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Epidemiological Investigations on Arthropod-Borne Diseases of Domestic Livestock and Wildlife

Johnson, Duane
Montana State University
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The goal of this project is to develop an integrated mosquito management program for coal bed natural gas (CBNG) impoundments that are responsible for increasing the risk of West Nile virus transmission to wildlife, humans and domestic livestock (equines).

The first objective will be to assess the effectiveness and practicality of using larvicides, larviciding oils and pond construction on mosquito production.

The second objective will be to quantitatively compare mosquito consumption rate larvivorous fish species in screened enclosures in CBNG impoundments. Screened enclosures are considered a "bridge" in scale between laboratory aquaria and large CBNG impoundments. Subobjectives are to assess the effects of combining species and the influence of different densities of aquatic vegetation on predation rate.

The third objective will be to initiate and monitor field releases of the most effective species in impoundments.

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NON-TECHNICAL SUMMARY: Water produced during the extraction process of coal bed natural gas (CBNG) in the Powder River Basin, Wyoming has been identified as a source for mosquito production, particularly Culex tarsalis a primary vector of West Nile virus (M. Doherty 2007. MSU. M.S. Thesis). Because of the abundance of Cx. tarsalis, there are elevated concerns related to WNV transmission to humans, equines and wildlife in the area. As reported by Naugle et al. (2004), WNV was responsible for a sharp decline in the survival rate of the greater sage-grouse, Centrocercus urophasianus, in 2003 in the Powder River Basin. Controlling mosquitoes in CBNG ponds is essential to minimizing the risk of future WNV epizootics and other encephalitic viruses (western equine encephalitis, St. Louis encephalitis) reported from the area. There are benefits to a conventional approach to mosquito management but a long term, sustainable approach to mosquito management in the Powder River Basin is needed. One potential solution is the use of fish that prey on immature stages of mosquitoes. The plains killifish, Fundulus zebrinus, and fathead minnow, Pimephales promelas, were selected as potential candidates for use in CBNG impoundments. Both fish species are indigenous to Wyoming and are associated with shallow areas in streams, lakes and prairie pothole habitats. They are tolerant of low dissolved oxygen, high temperature and high salinity conditions and feed on a variety of aquatic invertebrates. Laboratory studies at MSU-Bozeman, completed in 2007, compared the predation rates of PKF and FHM. Results from replicated trials in glass aquaria and small wading pools indicated that each species was effective in consuming mosquito larvae and pupae. Both species are therefore considered suitable candidates for mosquito predation studies in the field.

APPROACH: CBNG impoundments selected for this study will have natural populations of mosquito larvae. Screened enclosures will encompass plots within each impoundment measuring 10' x 10' and will include one of three vegetation densities: none, medium and heavy. In one set of experiments, nontarget organisms (i.e., alternative prey for fish) will be removed from the enclosures by dipping and seining. If necessary, fish will be added to the enclosures for 24 to 48 hr to consume prey not collected and then removed. In another set of trials, no attempt will be made to remove alternative prey. Feeding experiments will be conducted in the enclosures comparing fish species and vegetation density. Each enclosure will be stocked with a single species of fish at a rate of 2 - 3 kg per ha which is generally around 15 - 20 fish per enclosure; control treatments will contain mosquitoes but no fish. Combined species trials will use half the stocking rate for each species. Fish will be placed in each enclosure 24 hr before feeding experiments for acclimation and to clear their stomachs of previously ingested food. To eliminate some of the natural variability in field conditions, feeding experiments will be conducted on clear days and at the same time of day (1000 - 1500 hr). Mosquito larvae and pupae will be collected from the impoundment and a known number added to each enclosure. (If necessary, non vector immature mosquitoes can be imported for this purpose). After 120 minutes, fish and remaining larvae will be removed from each enclosure. Fish will be euthanized for dissection and stomach content analyses. Fish lengths and weights will be taken. Predation rates will be calculated as larvae or pupae consumed per fish. Based on results from enclosure studies, a pilot study will be conducted in spring/summer 2010 where the most effective specie(s) will be released in an impoundment. Predation by larvivorous fish will be excluded in a portion of the impoundment and will serve as the control. Mosquitoes will be monitored in different densities of vegetation by larval dipping and will be used to estimate the effectiveness of fish in reducing immature mosquitoes.

Funding Source
Nat'l. Inst. of Food and Agriculture
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Risk Assessment, Management, and Communication
Viruses and Prions
Pesticide Residues
Bacterial Pathogens
Meat, Poultry, Game