Early-life exposures and Johne's disease risk in zoo ruminants

Johne's disease, caused by Mycobacterium avium subsp. paratuberculosis (MAP), is a chronic, progressive bacterial enteritis of ruminants that can cause serious losses in both livestock and exotic species. Infection risk in exotic ruminants is associated with maternal infection status, but the effect of other herdmates on risk of infection has not been reported, to our knowledge. We conducted a retrospective cohort study to evaluate the association between MAP infection status and early-life contact with infected herdmates. The study population included 3,234 individuals representing 128 species at San Diego Zoo Global facilities between 1991 and 2010. Animal movement, health, and pathology records were used to trace enclosure-sharing contacts between members of the study population and any MAP-infected animal. Contact-days were counted by age of the reference animal and the number of unique infected individuals contacted. Herdmate infection status was stratified by stage of infection (180 d prior to diagnosis), age, and whether relevant lesions were found at autopsy. Having an infected herdmate was a strong risk factor for infection (OR = 4.4; 95% CI: 1.9-10.3), and each method of defining herdmate infection status showed significant differences in infection risk. The best predictor was number of contact-days within the first week of life, with a 2-fold increase in risk associated with each doubling in the number of contact-days (OR = 2.1; 95% CI: 1.1-4.0). We conclude that early contact with infected animals is an important predictor of MAP infection risk, although the effect size is smaller than that previously described for maternal infection status.