Why is it that a cow that has tested negative for the disease on three consecutive herd tests can suddenly test positive on the fourth test?
The blood test relies on detecting antibodies in the blood. However, as mentioned above, an animal with Johne’s disease usually only starts to form these protective antibodies long after infection has been established. Some may never develop this immune response and so can remain completely undetected, although this is unusual. A cow can test negative for several consecutive tests and it is only when she gets to a stage in the disease that her body mounts an immune response to the bacteria that we can detect her as positive. The up side of this is that she usually turns positive when she is shedding few bacteria into the environment to infect others – so we can still facilitate control of the disease by concentrating on hygiene measures. In contrast, other diseases such as Leptospirosis and IBR cause the development of disease-specific antibodies within a few days of becoming infected so are much quicker and easier to diagnose.