Commercially available milk ELISA tests for Johne's disease (JD) are vary in sensitivity and specificity depending on the disease status of the cow. The objective of this study was to explore and describe subsequent test results from cows previously positive on the AntelBio Johne's Milk ELISA test. The cut point for a positive milk ELISA test was set at an optical density (OD) of 0.1.

The study was conducted retrospectively utilizing the CanWest DHI milk ELISA records from 2,398 cows in 128 herds in the Canadian provinces of Ontario, Manitoba, Saskatchewan, Alberta, and British Columbia. The study included all cows in these herds that had been tested at least twice between March 2005 and April 2007. The cows were in lactations one to eleven.

Of all cows tested, 87 cows had a positive test result and were tested on at least one subsequent occasion. Of these, 36 (41%) tested negative and 51 (58.6%) tested positive for JD at a second milk ELISA test (27 to 632 days after the first test). Nine cows were tested three times, with five of these testing positive at all three tests. Of the four remaining cows, two tested negative on test two and three, while one cow changed from positive to negative and one from suspicious to positive between test two and three. One cow had four and another cow five subsequent positive tests within the period of one year. The OD of the test score and subsequent changes in test status were independent (p=0.1852). Neither the lactation number nor the breed of the cows (Guernsey, Jersey, Holstein) were associated with the occurrence of changes in the test status (p > 0.1), nor were the differences in OD between tests of the 87 cows associated with interval between the tests (p > 0.6).

The variability in the milk ELISA test scores and their test interpretations, even in cows with initially high OD test scores, underscore the limitations of this test as a diagnostic tool for individual cows.