Persistent fecal Salmonella shedding in five dairy herds.

OBJECTIVE: To monitor patterns of Salmonella fecal shedding in naturally infected dairy herds, determine the association between fecal shedding and individual animal production measures, and evaluate potential risk factors for shedding of Salmonella organisms among cattle in dairy herds.
DESIGN: Longitudinal study. SAMPLE POPULATION: 5 Ohio dairy herds. PROCEDURE: For 3 herds, fecal samples were collected from all mature cows and unweaned calves 7 times during an 18-month period. For the remaining 2 herds, fecal samples were collected from 50 lactating cows 6 times during a 12-month period. Individual animal production records for 3 herds were used to examine associations between individual fecal Salmonella shedding status and 305-day mature-equivalent milk production, somatic cell count, milk fat content, and milk protein content. Multivariable logistic regression was used to test for associations between fecal shedding status and breed, lactation status, lactation number, and duration of lactation.
RESULTS: None of the adult animals had clinical signs of salmonellosis, but prevalence of fecal Salmonella shedding at individual collection times ranged from 0 to 99% for cows and from 0 to 67% for unweaned calves. Mature cows were more likely to be shedding Salmonella organisms than were unweaned calves. Within herds, lactation status and duration of lactation for individual animals were associated with Salmonella shedding status. Salmonella fecal shedding status was not associated with individual cow production measures. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that subclinical fecal Salmonella shedding can persist in dairy herds for up to 18 months with no measurable effects on health or production of individual cows.