horse tied to trailer

Equine Infectious Anemia

What is equine infectious anemia?


  • Equine infectious anemia (EIA) is a viral disease transmitted primarily by flies, contaminated instruments and equipment.
  • There is no vaccine for EIA and no known treatment.
  • Horses that survive the acute phase of infection become lifelong carriers that pose a transmission risk to other horses. EIA-positive horses must be identified and isolated (at least 200 yards) from other horses or euthanized to prevent the spread of the virus.
  • EIA – positive horses are quarantined by the state animal health officials with the California Department of Food and Agriculture.
  • A Coggins test is commonly used to determine if a horse has EIA.

Equine infectious anemia (EIA) is a viral disease caused by the equine infectious anemia virus. This virus belongs to the family of viruses that includes human immunodeficiency virus (HIV). The disease affects horses and ponies, but is not transmissible to humans. Donkeys and mules can contract EIA, but most do not develop severe clinical signs. Equine infectious anemia has been found worldwide and is a reportable disease in all U.S. states.

Blood-feeding flies (especially horse flies and deer flies), blood-contaminated needles, blood transfusions, and contaminated instruments can transmit the virus. Less common routes of infection include from mare to foal in utero, and in milk or semen. Infected horses can show no signs of disease or they may exhibit acute and/or chronic recurring clinical signs. The virus is present in white blood cells and plasma where levels can become elevated during periods of stress. EIA-positive horses that survive the acute stage of infection become lifelong carriers that can transmit the virus to other horses. There is currently no vaccine and no known treatment for EIA.

What are the clinical signs of equine infectious anemia?

Most horses remain asymptomatic. For those that do show clinical signs of EIA, these can be nonspecific, vary in severity, and classically progress through three stages after an initial incubation period of 15-45 (or more) days. The initial, acute episode, which lasts 1-3 days, is characterized by fever, a deficiency of platelets in the blood (thrombocytopenia), and lethargy. This is followed by a period of days to months in which clinical signs include recurring fever, thrombocytopenia, severe anemia (not enough red blood cells or hemoglobin in the blood), decreased appetite, and sudden death. Additional signs may include jaundice, rapid breathing, rapid heart rate, swelling of the limbs (edema), bleeding from the nose, red or purple spots on the mucous membranes, blood in the feces and overall general weakness. It is impossible to differentiate EIA from other diseases that cause fever, edema and/or anemia based solely on clinical signs. Owners may not realize that a horse is infected unless it is tested.

The strain and dose of the virus, as well as the health of the animal, influence the severity of clinical signs and the morbidity rate. Many horses show no signs, or very mild signs, when they are first exposed to the virus. Those that survive the acute phase become lifelong carriers. These horses may develop recurrent flare-ups following periods of stress, exhibiting signs that vary from failure to thrive to fever, anemia, weight loss, lethargy, and swelling (edema) of the legs, chest, and abdomen.

How is equine infectious anemia diagnosed?

In the U.S., EIA testing must be performed by USDA-approved laboratories. Testing is performed by serology via the agar gel immunodiffusion (AGID) test, commonly known as the Coggins test (named after its developer Dr. Leroy Coggins), or enzyme-linked immunosorbent assay (ELISA).

A positive result using either test indicates the presence of EIA-specific antibodies in the blood. The AGID/Coggins test may not detect antibodies until 2 or 3 weeks after the horse has been infected. The ELISA can detect antibodies earlier and is more sensitive but has a higher rate of false positives.

Positive results on ELISA are usually confirmed by AGID/Coggins or immunoblotting (Western blotting). These tests can be supplemented with polymerase chain reaction (PCR) tests to confirm results, especially in cases where there are conflicting results. Reverse transcription polymerase chain reaction (RT-PCR)  assays may also be used and are especially useful in foals born to infected mares as young animals may have maternal antibodies. A negative Coggins test, dated within the last 6-12 months (preferably within the last 60-90 days), is required when a horse is moved across state lines and is required in some states when a horse is sold, traded, or donated.

Testing laboratories must report positive EIA tests to local state or federal animal health officials within 24 hours. A regulatory veterinarian will then identify exposed horses, which will be placed under quarantine and tested for EIA. The USDA Animal and Plant Health Inspection Service maintains summary reports of confirmed EIA cases in the U.S.

The California Animal Health and Food Safety Laboratory offers both AGID and ELISA testing for the EIA virus.

How is equine infectious anemia treated?

There is no treatment or “cure” for EIA. If you suspect that your horse may be infected, call your veterinarian immediately, move the horse at least 200 yards away from other horses and reduce exposure to biting flies. Equine infectious anemia is a reportable disease. Positive cases must be reported by the testing laboratory to local state or federal animal health officials within two days of discovery.

What is the prognosis for equine infectious anemia?

In many cases, infected horses are clinically normal, or show nonspecific clinical signs. In severe cases, high fever, reduced platelet counts, and acute depression can lead to death. Regardless of the severity of clinical signs, infected animals are lifelong carriers. They must be permanently identified by tattoo, brand or microchip. Horses positive for EIA must be permanently isolated and quarantined at a minimum of 200 yards from all other horses or euthanized to prevent transmission of the disease.

How can equine infectious anemia be prevented?

Although EIA can result in serious outcomes, infections are rare. The rate of infection varies throughout the world based on location and is influenced by the number and species of flies, their habitats and the density of the horse population. The virus itself does not persist in the environment and cannot survive in soil or water. It is infective for up to 96 hours on contaminated needles and stays viable on the mouthparts of biting flies for less than 4 hours. It can be destroyed by most common disinfectants, such as bleach or alcohol (follow manufacturer’s recommendations and label instructions).

Since there is no vaccine available in the U.S., surveillance and testing are the best methods of prevention.

To reduce your horse’s chance of becoming infected with EIA:

  • Reduce exposure to biting flies through proactive management and insect control.
  • Never reuse needles or syringes; only use sterile needles and licensed blood products.
  • Use a sterile needle each time you puncture a multi-dose medication bottle.
  • Thoroughly disinfect surgical and dental equipment, bits and lip chains between horses.
  • Ensure that blood transfusions are only performed by licensed veterinarians using blood from confirmed EIA-negative donor horses.
  • Clean and cover open wounds.
  • Test every horse at least annually.
  • Separate horses with fevers, inappetence and/or lethargy from other horses and contact your veterinarian.
  • Require proof of a recent negative EIA test for new horses entering the premises or when purchasing a new horse.

For more information:

AAEP Infectious Disease Guidelines – Equine Infectious Anemia

CDFA – Equine Infectious Anemia

California Animal Health and Food Safety Laboratory tests and fees