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Equine Infectious Anemia (EIA) – Coggins Test in Horses

Equine Infectious Anemia – Coggins Test in Horses
American Association of Equine Practitioners
By: Robert N. Olgesby, DVM

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Equine Infectious Anemia - Coggins Test - Coggins - Equine Infectious - Infectious Anemia

Equine Infectious Anemia (EIA) 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 in-apparent 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 (EIA) is a disease that threatens the world’s horse, donkey and mule populations. Despite testing and measures to eradicate the equine infectious anemia virus (EIAV), more than 500 new cases are identified each year in the U.S. There is no cure for EIA. Although most infected horses show no symptoms, they remain contagious for life, endangering the health of other horses. For this reason, the United States Department of Agriculture (USDA, ) and state animal health regulatory agencies require euthanasia or strict lifelong quarantine for horses testing positive for EIAV.

Equine Infectious Anemia
understanding the importance of the Coggins Test

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.


Equine infectious anemia is a potentially fatal viral disease. EIAV reproduces in white blood cells that circulate throughout the body. The immune system, via antibodies, may attack and destroy red blood cells, leading to anemia. Inflammation associated with the viral infection may damage vital organs, such as bone marrow, liver, heart and kidney. Secondary infections (e.g. pneumonia) may occur due to subsequent immunosupression. EIAV-infected horses may die from the direct effects of the virus or from secondary infections. EIA generally has three forms:


  • Acute: Seen within one to two weeks after the horse’s first exposure to the virus, this phase is the most detrimental. It may be difficult to accurately diagnose acutely infected horses, as antibodies are not immediately produced and anemia is not present at this stage. However, the virus is active, multiplying and damaging the immune system and other organ systems.

Chronic: If the horse survives the acute phase, a subacute or chronic phase may occur. The classic signs of EIA, such as fever, depression, weight loss, anemia and petechial (pinpoint sized) hemorrhages on the mucous membranes, are most likely seen in this phase. Repeated flare-ups of clinical signs often occur. Such episodes are seen with recrudescence of the virus and viremia (virus present in the bloodstream) during periods of stress or the administration of corticosteroids.


  • Inapparent: Over time, the periodic episodes decrease in severity and frequency. Within a one-year period many horses begin to control the infection and show no clinical signs or problems. These unapparent carriers are infected for life and may be a source of infection for other horses. It is recommended that horse owners implement an EIA control plan for their premises.


EIA may be difficult to diagnose because the symptoms are not specific and may vary from horse to horse. Additionally, individuals may demonstrate no obvious signs (inapparent carriers). Signs may include one or more of the following:

  • Fever (temperature may even exceed 105 degrees F)
  • Depression
  • Mucosal petechial hemorrhages
  • Decreased platelet numbers (thrombocytopenia)
  • Decreased red blood cell numbers (anemia)
  • Swelling of legs, lower chest and abdomen (edema)
  • Decreased appetite (anorexia)
  • Fatigue, reduced stamina or weakness


The only way to accurately determine whether a horse is infected with the EIA virus is by identifying antibodies in the blood via agar gel immunodiffusion (AGID) or competitive enzyme linked immunoadsorbent assay (C-ELISA) tests. The AGID method is considered the “gold standard” and is commonly known as the Coggins test. This test was developed 25 years ago by veterinary researcher Dr. Leroy Coggins. A negative Coggins test means there are no detectable antibodies at the time of testing. A positive test indicates the horse is infected and a carrier of the virus. C-ELISA tests offer the advantage of rapid results. However, false-positive results are more common with the C-ELISA tests and positive results should be verified by a standard Coggins (AGID) test. Foals may be false positive due to maternal antibodies passed via colostrum for as long as six months with either test.


There is no effective treatment for EIA. There is no vaccine to prevent it. There is no cure. However, good management can reduce the potential of infection. The following guidelines will help:

  • Use disposable needles and syringes (one per horse) when administering vaccines and medications.
  • Sterilize dental tools and other instruments before using them on another horse.
  • Test all horses for EIA at least annually.
  • Test horses at the time of purchase examination.
  • Stable owners, horse show and event managers should require and verify current negative Coggins certificates for all horses entering the premises.
  • New horses should be quarantined for 45 days and observed for any signs of illness, including elevated temperatures, before introducing them to the herd. They should be retested if exposure to EIA is suspected at a 45-day interval.
  • All stable areas should be kept clean, dry and waste-free. Good pasture management techniques should also be practiced. Remove manure and provide adequate drainage to discourage breeding sites for pests.
  • Horses that are at greater risk (such as animals who are in frequent contact with outside horses or who live or travel in geographic regions known for EIA outbreaks) should be tested more frequently, ideally every four to six months.

Equine Infectious Anemia – Coggins Test
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