Rural Heritage Vet Clinic

Coggins Testing—Is It Necessary?

by Beth A. Valentine, DVM, PhD

Equine infectious anemia. Does that term ring a bell? How about Coggins test? EIA—also known as swamp fever—is what the Coggins test looks for. Okay, so what? Is it as important as your veterinarian makes it seem?

Yes, it is. No horse ever recovers from EIA; the retroviral infection persists for the horse's life, and no effective vaccine or treatment has been found. The virus is related to viruses that cause bovine leukemia, feline leukemia, and HIV infection. The blood test that detects the virus was designed by Dr. Leroy Coggins, a prominent veterinary virologist.

Since the advent of rigorous testing, EIA has become a rare disease in most parts of the world, but where it does occur it can have a tremendous impact on the horse industry. Testing and reporting of results are overseen by veterinary regulatory agencies. Your veterinarian must fill in forms with details including all the horse's identifying marks, such as white markings and areas where the hair grows in whorls.

In many states transporting a horse within the state without a negative Coggins test is illegal. Most, if not all, states require a negative Coggins test before a horse may be transported across state lines. Many competitions and sales require a negative Coggins test for all horses entered. Depending on the state or competition, this negative test must have been obtained between the previous six months and two years. Any positive test is immediately reported to the state veterinarian. Positive horses must be destroyed or isolated for life, and all potentially exposed horses must be tested.

Although this disease is sometimes referred to as equine AIDS, the description is not accurate. The virus of EIA is only a distant cousin to the HIV virus that causes AIDS. The EIA virus is spread through the blood of infected horses, and infects only horses. Biting insects, especially horse flies and deer flies, carry the virus from horse to horse. Reusing needles when injecting one horse after another can spread the virus.

Most veterinarians have little experience with this disease and many horse owners feel that the required test is mainly a bureaucratic exercise. Not so. Despite rigorous testing, EIA virus is still around. Outbreaks of the disease have been found in wild mustang herds and in parts of the western United States and Canada. Last year a positive horse was detected after a sale in Pennsylvania, where a large number of potentially exposed horses had to be quarantined and tested. This spring four horses in West Virginia tested positive, one of them having come from the same Pennsylvania sale barn.

Foals born to EIA positive mares may test positive due to antibodies from the mare that pass to the foal in the mare's first milk (colostrum). Such foals must be placed in isolation facilities and retested for up to 6 months to find out whether or not they are infected.

How can you tell if your horse has been infected by EIA virus? The disease has three forms:
  1. Some horses develop a sudden onset of fever and depression within one to four weeks after being infected. These horses go off feed and develop small areas of bleeding (hemorrhages) in the gums, but are not anemic. Such horses often appear to recover, but unfortunately they are developing the second form of EIA, anemia.
  2. When anemia develops, it may be accompanied by fever. The anemia generally waxes and wanes, with some horses appearing to recover somewhat only to have the signs recur later. During this second stage of EIA infection the horse may die.
  3. The third form of EIA is the carrier horse that is infected but shows no signs of fever, anemia, or other problems. Although most horses with this chronic form of EIA appear to be normal, we veterinarians suspect the infection reduces their athletic performance to some degree. Such horses serve as a continuous source of infection for other horses within the range of flying insects that feed on blood. The longer an EIA positive horse goes undetected, the more likely other horses will be infected.
The Coggins test is simple, inexpensive, and well worth the peace of mind that comes from knowing your horses remain negative for EIA, even if they never leave your farm and even if your state or the competitions you enter do not require a negative Coggins test. Drawing blood for a Coggins test should be a routine part of each horse's annual veterinary examination.


Beth A. Valentine, DVM, PhD, is a diagnostic pathologist at Oregon State University in Corvallis, this site's virtual vet and co-author of Draft Horses, an Owner's Manual. This article appeared in the Summer 2000 issue of Rural Heritage.

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26 October 2011 last revision