Epidemiology Of Listeria Monocytogenes On A New York State Dairy Farm

Listeria monocytogenes is a zoonotic pathogen that causes listeriosis in humans. Dairy cattle are a healthy reservoir of L. monocytogenes and the presence of the pathogen on dairy farms has been frequently described. The present study had as main objective to study the molecular epidemiology of L. monocytogenes on a New York State dairy farm by using data collected between February 2004 and June 2008, and Pulsed-field gel electrophoresis (PFGE) typing of L. monocytogenes isolates. Fecal samples were collected every 6 months from all lactating cows. Approximately 20 environmental samples were obtained every 3 months. Bulk tank milk samples and in-line milk filter samples were obtained weekly. Samples from milking equipment and the milking parlor were obtained in May 2007, and in January and February 2008. A high prevalence of L. monocytogenes in bulk tank milk (25.4%) and in milk filters (66.6%) was observed, contrasting with a low prevalence of the pathogen in fecal (6 %) and environmental (7%) samples. The presence of a L. monocytogenes-containing biofilm in milking equipment was suggested as a potential source of bulk tank milk contamination. Furthermore, the presence of three predominant and persistent L. monocytogenes strains (PFGE types T, D, and F accounting for 28.6%, 22.6%, and 14.9% of L. monocytogenes isolates, respectively) was observed in the milking line. Predominant and persistent strains showed high adherence ability in an in-vitro biofilm assay. Our results suggest that the milking system was exposed to several L. monocytogenes types from different sources. Only 3 PFGE types, however, were successful in persisting within the milking system, suggesting that some strains are more suitable to that particular ecological environment. Finally, the risk of listeriosis due to consumption of raw milk sold by permitted dealers and due to consumption of raw milk on farms was estimated. Overall, the annual number of listeriosis cases due to raw milk consumption was predicted to be low by our model. A reduction in the number of cases per year in all populations was observed when a raw milk testing program was in place, especially when routine testing and recalling of milk was conducted