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Swine Production, Salmonella and the Salmonella Infection Cycle


The proposed study is part of a continuum of research with the overall objective of defeating the microbial threat to the food supply posed by Salmonella and other food-borne pathogens. The principal objective of the proposed study is to gain a better understanding of Salmonella infections in swine and the agricultural and animal husbandry practices that perpetuate these infections. <P> The principal objective of the project will be accomplished by pursuing four specific research objectives. The first research objective is determination of the prevalence of Salmonella in the environment of commercial pigs. Salmonella is a stealthy pathogen and can present a considerable challenge in maintaining the health of intensively reared pigs. Although clinical disease outbreaks are well described, sub-clinical infections and healthy carriers appear to have a significant role in maintaining the presence of Salmonella on pig farms and perpetuating the threat to the food supply. <P> The second research objective is determination of the prevalence of Salmonella in manure storage facilities on swine farms. <P> The third research objective is examination of the presence of Salmonella in soils used for propagation of crops to be fed to livestock (pigs). <P> The fourth research objective is examination of the role that the feed mill exerts in the introduction of Salmonella into the production environment of pigs. <P> The second, third and fourth objectives fall under a collective hypothesis that the use of manure from livestock operations to fertilize agricultural land is facilitating introduction of Salmonella into the human and animal food chain. Cultivation of cereal grains and oilseed by-products destined for livestock consumption on Salmonella-laden soils is believed to have a carry-over effect into the feed mill and present an ongoing contamination risk to feed preparation facilities and facilitate continuous re-introduction into the swine production environment.

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NON-TECHNICAL SUMMARY: Salmonella represents a persistent threat to the wholesomeness of the food supply for humans and animals. Salmonella contamination of the human food supply is estimated to cause food-borne outbreaks of disease that approach 1.3 million cases per year with mortalities approaching 600 deaths per year. Economic losses have been estimated at 2.9 billion dollars per year and are attributed to costs associated with hospitalization, medical therapy, time lost at work and reduced productivity. The purpose of the proposed project is description of the cycle of Salmonella infection in swine and clarification of the numerous factors that conspire to promote infection and persistence of Salmonella in intensive swine environments. This information will enable development of strategies that may be employed to mitigate the influence of Salmonella in the production environment and effectively curtail entry into the food supply of humans and animals.

APPROACH: A cross-sectional epidemiologic survey will be utilized to satisfy the described research objectives. Environmental sampling will be used to detect Salmonella in buildings where swine are reared, in manure storage facilities, in agricultural fields where manure is applied as fertilizer and in associated feed mills where diets are prepared for feeding to livestock. With regards to sampling the physical environment of pigs, multiple samples will be obtained from breeding/gestation and grower/finisher sites and cultured for Salmonella. Multiple samples of effluent from manure storage facilities and soils that have been fertilized with manure will be obtained for culture and multiple samples from the feed mill will be collected for culture to detect Salmonella contamination. Samples will be pre-enriched and plated on selective media for growth of Salmonella. Salmonella isolates will be identified to the species level and subjected to antibiotic susceptibility testing. Culture results will be counted and subjected to non-parametric tests to evaluate the significance of Salmonella prevalence in the sampling environments. Logistical regression will be used to evaluate associations between the different sampling environments. Antimicrobial susceptibility data will be tabulated and means subjected to the least significant difference test as the mean separation procedure.


PROGRESS: 2007/10 TO 2008/09<BR>
OUTPUTS: The purpose of the experiment was to examine whether sows shed salmonella during and/or immediatley after parturition, thereby posing an infectious hazard to nursing pigs. The experiment was conducted in 8 farrowing rooms and each room contained 16 farrowing crates. The experiment was replicated once for a total of 2 replicates. The farrowing facilities were managed in accordance with accepted biosecurity principles. The farrowing rooms were thoroughly cleaned with a high pressure sprayer and permitted to dry. The farrowing rooms were then disinfected and allowed to dry completely before moving sows into the farrowing crates. Rectal swabs were collected from a total of 256 sows within 24 hours post-parturition for salmonella cutlure. After collection, rectal swabs were transported to the microbiology laboratory and suspended in tetrathionate broth for 24 hours for microbial enrichment. The enriched samples were then plated onto MacConkey agar for growth of salmonella. <BR>TARGET AUDIENCES: Target audiences constitute swine producers, veterinarians and other individuals involved in intensive swine production and swine health maintenance. Information will be disseminated to these groups through presentation at continuing education meetings and training sessions. <BR><BR>

IMPACT: 2007/10 TO 2008/09<BR>
The recovery of salmonella from recently farrowed sows was low. Of the 256 rectal swabs collected during the experiment, salmonella was isolated from only 2 rectal swabs (0.78% recovery rate). The litters of salmonella-positive sows did not develop clinical disease suggestive of salmonella infection (diarrhea, septicemia). Based on the culture results, we concluded that the nursing pigs failed to become colonized with salmonella or the two salmonella-positive sows constituted false positives and never posed an infectious hazard to their pigs. The results imply that sows do not shed salmonella in amounts that pose a hazard to the health of their pigs in the period immediately after parturition.
PROGRESS: 2006/10/01 TO 2007/09/30<BR>
OUTPUTS: Salmonella is theorized to gain access to the human and animal food supply through contamination of the soil when animal manure is applied to arable land as fertilizer. Contamination of plants and carry over of this contamination into processing and preparation of foodstuffs for humans and animals is considered to result in colonization and foodborne infections. A pilot experiment was conducted where arable soil samples were collected and cultured after manure originating from swine production facilities was applied to the soil. Culture of manure slurries for salmonella prior to land application was not performed. Seven fields were sampled as part of the experiment. Four 50-gram samples of soil was collected from each field (28 samples total). After thorough mixing, 5 grams of each soil sample was suspended in tetrathionate broth for 24 hours. The enriched soil samples were then plated onto MacConkey agar for cultivation of salmonella. Initial culture of the soil samples failed to recover salmonella and repeat of the procedures on all samples failed to recover salmonella a second time. <BR>TARGET AUDIENCES: Farmers and others applying swine manure to fields
IMPACT: 2006/10/01 TO 2007/09/30<BR>
The poor recovery of salmonella suggests that manure destined for application to the soil as fertilizer may not be a major cause of soil contamination with the organism. Although salmonella was not recovered, its viability in the manure slurries could have been impacted by numerous factors before and after application to the soil if the organism was present. More research is warranted to investigate the possible relationship between manure fertilizer and salmonella contamination of plants for human and animal consumption.
PROGRESS: 2005/10/01 TO 2006/09/30<BR>
The initial objective of this project was to characterize the prevalence of salmonella in the on-farm production environment. Facilities that housed pigs in the grower/finisher stage of production were selected for sampling because clinical disease in pigs is typically associated with older animals. A pilot project was performed where the physical environment of six grower/finisher facilities were sampled using the technique of environmental sampling. Two grower/finisher rooms were sampled on each farm at different times and four swabs were collected from each room. Samples were transported to the microbiology laboratory and plated onto selective media for growth of salmonella. Culture of the grower/finisher rooms failed to recover salmonella from the production environment of the farms sampled. Sampling of grower/finisher facilities will be repeated with an increase in the number of swabs collected to improve the possibility of recovering salmonella.
IMPACT: 2005/10/01 TO 2006/09/30<BR>
The difficulty in recovering salmonella from the grower/finisher facilities is in agreement with reports from other investigators who have observed that isolation of the organism from the on-farm production environment is inconsistent. Whereas the process of transporting and holding pigs in slaughter facilities prior to processing induces sufficient physiological changes that promote shedding and excretion of salmonella from pigs that failed to show clinical signs of salmonella infection on the farm. It appears that a critical area of consideration with regards to minimizing the impact of salmonella on the food supply would be pre-harvest interventions that effectively reduce post-harvest shedding.

Ragland, Darryl
Purdue University
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