Evaluation of microbial culture of pooled fecal samples for detection of Mycobacterium avium subsp. paratuberculosis in large dairy herds

Objective - To evaluate the sensitivity of microbial culture of pooled faecal samples for detection of Mycobacterium avium subsp. paratuberculosis (MAP) in large dairy herds and assess the use of the method for estimation of MAP prevalence. Animals - 1,740 lactating cows from 29 dairy herds in California. Procedure - Serum from each cow was tested by use of a commercial ELISA kit. Individual faecal samples were cultured and used to create pooled faecal samples (10 randomly selected faecal samples/pool; 6 pooled samples/herd). Sensitivity of MAP detection was compared between Herrold's egg yolk (HEY) agar and a new liquid culture method. Bayesian methods were used to estimate true prevalence of MAP-infected cows and herd sensitivity. Results - Estimated sensitivity for pooled faecal samples among all herds was 0.69 (25 culture-positive pools/36 pools that were MAP positive). Sensitivity increased as the number of culture-positive samples in a pool increased. The HEY agar method detected more infected cows than the liquid culture method but had lower sensitivity for pooled faecal samples. Prevalence of MAP-infected cows was estimated to be 4% (95% probability interval, 2% to 6%) on the basis of culture of pooled faecal samples. Herd-level sensitivity estimate ranged from 90% to 100% and was dependent on prevalence in the population and the sensitivity for culture of pooled faecal samples. Conclusions and Clinical Relevance - Use of pooled faecal samples from 10 cows was a cost-effective tool for herd screening and may provide a good estimate of the percentage of MAP-infected cows in dairy herds with a low prevalence of MAP.