The effect of subclinical Mycobacterium paratuberculosis infection on daysopen in Michigan, USA, dairy cows

A prospective cohort study design was used to evaluate the impact of subcli nical Mycobacterium paratuberculosis infection on days open in a sample of Michigan dairy herds with a history of cows positive for M. paratuberculosi s diagnosed by fecal culture. Participating herds were tested and productiv ity and reproduction records were monitored for 18 months. All cows greater than or equal to 24 months old were tested for M. paratuberculosis infecti on using the ELISA and radiometric fecal-culture (RFC) techniques. Test-neg ative cows were re-tested at the conclusion of the monitoring period. Multi variable regression models were used. Using both tests in parallel, the ove rall sample apparent prevalence for M. paratuberculosis infection was 41.8% (223/533 animals tested). Adjusting for diagnostic sensitivity and specifi city, this resulted in a calculated sample true prevalence of 59.9%, ELISA- positive cows (on average) had a 28-day increase in days open when compared to ELISA-negative cows (p=0.02). The diagnostic method used to define a ca se altered the apparent association between paratuberculosis test status an d days open. Fecal culture was a less-effective diagnostic tool for use in herds with a high prevalence of infected animals. The increase in days open in the ELISA-positive cows was an indication that perhaps reduced estrus e xpression or an increased post-partum anestrous period occurred in the subc linically infected ELISA-positive animals. This might have been due to a ne gative energy balance associated with M. paratuberculosis infection. (C) 20 00 Elsevier Science B.V. All rights reserved.