Equine Infectious Anemia (EIA) and the Coggin's Test in Horses

by Robert N. Oglesby, DVM

Introduction

Equine Infectious Anemia is a viral disease of horses for which there is no cure, nor is there a vaccine to prevent this disease. Equine infectious anemia is a disease of horses characterized by three clinically distinct forms. That range from a rapidly worsening febrile disease that results in death to a unapparently ill chronic carrier form. First described over 150 years ago in Europe, the disease has been seen in the United States for more than 75 years. In 1970 an agar-gel immunodiffusion (AGID) test, the Coggins test, capable of detecting inapparent virus carriers. This test, along with improved knowledge about EIAV transmission, has made effective control of the infection among horse populations a reality. This article discusses how horses get this disease, clinical signs, testing for EIA and its reliability, and the handling of infected horses.


 
Equine Infectious Anemia
understanding the importance of the Coggins Test
by Mark A. Crisman, D.V.M.
An EIA (Coggins) blood test is required on horses before most organized events, and for transporting a horse from one state to another. Nearly everyone has dealt with it, but many owners don't know exactly what it is.
 
The test is a check for equine infectious anemia, a contagious disease that affects horses worldwide. It is also known as "swamp fever" because of the higher incidence on the Gulf Coast of the United States, where increased humidity and temperature are favorable for transmission of the disease.
 
There is no treatment known to cure or eliminate the virus. Additionally, studies to develop an effective vaccine have been disappointing.  EIA is caused by a virus that was first identified in 1904. Although this disease has affected horses for a long time, researchers have recently grown more interested because the EIA virus is closely related to the human immunodeficiency virus (HIV).
 
EIA is transmitted primarily by bloodsucking insects. The horsefly is the most common. The large size and painful bites of these flies make it unlikely that a horse would permit them to complete a blood meal uninterrupted; by tail swishing or twitching, they send the biting fly on its way. This is the problem, however, because the virus is transmitted to other horses when the horsefly starts his meal on an infected horse and completes it on an uninfected one.
EIA has three common clinical forms.
 
In the acute or early form, the horse will be depressed, uncoordinated and feverish. Horses are rarely anemic during this stage. This phase may last several days and is the stage during which the horse is most likely to transmit the disease to nearby horses.
 
The second phase is characterized by weight loss, recurring fevers and general weakness. Anemia is likely to be present, and mares can abort during this stage.
If horses survive the first two stages, they enter the final or chronic stage, where they often appear normal. An owner may report that a horse is a poor keeper, and the animal may be mildly anemic. Infected mares can transmit the disease to their foals.
 
A horse infected with the EIA virus will be a carrier of the disease for life. Although horses in the chronic stage appear normal, they become ill again if subjected to stress, such as shipping or severe weather.  Carriers pose a health threat to the equine community. Since the early 1960's, several outbreaks of EIA have occurred at either race tracks or large breeding farms, resulting in the deaths of many horses.
 
The Coggins test, developed in the 1960's by Dr. Leroy Coggins, is based on the detection of antibodies in the horse's blood. Once a horse is infected with EIA the antibodies will be present in the blood for life. THE USDA does not have an eradication program for EIA--however, a national approved laboratory system has been established to perform the Coggins test.
Even if you don't show your horse, it should have an annual Coggins test. Horses that are shown or transported regularly may require more frequent testing.
 
Most states require that horses entering or being transported across their lines have a negative Coggins Test no more than six months old. Interstate transport of horses with a positive Coggins test is prohibited. Likewise, many horse shows and other events require a negative Coggins for your horse.
 
If a horse tests positive, strict quarantine measures are imposed. These include confinement in a screened stall and a 200-yard buffer from all other horses. Often, owners elect to euthanize their horses rather than subjecting them to such strict isolation.
 
Because of the relatively low incidence of EIA in the United States, most horse owners and veterinarians have decided that additional regulations are not necessary. Last year, 76,000 horses were tested in Kentucky, with 20 horses testing Coggins-positive. In Virginia, 40,000 were tested, with eight positives. Nevertheless, if you plan to buy a horse or board a horse, require either a recent negative Coggins from the owner, or isolate the horse until the test is performed.

 

 


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