and levels of viremia, remain unknown. To better understand WNV infections in fox squirrels (S. niger), we subcutaneously inoculated fourteen fox squirrels with WNV. Peak viremias ranged from 10(4.00) plaque-forming units (PFU)/mL of serum on day 2 post-infection (DPI) to 10(4.98) PFU/mL on 3 DPI, although viremias varied between individuals. Oral secretions of some fox squirrels were positive for WNV viral RNA, occasionally to moderate levels (10(3.2) PFU equivalent/swab). WNV PFU equivalents in organs were low or undetectable on 12 DPI; gross and histologic lesions were rare. The viremia profiles of fox squirrels indicate that they could serve as amplifying hosts in nature. In addition, viral RNA in the oral cavity and feces indicate that this species could contribute to alternative WNV transmission in suburban communities.
Descriptors: rodent diseases, squirrels, West Nile fever, West Nile virus, viral biosynthesis, brain pathology, kidney pathology, liver pathology, myocardium pathology, viral RNA analysis, random allocation, reverse transcriptase polymerase chain reaction, virus shedding, mortality, pathology.
Savage, H.M., M. Anderson, E. Gordon, L. McMillen, L. Colton, D. Charnetzky, M. Delorey, S. Aspen, K. Burkhalter, B.J. Biggerstaff, and M. Godsey (2006). Oviposition activity patterns and West Nile virus infection rates for members of the Culex pipiens complex at different habitat types within the hybrid zone, Shelby County, TN, 2002 (Diptera: Culicidae). Journal of Medical Entomology 43(6): 1227-1238. ISSN: 0022-2585.
Abstract: Oviposition activity and West Nile virus (family Flaviviridae, genus Flavivirus, WNV) infection rates were assessed for members of the Culex pipiens complex from July through December 2002 by using gravid traps placed at four ecologically different sites in the southern portion of the hybrid zone in Shelby County, TN. Molecular assays identified three members of the Cx. pipiens complex: Cx. pipiens pipiens L., Cx. p. quinquefasciatus Say, and Cx. p. pipiens-Cx. p. quinquefasciatus hybrids (hybrids). The Cx. pipiens complex accounted for 90% of mosquitoes collected in gravid traps. All 285 WNV-positive mosquitoes were Culex mosquitoes, and 277 (97%) were Cx. pipiens complex mosquitoes. Infection rates among members of the Cx. pipiens complex were not significantly different. Infection rates were significantly higher at two urban sites than at a rural site, and WNV was not detected at a forested site. At urban sites, abundances of members of the Cx. pipiens complex corresponded to a simple latitude model of the hybrid zone. Cx. p. quinquefasciatus was most abundant (46.4%), followed by hybrids (34.1%) and Cx. p. pipiens (19.5%). The relative abundances at a rural site were reversed with Cx. p. pipiens (48.4%) being most abundant. This demonstrates that spatial habitat variation may profoundly influence the distribution of members of the Cx. pipiens complex within the hybrid zone. Members of the Cx. pipiens complex did not display different oviposition patterns. However, oviposition patterns assessed hourly at urban and rural sites were significantly different. At urban sites, oviposition activity of Cx. pipiens complex mosquitoes was bimodal with an evening peak associated with sunset and a morning peak associated with sunrise. At the rural site, the evening peak was pronounced and the morning peak weak and similar to nighttime activity.
Descriptors: West Nile virus, Culex mosquitoes, traps, habitat variation, infection rates, oviposition activity, evening peak, morning peak.
Shaman, J., J. Day, and M. Stieglitz (2005). Drought-induced amplification and epidemic transmission of West Nile virus in southern Florida. Journal of Medical Entomology. 42(2): 134-141. ISSN: 0022-2585.
Abstract: We show that the spatial-temporal variability of human West Nile (WN) cases and the transmission of West Nile virus (WNV) to sentinel chickens are associated with the spatial-temporal variability of drought and wetting in southern Florida. Land surface wetness conditions at 52 sites in 31 counties in southern Florida for 2001-2003 were simulated and compared with the occurrence of human WN cases and the transmission of WNV to sentinel chickens within these counties. Both WNV transmission to sentinel chickens and the occurrence of human WN cases were associated with drought 2-6 mo prior and land surface wetting 0.5-1.5 mo prior. These dynamics are similar to the amplification and transmission patterns found in southern Florida for the closely related St. Louis encephalitis virus. Drought brings avian hosts and vector mosquitoes into close contact and facilitates the epizootic cycling and amplification of the arboviruses within these populations. Southern Florida has not recorded a severe, widespread drought since the introduction of WNV into the state in 2001. Our results indicate that widespread drought in the spring followed by wetting during summer greatly increase the probability of a WNV epidemic in southern Florida.
Descriptors: West Nile virus, virus transmission, drought, disease outbreaks, sentinel animals, chickens, humans, spatial variation, temporal variation, insect vectors, Culex nigripalpus, Culicidae, simulation models, Florida, virus-amplification, land-surface-wetting, hydrology-models.
Steinman, A., C. Banet Noach, L. Simanov, N. Grinfeld, Z. Aizenberg, O. Levi, D. Lahav, M. Malkinson, S. Perk, and N.Y. Shpigel (2006). Experimental Infection of Common Garter Snakes (Thamnophis sirtalis) with West Nile Virus. Vector Borne and Zoonotic Diseases 6(4): 361-368. ISSN: 1530-3667.
Abstract: The role of various reptilian species in the infectious cycle of several arboviruses is documented, but their role in that of West Nile virus (WNV) is uncertain. Common garter snakes (Thamnophis sirtalis) were infected subcutaneously with 10(5) plaque forming units (PFU) WNV-Isr 98, five of nine snakes became viremic, and five exhibited persistent low levels of neutralizing antibodies. Four of the parentally infected snakes died and high titers of virus were found in multiple organ samples. In contrast, orally infected garter snakes did not become viremic, but viral RNA was detected in cloacal swabs. Since oral infection of predator birds by WNV is known, their ingestion of infected snakes may also result in their becoming infected.
Descriptors: reptilian species, West Nile virus, garter snakes, subcutaneous infection, oral infection, viremic, RNA, predator birds.
Stephen, C., N. Plamondon, and P. Belton (2006). Notes on the distribution of mosquito species that could potentially transmit West Nile virus on Vancouver Island, British Columbia. Journal of the American Mosquito Control Association 22(3): 553-556.
Abstract: We record the spatial and temporal distribution of 8 potential vectors of West Nile virus (WNV) on Vancouver Island in 2003 and 2004. Five species were widely distributed, but the other 3 were restricted to specific larval habitats. Adults were trapped from early April to September. The findings indicate a potential for WNV transmission if the virus arrives on the island. The results extend the published range of 5 mosquito species.
Descriptors: Culicidae virology, West Nile virus, Aedes, Anopheles, British Columbia, insect vectors, larva, population surveillance, West Nile fever transmission.
Sullivan, H., G. Linz, L. Clark, and M. Salman (2006). West Nile virus antibody prevalence in red-winged blackbirds (Agelaius phoeniceus) from North Dakota, USA (2003-2004). Vector Borne and Zoonotic Diseases 6(3): 305-309. ISSN: 1530-3667.
Abstract: This study was designed to explore the role that red-winged blackbirds (Agelaius phoeniceus) may have played in disseminating West Nile virus (WNV) across the United States. Using enzyme-linked immunosorbent assays designed to detect WNV antibodies in avian species we were able to determine the WNV antibody prevalence in a cohort of red-winged blackbirds in central North Dakota in 2003 and 2004. The peak WNV antibody prevalence was 22.0% in August of 2003 and 18.3% in July of 2004. The results of this study suggest that red-winged blackbird migratory populations may be an important viral dispersal mechanism with the ability to spread arboviruses such as WNV across the United States.
Descriptors: West Nile virus, red winged black birds, antibody prevalence, migratory populations, disseminating, disperal, spread.
Tachiiri, K., B. Klinkenberg, S. Mak, and J. Kazmi (2006). Predicting outbreaks: A spatial risk assessment of West Nile virus in British Columbia. International Journal of Health Geographics 5: 21.
Abstract: BACKGROUND: West Nile virus (WNv) has recently emerged as a health threat to the North American population. After the initial disease outbreak in New York City in 1999, WNv has spread widely and quickly across North America to every contiguous American state and Canadian province, with the exceptions of British Columbia (BC), Prince Edward Island and Newfoundland. In this study we develop models of mosquito population dynamics for Culex tarsalis and C. pipiens, and create a spatial risk assessment of WNv prior to its arrival in BC by creating a raster-based mosquito abundance model using basic geographic and temperature data. Among the parameters included in the model are spatial factors determined from the locations of BC Centre for Disease Control mosquito traps (e.g., distance of the trap from the closest wetland or lake), while other parameters were obtained from the literature. Factors not considered in the current assessment but which could influence the results are also discussed. RESULTS: Since the model performs much better for C. tarsalis than for C. pipiens, the risk assessment is carried out using the output of C. tarsalis model. The result of the spatially-explicit mosquito abundance model indicates that the Okanagan Valley, the Thompson Region, Greater Vancouver, the Fraser Valley and southeastern Vancouver Island have the highest potential abundance of the mosquitoes. After including human population data, Greater Vancouver, due to its high population density, increases in significance relative to the other areas. CONCLUSION: Creating a raster-based mosquito abundance map enabled us to quantitatively evaluate WNv risk throughout BC and to identify the areas of greatest potential risk, prior to WNv introduction. In producing the map important gaps in our knowledge related to mosquito ecology in BC were identified, as well, it became evident that increased efforts in bird and mosquito surveillance are required if more accurate models and maps are to be produced. Access to real time climatic data is the key for developing a real time early warning system for forecasting vector borne disease outbreaks, while including social factors is important when producing a detailed assessment in urban areas.
Descriptors: Culex sp., forecasting methods, West Nile virus, British Columbia, geographic information systems (GIS), theoretical models, population density, population dynamics, risk, temperature.
Ternovoi, V.A., E.V. Protopopova, S.G. Surmach, M.V. Gazetdinov, S.I. Zolotykh, A.M. Shestopalov, E.V. Pavlenko, G.N. Leonova, and V.B. Loktev (2006). [The genotyping of the West Nile virus in birds in the far eastern region of Russia in 2002-2004]. Molekuliarnaia Genetika, Mikrobiologiia i Virusologiia(4): 30-35.
Abstract: Samples from 20 species of trapped and dead birds were collected in the Far Eastern Region in 2002-2004 and were analyzed by the anti-WNV MAb-modified immunoenzyme assay for antigen detection and RT-PCR for viral RNA detection. Five positive samples from cinereous vultures (Aegypius monachus) and two positive samples from cattle egret (Bubulcus ibis) were found in both tests. The sequencing of the 322 bp fragments of protein E gene showed 99-99.67% homology with the strain WNV/LEIV-VlgOO-27924 of the WNV isolated in Volgograd, Russia, 2000. Additionally, five positive samples from birds (Pica pica, Corvus macrorhynchos, Larus crossirostris, Parus minor, Emberiza spodocephala) collected in autumn 2004 were found during screening with anti-WNV MAb-modified ELISA. These results confirm that the WNV is circulating in the Far Eastern Region of Russia and outbreaks of WN fever in humans may be possible. This demonstrates that the genotype 1a of the West Nile virus could spread in the southern regions of the Far East by migrating birds and introduction of the WNV into other southern regions of the Asian part of Russia are probably.
Descriptors: birds, West Nile fever, viral genotype, immunoenzyme techniques, reverse transcriptase polymerase chain reaction, Russia, West Nile virus classification.
Language of Text: Russian.
Tiawsirisup, S., K.B. Platt, R.B. Evans, and W.A. Rowley (2005). A comparision of West Nile Virus transmission by Ochlerotatus trivittatus (COQ.), Culex pipiens (L.), and Aedes albopictus (Skuse). Vector Borne and Zoonotic Diseases 5(1): 40-47. ISSN: 1530-3667.
Abstract: Transmission of West Nile virus (WNV) by Ochlerotatus trivittatus, Culex pipiens, and Aedes albopictus were compared 14 days after taking blood meals from viremic chickens with titers ranging from 10(2.5) to 10(9.5) cell infective dose (50)s (CID50s)/mL serum. Transmission occurred in one of four (25%) Oc. trivittatus and one of 25 (4%) Cx. pipiens that fed on chickens with titers of 10(5.5) CID50s/mL. No transmission occurred among two of 16 (13%) Oc. trivittatus or one of 25 (4%) Cx. pipiens that became infected after blood meals with titers of 10(5.0) and 10(4.5) CID50s/mL, the next lowest blood meal titers evaluated. Seventeen of 28 (61%) Ae. albopictus transmitted WNV after blood meals with titers of 10(7.0) CID50s/mL, but no infection or transmission was observed among 21 Ae. albopictus that fed on chickens with titers of 10(5.0) CID50s/mL, the next lowest titer evaluated. Transmission by all three species increased dramatically after blood meals with WNV titers of > or = 10(5.5) CID50s/mL. No significant differences occurred in dissemination and transmission rates of the three species after taking blood meals with titers of > 10(7.0) CID50s/mL. The cumulative mean +/- SE transmission rates of Oc. trivittatus, Cx. pipiens, and Ae. albopictus after blood meals with titers of > or = 10(7.0) CID50s/mL were 45.5 +/- 4.1%, 46.8 +/- 4.5%, and 72.4 +/- 5.5%. The cumulative mean dissemination rates of the three species were 78.3 +/- 6.7%, 74.8 +/- 2.6%, and 88.6 +/- 2.1%. The rates of transmission by the three species that developed disseminated infections after blood meals with titers of > or = 10(7.0) CID50s/mL were 58.8 +/- 4.4%, 62.6 +/- 5.8%, and 81.6 +/- 5.4%, respectively. In a previous study, we found that susceptibility of the three species to WNV was essentially the same when fed on chickens with WNV titers of > 10(7.0) CID50s/mL, but Oc. trivittatus and Cx. pipiens were more susceptible than Ae. albopictus to WNV at lower virus titers. The current study strongly suggests that Ae. albopictus is a more efficient vector than Oc. trivittatus and Cx. pipiens when fed blood meals with titers of > 10(7.0) CID50s/mL. However, Oc. trivittatus and Cx. pipiens might be more efficient as vectors when infected by blood meals with titers of < 10(7.0) CID50s/mL.
Descriptors: insect vectors, West Nile fever transmission, West Nile virus isolation and purification, chickens, culex mosquitoes, disease susceptibility, disease vectors, ochlerotatus, species specificity, viral load.
Tiawsirisup, S., K.B. Platt, B.J. Tucker, and W.A. Rowley (2005). Eastern cottontail rabbits (Sylvilagus floridanus) develop West Nile virus viremias sufficient for infecting select mosquito species. Vector Borne and Zoonotic Diseases 5(4): 342-350. ISSN: 1530-3667.
Abstract: The potential of the eastern cottontail rabbit (CTR; Sylvilagus floridanus) to contribute to an enzootic West Nile virus (WNV) cycle was demonstrated by characterizing the WNV viremia profile of 15 CTRs and demonstrating that mosquitoes could become infected by feeding on these CTRs. Eight CTRs were infected with a titer of 10(5.0) cell-infectious dose 50% endpoints (CID50s) of WNV (NY99-Crow) by needle and seven CTRs by bite of one or more WNV-infected mosquitoes. There were no marked differences between the WNV viremia profiles of CTRs infected by either method. West Nile virus was detected in serums of all CTRs by 24 h p.i. The daily mean titers of all 15 CTRs on days 1-4 p.i. were 10(4.1+/-0.4), 10(4.7+/-0.3), 10(4.1+/-0.6), and 10(3.7+/-0.6) respectively, declining to 10(1.2+/-0.1) CID50s/ml of serum by day 6 p.i. No virus was detected in the blood of any CTR on day 7 p.i. The average duration of WNV titers of >or=10(4.3) and <10(5.0) CID50s/mL for all CTRs was 2.2 +/- 0.6 and 1.0 +/- 0.1 days, respectively. The minimum estimated infection rates (MEIRs) of Culex pipiens (L.) and Culex salinarius (Coq.) that fed on CTRs with titers of >or=10(4.3) and >10(5.0) were 11.5 +/- 5.5 and 21 +/- 6.0%, respectively. These rates increased to 20.5 +/- 6.4% and 25.0 +/- 3.0% when CTR serum titers were >10(5.0) CID50s/mL. Neither Aedes aegypti (L.) nor Aedes albopictus (Skuse) were infected by feeding on CTRs with titers of <10(5.0) CID50s/mL. The MEIRs of these two species were 11.5 +/- 3.5% and 1.5 +/- 0.5% after feeding on CTRs with titers of >10(5.0) CID50s/ml. None of the CTRs infected by mosquito bite or by needle showed any symptoms of WNV disease.
Descriptors: cottontail rabbit, West Nile virus, viremia, Culex mosquitos, infection rates.
Turell, M., D. Dohm, M. Sardelis, M. O' Guinn, T. Andreadis, and J. Blow (2005). An update on the potential of North American mosquitoes (Diptera: Culicidae) to transmit West Nile virus. Journal of Medical Entomology. 42(1): 57-62. ISSN: 0022-2585.
Abstract: Since first discovered in the New York City area in 1999, West Nile virus (WNV) has become established over much of the continental United States and has been responsible for >10,000 cases of severe disease and 400 human fatalities, as well as thousands of fatal infections in horses. To develop appropriate surveillance and control strategies, the identification of which mosquito species are competent vectors and how various factors influence their ability to transmit this virus must be determined. Therefore, we evaluated numerous mosquito species for their ability to transmit WNV under laboratory conditions. This report contains data for several mosquito species not reported previously, as well as a summary of transmission data compiled from previously reported studies. Mosquitoes were allowed to feed on chickens infected with WNV isolated from a crow that died during the 1999 outbreak in New York City. These mosquitoes were tested approximately equal to 2 wk later to determine infection, dissemination, and transmission rates. All Culex species tested were competent vectors in the laboratory and varied from highly efficient vectors (e.g., Culex tarsalis Coquillett) to moderately efficient ones (e.g., Culex nigripalpus Theobald). Nearly all of the Culex species tested could serve as efficient enzootic or amplifying vectors for WNV. Several container-breeding Aedes and Ochlerotatus species were highly efficient vectors under laboratory conditions, but because of their feeding preferences, would probably not be involved in the maintenance of WNV in nature. However, they would be potential bridge vectors between the avian-Culex cycle and mammalian hosts. In contrast, most of the surface pool-breeding Aedes and Ochlerotatus species tested were relatively inefficient vectors under laboratory conditions and would probably not play a significant role in transmitting WNV in nature. In determining the potential for a mosquito species to become involved in transmitting WNV, it is necessary to consider not only its laboratory vector competence but also its abundance, host-feeding preference, involvement with other viruses with similar transmission cycles, and whether WNV has been isolated from this species under natural conditions.
Descriptors: Culicidae, Culex, mosquitoes, insect vectors, West Nile virus, vector potential, vector competence, disease transmission, infection, vectorial capacity, infection-rate, dissemination-rate, transmission-rate.
Van Dyken, M., B. Bolling, C. Moore, C. Blair, B. Beaty, W.I. Black, and B. Foy (2006). Molecular evidence for trypanosomatids in Culex mosquitoes collected during a West Nile virus survey. International Journal for Parasitology. 36(9): 1015-1023. ISSN: 0020-7519.
Abstract: Adult mosquitoes were previously collected and tested for West Nile virus during an intense WNV outbreak in 2003-2004 along the Cache la Poudre River in Colorado, USA. A subset of these mosquitoes was also tested for infection with trypanosomatids using nested PCR to amplify 18S rRNA. Of the 69 pools of Culex pipiens that were screened for both pathogens, 4.3% were positive for WNV and 11.6% tested positive for trypanosomes; no pools were found to be co-infected with both pathogens. One hundred and forty-three pools of Culex tarsalis, considered to be the principal WNV vector in this area, were tested in the same manner. 7.7% were positive for WNV and 20.3% of these pools tested positive for trypanosomes. Five pools of C. tarsalis were found to be co-infected with both pathogens, which was approximately 2.2 times more frequent than would be expected if these pathogens are independent of each other. Sequencing and maximum parsimony analysis of 18S rRNA revealed that four of the isolates arise in or near clades of described avian trypanosomes, likely indicating that these are vectored pathogens between birds and mosquitoes. Unexpectedly, the majority (24/28, 86%) of our positive samples form their own separate clade within the order Trypanosomatida with 100% bootstrap support. We have identified a potential new clade of trypanosomatids that exist within important mosquito vectors and discuss the potential ecological connections between these trypanosomes, arboviruses and mosquitoes.
Descriptors: Culex pipiens, Culex tarsalis, Trypanosoma, disease detection, pathogen identification, ribosomal DNA, nucleotide sequences, phylogeny, insect vectors, mixed infection, trypanosomiasis, West Nile virus, disease surveillance, Colorado, Internet-resource.
Ward, M.P., A. Raim, S. Yaremych Hamer, R. Lampman, and R.J. Novak (2006). Does the roosting behavior of birds affect transmission dynamics of West Nile virus? American Journal of Tropical Medicine and Hygiene 75(2): 350-355. ISSN: subcutaneous.
Abstract: The potential role of many urban passerine birds in the transmission of West Nile virus (WNV) is well-documented by studies on host competency, seroprevalence in wild birds, and identification of vector blood meal source. In contrast, the impact of bird behavior on transmission dynamics is largely unexplored. Bird roosting (perching) behavior may be a critical component regulating WNV transmission because of the crepuscular/nocturnal feeding behavior of Culex mosquitoes, the primary vectors of WNV. We used radio telemetry to determine the roosting behavior of American crows (Corvus brachyrhynchos) and northern cardinals (Cardinalus cardinalus). On average, healthy crows moved slightly shorter distances between roosts than viremic crows, 1,038.3 meters versus 1,255.5 meters, while cardinals only moved 54.7 meters. Given the average movements of crows and cardinals between roosts, crows, which are viremic for five days, could spread the virus throughout a mean +/- SE area of 20.84 +/- 0.79 km(2), while viremic cardinals would, on average, only spread the virus over a mean +/- SE area of 0.03 +/- 0.01 km(2). Because the crow population in Illinois is decreasing at a rate of 11.5% per year and up to 35.6% per year in certain locations, crows are becoming scarce in some areas, thus reducing their role as wild bird sentinels. We suggest that if crows are important in dispersing WNV, large decreases in their abundance will shift transmission cycles to a more focal nature because of the differences in roosting behavior of crows compared with other urban birds, such as cardinals.
Descriptors: transmission of bird diseases, passeriformes physiology, West Nile fever, animal behavior and physiology, crows, Illinois, population dynamics, radio waves, time factors, West Nile virus.
Ward, M.R., D.E. Stallknecht, J. Willis, M.J. Conroy, and W.R. Davidson (2006). Wild bird mortality and West Nile virus surveillance: biases associated with detection, reporting, and carcass persistence. Journal of Wildlife Diseases 42(1): 92-106. ISSN: 0090-3558.
Abstract: Surveillance targeting dead wild birds, in particular American crows (Corvus brachyrhynchos), plays a critical role in West Nile virus (WNV) surveillance in the United States. Using crow decoy surrogates, detection and reporting of crow carcasses within urban and rural environments of DeKalb County, Georgia were assessed for potential biases that might occur in the county's WNV surveillance program. In each of two replicated trials, during July and September 2003, 400 decoys were labeled with reporting instructions and distributed along randomly chosen routes throughout designated urban and rural areas within DeKalb County. Information-theoretic methods were used to compare alternative models incorporating the effects of area and trial on probabilities of detection and reporting. The model with the best empirical support included the effects of area on both detection and reporting of decoys. The proportion of decoys detected in the urban area (0.605, SE = 0.024) was approximately twice that of the rural area (0.293, SE = 0.023), and the proportion of decoys reported in the urban area (0.273, SE = 0.023) was approximately three times that of the rural area (0.103, SE = 0.028). These results suggest that human density and associated factors can substantially influence dead crow detection and reporting and, thus, the perceived distribution of WNV. In a second and separate study, the persistence and fate of American crow and house sparrow (Passer domesticus) carcasses were assessed in urban and rural environments in Athens-Clarke, Madison, and Oconee counties, Georgia. Two replicated trials using 96 carcasses of each species were conducted during July and September 2004. For a portion of the carcasses, motion sensitive cameras were used to monitor scavenging species visits. Most carcasses (82%) disappeared or were decayed by the end of the 6-day study. Carcass persistence averaged 1.6 days in rural areas and 2.1 days in urban areas. We analyzed carcass persistence rates using a known-fate model framework in program MARK. Model selection based on Akaike's Information Criteria (AIC) indicated that the best model explaining carcass persistence rates included species and number of days of exposure; however, the model including area and number of days of exposure received approximately equal support. Model-averaged carcass persistence rates were higher for urban areas and for crow carcasses. Six mammalian and one avian species were documented scavenging upon carcasses. Dead wild birds could represent potential sources of oral WNV exposure to these scavenging species. Species composition of the scavenger assemblage was similar in urban and rural areas but "scavenging pressure" was greater in rural areas.
Descriptors: aves, ecological techniques, wild bird mortality monitoring and application in West Nile virus surveillance, biases, pathological techniques, viral diseases, West Nile virus, wild bird mortality monitoring application in disease surveillance, mortality, Georgia USA.
Ward, M. (2005). Epidemic West Nile virus encephalomyelitis: A temperature-dependent, spatial model of disease dynamics. Preventive Veterinary Medicine. 71(3-4): 253-264. ISSN: 0167-5877.
Descriptors: West Nile virus, viral encephalitis, disease outbreaks, ambient temperature, epidemiological studies, epidemiology, spatial distribution, horses, horse diseases, heat sums, risk factors, geographic information systems, biogeography, disease transmission, models, Indiana.
Notes: In the Special Issue: Proceedings of GISVET'04/edited by W. Martin and P. Durr. Proceedings of a conference held June 23-25, 2004, Ontario, Canada.
Ward, M., B. Ramsay, and K. Gallo (2005). Rural cases of equine West Nile virus encephalomyelitis and the normalized difference vegetation index. Vector Borne and Zoonotic Diseases. 5(2): 181-188. ISSN: 1530-3667.
Descriptors: horses, West Nile virus, viral encephalitis, disease outbreaks, vegetation cover, spatial distribution, spatial variation, temporal variation, remote sensing, satellites, image analysis, rural areas, Indiana, disease-foci.
White, B.J., D.R. Andrew, N.Z. Mans, O.A. Ohajuruka, and M.C. Garvin (2006). West Nile virus in mosquitoes of northern Ohio, 2003. American Journal of Tropical Medicine and Hygiene 75(2): 346-349. ISSN: subcutaneous.
Abstract: From June 19, 2003 to August 18, 2003, we surveyed the mosquitoes of Oberlin, OH, for West Nile Virus (WNV) infection using reverse transcriptase-polymerase chain reaction. A total of 12,055 mosquitoes, representing 17 species or species groups and 4 genera, were collected in gravid traps at seven sites throughout the city, with Culex pipiens/restuans being the most abundant and showing the highest minimum infection rate (MIR) of 0.78. This represents a decrease in WNV enzootic activity from the previous year. Both Cx. pipiens/restuans abundance and MIR increased significantly with date. However, we found no correlation between Cx. pipiens/restuans abundance and MIR.
Descriptors: culex mosquitos, insect vectors, West Nile virus isolation and purification, culicidae classification, Ohio, population density, population surveillance methods, reverse transcriptase polymerase chain reaction methods, time factors, West Nile fever transmission.
Wonham, M.J., M.A. Lewis, J. Renc+or awowicz, and P.v.d. Driessche (2006). Transmission assumptions generate conflicting predictions in host-vector disease models: a case study in West Nile virus. Ecology Letters. 9(6): 706-725. ISSN: 1461-023X.
Abstract: This review synthesizes the conflicting outbreak predictions generated by different biological assumptions in host-vector disease models. It is motivated by the North American outbreak of West Nile virus, an emerging infectious disease that has prompted at least five dynamical modelling studies. Mathematical models have long proven successful in investigating the dynamics and control of infectious disease systems. The underlying assumptions in these epidemiological models determine their mathematical structure, and therefore influence their predictions. A crucial assumption is the host-vector interaction encapsulated in the disease-transmission term, and a key prediction is the basic reproduction number, R[subscript 0]. We connect these two model elements by demonstrating how the choice of transmission term qualitatively and quantitatively alters R[subscript 0] and therefore alters predicted disease dynamics and control implications. Whereas some transmission terms predict that reducing the host population will reduce disease outbreaks, others predict that this will exacerbate infection risk. These conflicting predictions are reconciled by understanding that different transmission terms apply biologically only at certain population densities, outside which they can generate erroneous predictions. For West Nile virus, R[subscript 0] estimates for six common North American bird species indicate that all would be effective outbreak hosts.