Risk factors associated with fecal shedding of Listeria monocytogenes by dairy cows and calves

BACKGROUND: Listeria monocytogenes (LM) is an important foodborne pathogen affecting animals and humans. Listeriosis outbreaks in humans caused by consumption of unpasteurized dairy products are of serious concern. OBJECTIVE: To determine risk factors associated with fecal shedding of LM in family dairy farms. ANIMALS: Fecal samples were collected from cows and calves on 20 family dairy farms in 2‐week intervals for a period of 1 year. METHODS: Longitudinal study. LM was detected using qPCR. Univariate mixed effect model and multivariate analyses were performed to associate risk factors (dietary change, breed, mastitis, other diseases, antibiotic treatment, other treatments, heat index, and meteorological season) with fecal shedding of LM. RESULTS: LM was isolated from all farms on at least 1 sampling day. The average yearly prevalence was 18.2% (98/540) and 8.4% (43/511) in cows and calves, respectively. Heat index (P = .05) and meteorological season (P = .04) affected fecal shedding of LM on a farm level. Meteorological season only influenced fecal shedding of LM in cows (P = .04), whereas heat index (P = .01) influenced fecal shedding of LM in calves. Spring season was identified as the major risk factor associated fecal shedding of LM on a farm level (P = .01) and in cows (P = .01). Dietary changes were associated with lower odds for fecal shedding of LM in calves (P < .01). CONCLUSIONS AND CLINICAL IMPORTANCE: Fecal shedding of LM is associated with environmental temperatures and the meteorological season. Farmers and veterinarians should use this information when implementing strategies to reduce risks for LM dissemination in animals and in the community.