When I asked Denna Carrier (of the Collinsville Round Up Club) to explain "Negative Coggins" to a city kid (me), I got the following reply. -- Ted

Coggins is a disease found in horses. It is transmitted by flies/mosquitoes from animal to animal if they are in close proximity. This disease isn't usually noticeable in most horses, but in some cases the horses muscles will degenerate progressively until death. There is no cure for Coggins, and any horse who tests positive should be put down immediately.
This test is required every year for any horse old enough to wean, and is being transported or moved out of their "home pasture". It is illegal to transport horses without a current veterinary test result document or "Negative Coggins" which will include the owners information along with the horses name, age, and description (color, distinguishing markings, etc).