During the first year after infection, the horse will show symptoms in cycles, with period of weeks to months separating them. During the time the horse has symptoms, virus is readily detected in the blood and infection easily spread by biting insects. The fatality rate in the first year of infection varies from 30% to as high as 60%, being influenced by such things as the general health and immunocompetence of the horse, and the dose of virus that was received.
After the first year or so, a horse that survives may enter a long period where it is free of symptoms, maybe for life. Virus may not be detectable in the blood at this point, but research has clearly shown that the horse is still infected. The virus is present in all the body tissues, and in the white blood cells, called macrophages. As long as a horse in the chronic, asymptomatic phase remains healthy, there is minimal risk of transmitting the disease to other horses by biting flies, but researchers have shown that as little as 1 cc of whole blood (1/5th teaspoon) from such a horse, blood containing the cells, not just serum, can transmit the infection to another horse.
If this horse has an injury, and its blood comes into contact with an area of open skin/abrasion on another horse, transmission may occur. Even more importantly, if the carrier horse is stressed in any way by another illness, injury, shipping, use of corticosteroid drugs, etc., his immune system can lose the precarious control it has over multiplication of the EIA viruses in his tissue and begin again to have virus circulating in the blood. At this point, the horse is again a threat to other horses via biting insects. These are not hypotheticals. The scenarios have been confirmed by research. This is why a positive Coggins test for antibodies is the most sensitive test for infection.
As agonizing and pointless as it seems to lose the life or use of an apparently healthy horse with a positive Coggins test, it's important to understand that the horse poses a threat to the life and health of horses around him. It's impossible to predict when the horse may have a stressor that causes his body to lose control over virus circulating freely in the blood, or to guarantee that it will never happen.
In addition to the options Mr. Lichtenstein mentioned (euthanasia or sale for meat), states also have an option of donating the horse to a research facility, or the quarantine of the horse under specific conditions if the owner does not want to euthanize. The quarantine does require branding, isolation from other horses, and obviously the horse cannot be moved. A specific state's statues regarding EIA can be viewed by logging onto the Internet site: http://tarlton.law.utexas.edu/dawson/eia/eia.htm
Research is progressing rapidly on a vaccine for EIA, thanks in large part to interest in one for AIDS. China already claims to have eliminated EIA in that country by widespread mandatory use of a modified live vaccine, but researchers have been unable to get sufficient details to confirm this.
Vaccine-induced antibodies causing a "false positive" Coggins test is really the least of the worries and could be handled in a variety of ways, such as serial negative Coggins tests before the vaccination is done, or antibody testing for the vaccine strain of the virus to make sure the antibodies are specifically for that.
The difficulty with producing an effective vaccine is the same one that plagues AIDS vaccine development, and also explains how the virus manages to elude complete elimination by the immune system. These viruses have the ability to change their outer coating easily. When the immune system has developed antibodies that recognize one makeup and drive the virus to seek shelter inside blood cells, it works on changing its coating to one that the antibodies do not recognize. This is what causes the initial cycling of symptomatic and asymptomatic periods.
Eventually the horses that survive to enter the prolonged, apparently healthy stage have a sufficient array of different antibodies to keep the virus at bay as long as they avoid significant stresses on their immune system. However, the antibodies can't destroy it completely in its hiding places. The task before EIA researchers is to find a vaccine strain, or combination of strains, that covers enough of the possible different outer coating arrays to stop the virus in its tracks.
