Why are three (3) negative tests required for affected herds?
Studies of known “positive” bulls have shown that the culture method will miss about 10–20% of infected bulls if we only test them once; testing the herd (all the bulls in the herd) once gives us an 80–90% chance of finding the disease if it’s there. If no infected bulls are found on the basis of a single culture of all bulls, then we can be 80–90% sure that the herd is “clean” (free of trichomonosis). At the second test, we are trying to find infected bulls that were not detected on the first test; this means we are looking for bulls called negative on the first test when they were infected (false negatives). After a second test we can be 80–90% + 8–9% more certain we have found all positive infected bulls, so 88–99% sure the herd is not affected with trich. A third consecutive test of all herd bulls allows us to be more than 99% sure that the entire bull herd is free of disease.
