Mr. David Graham:

Essentially, there are two types of test. We can look for evidence of antibody, which is typically done on blood but can also be done on a milk sample. That is looking for the immune marker the animal produces. Alternatively, we can use a direct test for the bug itself. The latter has its limitation in that, typically, we believe the majority of infections occur in the first weeks to months of life. Even though an animal is infected at that stage, it will not test positive, either in direct testing of dung for the organism itself or in a blood or milk test, until the animal is two, three or four years of age. There is that silent period where the animal is potentially infected and is increasingly contaminating the environment and possibly transmitting infection to its offspring, but only later on, through use of either of those tests, is it being flagged as infected. As one moves through those various phases for both of the tests, the performance characteristics and accuracy of the test will improve in terms of its ability to detect a genuinely infected animal.