Epidemiologic studies on the detection and clinical risk of bovine paratuberculosis on California dairies

Paratuberculosis, or Johne's disease, is a chronic, granulomatous, enteric disease of ruminants caused by Mycobacterium avium ssp. paratuberculosis (MAP). Infection with MAP is common on California dairies, and causes financial losses through decreased milk production, premature culling, and decreased carcass values.; Factor analysis was used to analyze a paratuberculosis risk assessment questionnaire completed on 815 U.S. dairies during the National Animal Health Monitoring System's Dairy 2002 survey. Two-thirds of the variance in the original 38 risk assessment questions was explained by 11 factors. Responses to many of the questions were strongly correlated, suggesting that a shorter risk assessment instrument could be used without a substantial loss of information by removing or combining closely related questions.; Environmental sampling was evaluated as a method of determining herd infection status by culturing manure samples collected from bam alleyways and wastewater lagoons on 23 California dairies. Samples of lagoon water were most likely to contain MAP, and the proportion of positive environmental samples was correlated with the proportion of seropositive animals. Environmental sampling identified a similar proportion of infected herds as did serum ELISA testing and fecal culture of 60 animals in each herd, and was less expensive.; Paired samples of waste- and bulk-milk were collected from 20 dairies. Wastemilk had significantly higher total bacterial and somatic cell counts than did bulk milk collected from the same farms, and was more likely to contain MAP DNA as identified by polymerase chain reaction. There was no significant difference between milk types with respect to prevalences of Salmonella, Mycoplasma, or Listeria as determined by organism-specific culture methods.; Information was collected from two dairies enrolled in a paratuberculosis demonstration herd project to determine whether cow-level covariates could be used with serum ELISA results to improve prediction of fecal culture status in cows tested for MAP infection at the end of lactation. Of the predictors that were evaluated, age and a herd-standardized version of 305-day mature equivalent milk production significantly improved the prediction. For a given ELISA result, older cows and those with lower milk production were more likely to be shedding MAP in their feces.