Contribution of environmental mycobacteria to false-positive serum ELISA results for paratuberculosis.

OBJECTIVE To evaluate the effect of exposure to environmental mycobacteria on results of 2 commercial ELISAs for paratuberculosis in cattle. DESIGN Experimental trial. ANIMALS 19 weaned crossbred beef calves. PROCEDURES Calves were inoculated SC with 1 of 5 mycobacterial isolates (3 calves/isolate) derived from herds with high proportions of false-positive serologic reactions for paratuberculosis, Mycobacterium avium subsp paratuberculosis (MAP; positive control inoculum; 2 calves), or mineral oil (negative control inoculum; 2 calves). Sera were assessed at intervals by use of 2 ELISAs (A and B) for paratuberculosis in cattle, and all calves underwent tuberculosis testing at the end of the study. RESULTS Neither mineral oil-inoculated calf had positive results with either ELISA during the study. Both MAP-inoculated calves were identified as seropositive via ELISA-A, and 1 calf was identified as seropositive via ELISA-B. By use of ELISA-A, > or = 1 false-positive reaction over time was detected in 2, 3, 3, and 1 of the 3 calves injected with Mycobacterium avium, Mycobacterium intracellulare, Mycobacterium scrofulaceum, or Mycobacterium terrae, respectively. By use of ELISA-B, only M scrofulaceum induced false-positive reactions (2/3 calves). Calves that had at least 1 positive ELISA-A result were more likely to be classified as suspect reactors via the caudal fold tuberculosis test. CONCLUSIONS AND CLINICAL RELEVANCE False-positive serologic reactions may occur during use of commercially available ELISAs for paratuberculosis in calves experimentally exposed to environmental mycobacteria; naturally occurring exposures with these mycobacteria may represent a cause for high proportions of false-positive serologic reactions for paratuberculosis in some cattle herds.