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Bovine tuberculosis (bTb) was diagnosed in 22 cattle herds in the northeast comer of Michigan's lower peninsula. Of these 22 herds, 494 animals in 7 herds were examined by gross necropsy, histopathologic exam, mycobacterial culture, and polymerase chain reaction (PCR) assay performed only on samples that were histologically compatible for bTb. Results of culture and PCR assay interpreted in parallel were used as the reference test for calculation of the sensitivity of 1) the caudal fold test (CFT), 2) the caudal fold and comparative cervical skin tests used in series (CFTCCTSER), and 3) gross necropsy. Mycobacterium bovis was isolated from 43 animals. Using all 7 herds, the sensitivities of the CFT, the CFTCCTSER, and gross necropsy were 93.02%, 88.37%, and 86.05%, respectively. When the data were stratified by low- and moderate-prevalence herds, the sensitivities were 83.33%, 75.0%, and 83.33% in low-prevalence herds and 96.77%, 93.55%, and 87.10% in moderate-prevalence herds. The sensitivities of the 2 skin tests were slightly higher when 2 or more gross lesions were present, and the sensitivity of gross necropsy was significantly higher (P = 0.049). The sensitivity of the CFT was found to be notably higher than most estimates in other studies; however, a direct comparison was not possible because the amount of purified protein derivative and the reference methods were different in this study compared with other published studies. Although the sensitivities are high, 2 of the 7 herds (29%) would have had 1 or more positive animals left in the herd if a test-and-removal program had been used. This suggests that when positive herds are identified, selective culling of skin test reactors is a less acceptable disease control strategy than is complete depopulation.

Bovine tuberculosis skin tests have been used successfully for control and eradication of bovine tuberculosis in the United States since the eradication program was begun in 1917. In 1992, there were only 10 states that had not achieved tuberculosis-free status. Unfortunately, bovine tuberculosis has become endemic in the cattle population in certain areas in the United States, including the northern part of the Michigan's Lower Peninsula. The caudal fold test is a key element of herd-level and individual animal screening. The specificity of the caudal fold tuberculin test is not perfect, and has been estimated to be approximately 89-96%. It is also evident that the specificity of the caudal fold tuberculin test is not a constant in different cattle populations. Variation in specificity may be due to differences in the amount and concentration of the tuberculin used for the test, regional variation in cross-reacting mycobacteria that cause false-positive results on the caudal fold tuberculin test, and variation in the interpretation of the test results by the testing veterinarians. The objective of this study was to use available data from the bovine tuberculosis testing program in Michigan to evaluate possible associations between the proportion of false-positive results on the caudal fold tuberculin test, and factors regarding geography and type of herd.

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