– William Shulaw, Extension Veterinarian, Cattle and Sheep, Ohio State University
Over the last thirty years, a number of test methods have been developed in an effort to provide producers with a better diagnostic test. The current culture method available at Ohio’s Animal Disease Diagnostic Laboratory (ADDL) in Reynoldsburg uses liquid culture media and is perhaps the most sensitive method yet developed. Herd culture results are available in six to seven weeks, and individual positive cultures can be detected in as little as two weeks. This is a significant improvement over the previously used technology that took up to 12-16 weeks to complete. Culture of manure and tissue is still the “gold standard” for diagnosis of infection with MAP. Blood tests for Johne’s disease can be completed in less than a week and are much less expensive than culture. The current method used in most laboratories is referred to as an ELISA, and this test has been markedly improved in the past decade. However current research, including our observations in our demonstration herds, suggests that it is much less sensitive than culture. The ELISA only identifies 70-80% of cows that are heavy shedders, and perhaps only 10-30% of light and moderate shedders compared to manure culture. This test may help a producer determine whether they have Johne’s disease in their herd, but testing and culling based on ELISA results may leave animals remaining in the herd to contaminate the environment.
Culturing pools of five individual animals is now a well-accepted technique for detecting an infected herd, and it can be used to estimate the number of infected animals in a herd. In Ohio, veterinarians can now pool the manure samples from individual cows in their offices and submit them to the laboratory. (A recommended procedure is available from the ADDL) Of course, if one wishes to identify the infected animals in a culture-positive pool of five, the individual animals have to be cultured. However, in herds with a low prevalence of infected animals, or a herd with an unknown disease status, pooling can significantly reduce the cost of testing the herd. The work done with this technique in our demonstration herds suggests that it is much more sensitive than blood testing and reliably allows detection of pools containing heavy and moderate shedding cows and most pools containing one or more light shedders.
The current culture method in use at the ADDL provides results in the form of a number which reflects the number of days of incubation before a positive culture is detected. If they are not detected before, cultures are incubated up to 42 days at which time they are examined by staining and further testing if the stain is positive. Samples from cows which are positive in 25 days of incubation or less are considered “heavy” shedders, and those positive from 26 to 42 days are considered moderate to light shedders. The work we have done with culture of pools of five individual cow samples suggests that there is a close relationship between the days-to-positive of the pooled sample and that of the individual animal in the pool having the shortest days-to-positive. In the previous example in the herd just mentioned, the fecal sample from the two-year-old classed as a heavy shedder was positive at 16 days of incubation. The pool of five cows containing her manure actually had two positive cows, the second at 36 days, but the pooled sample was positive at 18 days of incubation. Another pool also had two culture-positive cows, each at 39 days-to-positive on their individual sample cultures, and the pooled sample was also positive at 39 days of incubation. Pooled sample culture allows the producer and the veterinarian to better characterize the extent of the infection in the herd than with blood testing and to determine if there are individual animals that should receive further testing to identify infected ones. This would also be a more sensitive, and a cost-effective, method of screening newly purchased animals to find infected animals than using blood testing by current ELISA.
Another observation that has emerged from work in the Demonstration Project herds is the need to carefully consider the timing of any diagnostic testing, especially in beef cattle herds. Prior to their enrollment in the project, both of our beef herds had been testing annually for several years using blood samples and fecal culture, however they collected the samples in the fall, and the older method of culture took 12-16 weeks for results to be available. This meant that by the time the results were received, the cows were near calving or had already calved. At that point, if there were fecal culture-positive cows, the calving area was already contaminated, and it was difficult to change the situation by culling or segregation of infected cows. If herds are interested in making meaningful progress toward controlling or eliminating Johne’s disease, we would advise using the most sensitive test, culture by individual or pooled sample, and scheduling the sample collection such that results will be available with enough time before calving season begins to cull or segregate infected cows and to remove potential contamination from the calving area before the new calves arrive.
Next week in Part 3 we’ll look at Control and Elimination of Johnes.