Equine Infectious Anemia (EIA) is an infectious disease that affects horses and other equids, such as donkeys and mules. The disease is caused by an RNA virus transmitted by blood-sucking insects. [1][2][3]

Many affected horses show no clinical signs and are asymptomatic carriers of the disease. However, stress or illness can cause signs to become apparent.

Horses that test positive for the virus must be isolated from other horses to prevent the spread of the disease. Horses infected with the EIA virus carry it for life and remain contagious. [1][2][3]

The best way to prevent the spread of Equine Infectious Anemia is to adopt good biosecurity policies in your barn and regularly test horses for the disease.

What is Equine Infectious Anemia?

Also known as swamp fever, Equine Infectious Anemia is an incurable viral disease affecting the horse’s immune system.

EIA is a lentivirus, a genus of retroviruses that have long incubation periods and that affect mammals.

The virus stimulates an immune response that can result in inflammation and tissue damage as antibodies attack the horse’s own cells. [1][2][3]

It can also result in equine anemia, which is characterized by a lack of red blood cells carrying oxygen through the body.

The extent of the immune response is related to the horse’s viral load. Most horses experience no clinical signs, but some horses with a high viral load are at risk of more serious symptoms.

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Natural transmission of EIA to horses occurs over relatively short distances via blood-feeding insects, such as horse flies, mosquitoes, gnats, deer flies, and stable flies. Insects can carry blood from an infected horse to another horse, spreading the disease.

The disease is also spread through direct contact with an infected horse or through contact with contaminated blood transfusions or medical equipment including unclean or reused needles, syringes, and dental instruments. [1][2][3]

A foal can also contract the disease in utero from its mare. [1][2][3] Transmission to other horses via a mare’s milk or stallion’s semen is possible, although less common.

The virus responsible for causing EIA is related to the Human Immunodeficiency Virus (HIV), but humans are not at risk of contracting infectious anemia from horses.

Risk of EIA Spread

Outbreaks commonly develop in late summer and early fall in temperate regions when populations of biting insects are at their highest levels. [2]

The risk of equine infectious anemia transmission is also increased when:

  • There is an EIA outbreak in the vicinity
  • New horses enter the pasture or stable and have not had a Coggins test
  • Attending horse events where negative Coggins tests are not required
  • Horses in pastured near swampy areas
  • Horses are not regularly tested for EIA


Horses that are infected can spread the virus even if they do not look sick. Infected horses must be quarantined to prevent the spread of the disease.

There is no cure for infectious anemia in horses, and there is no vaccine available to prevent it. However, horses with EIA can live long and healthy lives with proper management and support.

Despite this, infected animals are sometimes humanely euthanized to prevent further suffering and to protect other horses.


Equine infectious anemia affects members of the Equidae family (horses, ponies, zebras, mules, and donkeys) around the globe.

There is a higher incidence of this disease in warmer climates where blood-sucking insects are prevalent.

Although EIA has been recognized for centuries, cases of the disease began to rise in the 1930s and peaked between the 60s and 70s in the United States. [4]

Of the 600,000 to 900,000 horses tested annually since 1977 in the United States, 0.3% to 0.5% tested positive for the virus. [4] This is between 18,000 to 45,000 horses.

Types of Equine Infectious Anemia

There are three forms of EIA: acute, chronic and inapparent. [5][6]


Horses with an acute form of EIA have a rapid onset of symptoms and may die within two to three weeks. [5][6] An elevated body temperature may be the only apparent sign of the illness in horses because the condition appears so suddenly.

Horses in the acute phase of the disease are more likely to spread the infection because of the high level of virus present in their blood.

Horses that survive having the acute form of EIA may become chronically infected or inapparent carriers.


After surviving acute infection with EIA, horses can develop other signs of the disease as their immune system responds to the virus. The chronic state of the disease is the most diagnosed form in affected horses. [5]


Many horses infected with EIA are inapparent carriers, meaning that they show no signs of disease unless exposed to severe stress. [1][2][3]

Although the amount of virus in their blood is less compared to horses with the acute form of the illness, they continue to pose a risk to other horses.

Horses that are inapparent carriers of EIA are typically identified only when they are tested for the condition.

Signs & Symptoms

Signs of acute EIA may appear abruptly or gradually and can range from mild to severe. Common signs of the condition include fever, anemia, weakness, weight loss, and edema (swelling). [6][7]

Horses may also develop neurological signs, such as problems controlling movements, head pressing, or forelimb paralysis.

One case report involving a 7-year-old Quarter horse with EIA observed depression, disorientation, circling, knuckling at the fetlock, and cerebral brain dysfunction (hypermetria). [8]

Horses with infectious anemia may also be more vulnerable to other illnesses.

Additional clinical signs of equine infectious anemia include: [6][7]

  • Increased sweating
  • Rapid breathing
  • Irregular heartbeat
  • Thrombocytopenia (low platelet count)
  • Colic
  • Ataxia (loss of muscle coordination)
  • Jaundice or icterus (yellowing of the skin)
  • Anorexia (lack of appetite)
  • Abnormal bleeding
  • Abortion in pregnant mares
  • Pale mucous membranes
  • Emaciation (in chronic cases)
  • Small hemorrhages on mucous membranes
  • Blood-stained and or watery feces

In most horses, the frequency and severity of the clinical signs of EIA decrease over time, leading to an inapparent carrier form of the illness.


If you suspect your horse may have EIA, contact your veterinarian immediately.

EIA can be difficult to diagnose because many horses are asymptomatic. In horses that do display clinical signs, the symptoms can mimic those of other diseases.

A diagnosis of equine infectious anemia is typically made by testing the horse’s blood for the presence of the virus.

The Coggins test and the C-ELISA are used to detect equine infectious anemia. Testing is the best way to prevent the spread of EIA, and all horses should be tested at least once a year.


The Coggins test is a blood test used to detect Equine Infectious Anemia (EIA). Dr. Leroy Coggins first developed this test in 1972. [8]

Commonly used among horse industry professionals, the Coggins test (an agar-gel immunodiffusion test) looks for antibodies in the horse’s blood that are specific to the EIA virus. If these antibodies are present, it indicates that the horse has been exposed to the virus and is likely to be infectious.


More recently developed, enzyme-linked immunosorbent assays (ELISA tests) can be used to test for EIA. [8] This form of testing reduces the time for obtaining a lab result from a minimum of 24 hours to l