Introduction

Serum ELISAs are a key element of herd-based screening programs for paratuberculosis in cattle. The tests have a reported specificity of 96.8 to 99%; however, some herds have an apparent false-positive rate that exceeds what would be predicted based on these specificity estimates. The objective of this study was to evaluate the effect of inoculation of cattle with atypical mycobacterial isolates originating from feces of cattle in herds with demonstrated high false-positive rates using commercially available serum ELISAs.

Methods

Nineteen beef calves were selected from a single herd for use in this study. Calves were confirmed to be mycobacterial fecal culture negative using radiometric culture methods and seronegative for paratuberculosis with ELISA 1. All calves in the study had an S/P ratio of 0.00 at the start of the study. Calves were randomly assigned to one of seven treatment groups. Calves in six of the treatment groups were administered an injection of killed mycobacterial isolates in an oil adjuvant and the final group received an adjuvant only control (n=2). Isolates included Mycobacterium avium complex (n=3), M. intracellulare (n=3), M. scrofulaceum (n=3), M. celatum (n=3), M. terrea (n=3), and M. avium subsp. paratuberculosis (M. ptb.) (n=2). All calves received the injection subcutaneously.

Results

Serum ELISA 1 performed four weeks after administration of the isolates demonstrated increases of S/P ratios in all calves receiving Mycobacterium avium complex, scrofulaceum, intracellulare, and paratuberculosis. Using the recommended cut-off for positive test results (S/P > 0.25), the following proportions were classified as positive: M. avium complex (2/3), M. scrofulaceum (0/3), M. intracellulare (3/3), M. ptb. (2/2). Calves in the control group remained seronegative (S/P = 0.00). Samples evaluated using ELISA 2 remained negative in all calves.

Conclusions

Based on these results, it appears that atypical mycobacteria can induce humoral immune responses in cattle that contribute to false-positive serologic reactions.