Identification of potential on-farm sources of Listeria monocytogenes in herds of dairy cattle.

OBJECTIVE: To elucidate the ecology of Listeria monocytogenes on dairy cattle farms by determining the prevalence of the organism in various samples. SAMPLE POPULATION: Dairy cattle operations in central New York State. PROCEDURES: A repeated cross-sectional study design was used. Various samples were obtained from cattle (feces, composite udder milk, and udders), their environment (silage, feed bunks, water troughs, and floor bedding), inline milk filters, and bulk tank milk from 50 dairy farms. Samples were tested for L monocytogenes by use of a PCR assay with 2 steps of bacterial enrichment. Data were analyzed with mixed-effect logistic regression to control for the potential clustering of L monocytogenes on particular farms.
RESULTS: L monocytogenes was detected in composite milk, udder swab samples, and fecal samples at prevalences of 13%, 19%, and 43%, respectively. There was no significant clustering of the pathogen by farm. Listeria monocytogenes was more common in samples obtained from cattle and the environment during winter and summer versus the fall. The prevalence of L monocytogenes was twice as high in samples obtained from feed bunks, water troughs, and bedding, compared with that in samples obtained from silage (65%, 66%, 55%, and 30%, respectively). CONCLUSIONS AND CLINICAL RELEVANCE: L monocytogenes was more prevalent in samples obtained from dairy cattle and their environment than in milk samples. Strategies to control the pathogen in dairy operations should focus on cow hygiene and sanitary milk harvesting on the farm.