Again the information comes from Phil, the conclusions are mine. When a cow is in trouble, sick or dying, the first thing it experiences is peripheral circulatory shutdown. The peripheral, or outside circulation begins to shut down, this causes an immediate temperature reduction of the affected animals outside skin, or hide. When a helicopter flies low and slow over area pastures and has the infra-red scanner operating, it will notice a cow with a lower thermographic reading than the others in the herd. Find the cold cow, and you've identified a cow in trouble.
Somebody wants to know why that cow is in trouble.
Since the cow will probably die anyway, it's a good choice for sample extraction. It's also a good way to narrow down farms that might be affected by disease. That was always a hard question to answer, until now. There's no need to sample animals that don't appear to be at risk. If you are performing true epidemiology, you would go state to state and look for sick cows.