A Multi-state Molecular Epidemiological Survey of 16 Beef, Dairy, Poultry and Swine Farms to Facilitate Risk Assessment of Listeria monocytogenes

Listeria monocytogenes s an intracellular pathogen that poses serious risks for immuno-compromised individuals. On the farm risk assessment is needed to reduce exposure of such individuals to this pathogen. Base line epidemiological surveys for this pathogen are needed to identify common ecological reservoirs and sources. A 21 month survey of 4 animal farm types for L. monocytogenes in five different states was conducted to determine the level of occurrence in various environments. An overall occurrence of 1.4% (20/1432 samples) for L. monocytogenes was observed in this 21 month survey. Each sample type (soil, bedding/litter, feed/grass and animal rectal swabs) produced an isolate of L. monocytogenes. The Winter and Spring of 2003 produced the greatest number of isolates (18 of 20 total isolates). Feed and animal rectal swabs exhibited the highest number of isolates, eight and 10, respectively. The Simpson’s Index of Diversity for RiboPrinting™ of the isolates at 100% similarity was 0.9737, for PFGE with AscI 0.9684 and PFGE with ApaI 0.9842, indicating a parity amongst the two characterization methods. Clonal (100% similar) isolates were seen amongst samples taken from non-contiguous states and multiple isolates from the same farm with differing genetic compositions were isolated, yet no clear regional relationships were evident. Enrichment types and enrichment incubation times were not significant for isolating L. monocytogenes, having equivalent odds for isolating L. monocytogenes. Selective plating media was nearly significant with a point estimate of 1.569 (p=0.0806), which when the highest season (third) was analyzed alone the significance level dropped (p=0.0584).