Most dairy farmers have heard of Johne's disease. The Mycobacterium avium subsp. paratuberculosis (MAP) that causes Johne's disease (JD) is found on approximately 70% of dairy farms in the United States. The disease can have a significant economic impact on farms through lost milk production, premature culling, lower slaughter weights and values of infected cows – even before the cows start scouring and becoming thin.
Current control programs use best management practices to improve hygiene which decreases the risk of disease spread by MAP carried in cow manure, milk or colostrum. An important part of control programs is the testing of the entire mature herd (including 1st lactation cows and bulls) to identify how wide-spread the infection is within a herd and to help prioritize the need for making management changes to prevent introduction or spread of JD. If few cows are test-positive within a herd, the management should get a tune-up but things are probably going ok. If many cows are test-positive then the improvement of management practices needs to become a high priority.
There are two kinds of tests used in control programs for dairy herds: indirect tests that detect antibodies against the JD bacterium in milk or blood (but not the bacterium itself) and direct tests that detect the bacterium itself in manure. None of these tests can detect all infected animals at one time. Truly infected cows in the early stages of infection often have a negative test; more information is needed and that is why a current herd test is so important. The introduction of cows with unknown JD status is the greatest risk factor for bringing the disease onto a farm. If you want to buy a cow and you are told she is test-negative after a single testing, it is hard to trust that result as proof that she is truly uninfected without further information. However, if you ask and find out that this test-negative cow comes from a herd that has 30% of the mature animals with positive tests, then you have to be concerned that there is still a high probability that your one test-negative cow could be infected. On the other hand, if this test-negative cow comes from a herd where all the cows have been tested multiple times over several years and all have been test-negative, then you are inclined to trust that a cow's negative test is more likely to mean she is truly uninfected. The same is true for direct fecal tests for JD. A cow can be shedding MAP, but she might not shed enough MAP at the time the sample is taken to result in a positive test. So you can see that a single negative test of a cow is not very informative without further herd information.