Epidemiology and risk assessment of Listeria monocytogenes and Salmonella in New York state dairy herds

Listeria monocytogenes and Salmonella spp. are two important pathogens in veterinary medicine and public health. The illness resulted from ingestion of contaminated food varies from mild gastrointestinal disturbance to other severe forms. Both of these pathogens are prevalent in the dairy environment. Dairy products are one of the most common sources for listeriosis and salmonellosis outbreaks in the human population.; This project was proposed as a part of a long-term objective for a comprehensive risk assessment of L. monocytogenes and Salmonella spp. in dairy cows in New York State to develop the recommendations to effectively manage the risk. The specific objectives of the study aim towards determining the prevalence of L. monocytogenes and Salmonella spp. in the New York State dairy farms, identify the risk factors associated with its occurrence and suggests a practical intervention at the farm level to reduce the probability and the exposure of the organism.; A cross-sectional study was carried out to identify those risk factors for L. monocytogenes and Salmonella spp. occurrence in dairy farm in-line milk filter. The prevalence rate of the pathogens as determined using the farm milk filter was 12.6% and 1.5% respectively. Using multivariate logistic regression, we identified five statistically significant risk factors for L. monocytogenes: predipping, forestripping, type of milking system, dry cow therapy and J5 vaccination.; We proceeded with a nested case-control study to investigate the roles of feeding and silage management practices, for the occurrence of L. monocytogenes in the farm. From the findings in this study, several other possible explanations of the role of silage with the occurrence of L. monocytogenes in dairy premises were explored. Feeding and silage practices that were significantly associated with the likelihood of L. monocytogenes were component fed-herd, feeding leftovers to other cows, plastic type feed bunk, less frequency of feed bunk cleaning and feeding commercial dry grains to dairy herd.; The validity of sampling milk filter instead raw milk for the isolation of L. monocytogenes was investigated. We compare the two samples, and found that milk filter was more likely to capture the pathogen than raw milk.; In the final chapter, deterministic risk model complemented with stochastic model was used to illustrate the level of hazard that could be reduced from being introduced into the milk-processing environment through a proper intervention at the dairy farm level. Conditional probability from a logistic risk model based on pre-milking disinfectant practice on the farm created two scenarios for a risk simulation. Based on the simulation, we found that the probability and the number of L. monocytogenes that was introduced into the dairy processing environment was about three times more from farms that did not disinfect cows teats before milking.