Dairy farm reservoir of Listeria monocytogenes sporadic and epidemic strains

Identifying the reservoirs of a pathogen is vital for control of sporadic disease and epidemics. Listeria monocytogenes is a zoonotic foodborne pathogen that is responsible for 28% of food-related deaths in the United States annually, as well as a major cause of massive product recalls worldwide. To examine the role of the dairy farm as a potential source or reservoir for L. monocytogenes subtypes shown to cause human listeriosis, we compared the pulsed-field gel electrophoresis (PFGE) restriction enzyme digestion profiles of L. monocytogenes dairy farm-associated strains (milk, environmental, and bovine) to human sporadic and epidemic disease strains. Twenty-three percent of human sporadic strains had PFGE patterns identical to that of farm isolate(s). Additionally, three farm environmental strains and one human sporadic strain had a PFGE pattern identical to a strain of L. monocytogenes responsible for the 1985 California epidemic. These data indicate that this epidemic strain continues to cause sporadic human illness and has a potential dairy farm as a reservoir.