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 are 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 less than one hour.

Accurate testing enables horses that test positive to be removed from herds and to prevent the spread of EIA among tested populations.

Incubation Period

The incubation period is the time between when a virus is first contracted to the time that clinical signs are observable. The incubation period for EIA can range from two weeks to two months. [1][2][3]

The length of the incubation period depends on how much of the virus your horse is exposed to, the strain of the virus, and your horse’s immune function. [1][2][3]

If a horse with EIA is tested in the early stages of infection, there is a chance that they may test negative while still carrying the virus because its immune system hasn’t sufficiently responded to the virus for antibodies to be detected by the available screening tests.

For this reason, veterinarians recommend that owners test their horses each spring even if they have not been in contact with a horse that has tested positive.

False positives can also occur when testing blood samples collected from foals that have circulating colostral antibodies from drinking the milk of EIA-infected mares. These antibodies are no longer present when the foal is approximately six to nine months old. [9]


There is no specific treatment for equine infectious anemia. [1][2][3] Horses that test positive for EIA must be closely monitored by a veterinary professional.

Management of Horses with EIA

Horses that test positive for EIA need to be permanently quarantined to prevent transmission of the virus. [1][2][3]

Quarantined horses with EIA must be at least 200 yards away from other horses. If they are not going to be euthanized, they must be branded on the lip or the left shoulder. Special permits must be obtained to move the horse from the quarantine area. [10]

Infected horses should not be allowed to share the same food or water buckets that other horses use.

If you suspect your horse has contracted equine infectious anemia follow these steps immediately:

  • Contact your veterinarian
  • Move a horse suspected of having EIA at least 200 yards away from other horses
  • Reduce exposure to biting flies by using insect control products, protective horse wear, and keeping horses away from swampy areas.


In some cases, euthanasia is the most humane option for horses with EIA to prevent further suffering and spreading of the disease. [3]


Understanding how EIA is transmitted and implementing appropriate control measures can help limit the disease’s spread.

Key biosecurity measures to take to protect your horses from EIA include: [8]

  • Isolating any horse that tests positive for the virus
  • Routinely testing all horses in your herd for EIA
  • Avoiding contact with other horses that may be infected
  • Keeping your stable and equipment clean
  • Disinfecting all surfaces contaminated with blood or other bodily fluids
  • Reduce exposure to biting flies through management practices such as using foggers, electronic or automatic fly control systems in barns, and topical fly repellents.
  • Use only licensed and approved blood products
  • Use only new, clean needles with injectable medicines, syringes, and IV sets
  • Blood transfusions should be performed only by licensed veterinarians using blood obtained from horses that have tested negative for EIA and other infections transmitted through blood

When Should You Test Your Horse for EIA?

Testing for equine infectious anemia is recommended when horses are:

  • Attending exhibitions or competitive events
  • Being relocated to a new facility
  • Changing ownership
  • Entering horse auctions or sales


Equine infectious anemia is a serious condition that can lead to severe illness or death in affected animals.

There is no vaccine approved for EIA. Surveillance and testing are the best methods to prevent the spread of equine infectious anemia.

In countries that have adopted widespread testing strategies for EIA, the risk of exposure to the disease has been significantly reduced.

You can reduce your horse’s risk of infectious anemia by limiting exposure to biting insects and ensuring good biosecurity protocols.

If you think your horse may have EIA, contact your veterinarian immediately. Early diagnosis and treatment are critical to preventing transmission of the disease.

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  1. Cheevers, WP et al. Equine infectious anemia virus: immunopathogenesis and persistence. Rev Infect Dis. 1985. View Summary
  2. Cook, RF et al. Equine infectious anemia and equine infectious anemia virus in 2013: a review. Vet Microbiol. 2013.
  3. Sellon, DC. Equine infectious anemia. Vet Clin North Am Equine Pract. 1993. View Summary
  4. USDAEquine Infectious Anemia. United States Department of Agriculture. 2022.
  5. Cordes, T. et al. Equine Infectious Anemia. American Association of Equine Practitioners. 2020.
  6. Equine Infectious Anemia: What to Know. NC State Veterinary Medicine. 2022.
  7. Equine infectious anemia. Manitoba Agriculture. Accessed September, 2022.
  8. McIlwraith, CW et al. Neurologic signs and neuropathology associated with a case of equine infectious anemia. The Cornell veterinarian 68. 1978. View Summary
  9. Giedt, E. Equine infectious anemia. Oklahoma State University. 2017.
  10. Equine Infectious Anemia: Uniform Methods and Rules United States Department of Agriculture. 2007