RUMINANTS 896 Scientific Reports: Original Study JAVMA, Vol 230, No. 6, March 15, 2007 P aratuberculosis is a chronic, debilitating intestinal infection of ruminants caused by MAP. The disease is associated with notable economic losses in the beef and dairy industries because of increased culling, de- creased value of culled cattle, diagnostic and treatment costs, and decreases in production. 1-5 Serologic tests are an important component of paratuberculosis manage- ment programs when applied as herd-screening tools. Specificity estimates for commercially available paratu- berculosis ELISAs are varied; reported estimates range from 95.3% to 99.8% in uninfected dairy cattle. 6-11 Re- sults from herd screening performed by use of a com- mercially available ELISA in beef cattle in Texas have indicated that the proportions of false-positive results were greater than that expected on the basis of reported Contribution of environmental mycobacteria to false-positive serum ELISA results for paratuberculosis Jason B. Osterstock, DVM; Geoffrey T. Fosgate, DVM, PhD, DACVPM; Bo Norby, CMV, PhD, MPVM; Elizabeth J. B. Manning, MPH, MBA, DVM; Michael T. Collins, DVM, PhD, DACVM; Allen J. Roussel, DVM, MS, DACVIM From the Departments of Large Animal Clinical Sciences (Osterstock, Roussel) and Veterinary Integrative Biosciences (Fosgate, Norby), College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843; and the Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706 (Manning, Collins). Funded by USDA-APHIS-VS Award No. 04-9100-0792-GR. Presented in part at the 8th International Colloquium on Paratuberculosis, Copenhagen, August 2005, and the 86th Conference of Research Workers in Animal Diseases (CRWAD), St Louis, December 2005. Address correspondence to Dr. Osterstock. 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 paratuber- culosis, 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 tuber- culosis 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, ≥ 1 false-positive reac- tion over time was detected in 2, 3, 3, and 1 of the 3 calves injected with Mycobacterium avium, Mycobacterium intracellulare, Mycobacterium scrofulaceum, or Mycobacterium ter- rae, 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 classi- fied 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 paratu- berculosis in some cattle herds. (J Am Vet Med Assoc 2007;230:896–901) ABBREVIATIONS MAP Mycobacterium avium subsp paratuberculosis PPD Purified protein derivative S:P ratio Test sample to positive control sample ratio OD Optical density assay specificities. 12 A potential cause for these false- positive test results in beef cattle is their exposure to Mycobacterium spp that may have antigenic similarity to MAP and hence induce production of serum anti- bodies that cross-react with antigens in conventional serologic tests. These Mycobacterium spp are often re- ferred to as environmental or atypical mycobacteria. 13 In analyses of isolates obtained from bacterial cultures of feces from cattle and camelids, Cousins et al 14 identi- fied sufficient genetic homology among MAP and en- vironmental mycobacteria to cause false-positive reac- tions on PCR tests for paratuberculosis. Exposure to environmental mycobacteria has been associated with cross-reactions during intradermal tuberculin testing in both cattle and humans, and de- layed-type hypersensitivity reactions have been used to characterize the geographic distribution of environ- mental mycobacteria exposure among people in the United States. 15-18 The US Navy has measured respons-