Coggins Test and Equine Infectious Anemia (EIA)

Questions about the methods being used to confirm Equine Infectious Anemia. Perfect Horse Veterinary Editor Dr. Kellon responds.
Avatar:
Author:
Publish date:
Social count:
0
Questions about the methods being used to confirm Equine Infectious Anemia. Perfect Horse Veterinary Editor Dr. Kellon responds.

The perfect horse article "Equine Infectious Anemia Continues to Kill," was not complete and is somewhat misleading, as anyone who has endured an outbreak of EIA positives can attest.

Such an outbreak occurred circa 1990 in southeastern Pennsylvania. EIA as a disease was reported to be 30% fatal in symptomatic horses at the time. However, the Coggins test was 100% fatal-regardless of symptoms. While Dr. Eleanor Kellon describes the tests and symptoms accurately, she does not state that regardless of the outcome of other tests, a positive Coggins is a death sentence in all jurisdictions that use it.

The Coggins test does not test for the actual virus that causes EIA. It tests for antibodies to the virus. This virus is similar to HIV and, like any pathogen, causes the exposed body to generate antigens and antibodies. Most vaccines for humans and horses use killed viruses or viruses that are genetically altered not to be infectious. The first AIDS tests were for the antibodies and there were some who thought positive for antibodies should be a death sentence for the infected.

We cannot vaccinate against EIA because the only legally recognized test for the disease does not test for the disease itself, but for the antibodies the vaccine will stimulate. The statutes regarding EIA are what are killing horses, not the disease, in most cases.

The disease requires a serum transfer with a sufficient dosage of the live virus to cause active infection. The natural vector is the large horse fly. A horse with a large pool of the virus when bitten may coat the mandibles of the fly with enough serum to infect another horse if the fly bites while the serum is still wet and the virus viable. The latency seemed to be about 30 days.

To add insult to injury, the owner must pay his vet to draw and test the blood samples that will condemn his horse, pay to have the animal destroyed and disposed of, and cannot collect insurance or any compensation for the animal.

Even if the owner opts for a more expensive test that actually detects the virus, he cannot save an animal that has antibodies because the only test that counts is the Coggins test. In some jurisdictions, a two-in-a-row, 30-days-apart rule applies, but many owners cannot live with the quarantine signs on their barns for 60-90 days and just kill the horse or sell it for meat.

If you want to eradicate a disease, make it attractive for owners to do so. Make the tests free and mandatory on an annual basis. Test for the virus, not the antibodies, at least on the verification tests, and do not kill unless the virus is present. Compensate owners of horses as we compensate cattle owners, chicken owners, and others who lose animals to mandated destruction programs.

The disease did not kill my horse, a government order did. A symptomatic horse survived in quarantine, and another came clear on the second and third tests. All were in the same barn, same paddocks, and we rode together for years. There were no other positives in an 80-stall barn over a 120-day period that surveillance was in place.

New Vaccine May Offer Reprieve
By Eleanor Kellon, VMD

Mr. Lichtenstein very eloquently expresses the anguish, confusion and anger that every owner feels if his or her apparently healthy horse turns out to have a positive Coggins test. However, a few more details about this disease may help owners understand why the Coggins test continues to be used.

The virus that causes equine infectious anemia is a lentivirus, part of the same family of viruses that causes AIDS in people. One of the most important things to realize is that once infected with this virus, the horse remains infected for life. The first wave of symptoms occur 3 to 4 weeks after initial infection, ranging from a fever that may go undetected to more severe signs of fever, lethargy, edema, anemia, low platelet counts, and even diarrhea.

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.