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Key takeaways

Equine Infectious Anemia (EIA), or swamp fever, is a viral disease in horses.

  • It is transmitted by blood, either via biting insects or contaminated medical equipment
  • Symptoms include recurrent fever, lethargy, yellow gums, swelling, and muscle weakness
  • Once infected, disease transmission is possible whether symptomatic or not
  • Horses suspected of having EIA require emergency veterinary attention and isolation
  • Diagnosis is based on blood tests, typically a Coggins test. - There are strict rules about reporting this disease and government policies about management
  • There is no cure for EIA
  • Typically, the horse must be euthanized
  • In some countries, it is possible to permanently quarantine the horse
  • Prevention involves only attending events that require a negative Coggins test, regular testing of all horses, insect management, and sterilization of all medical instruments
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A closer look: Equine Infectious Anemia

Equine infectious anemia (EIA) is a geographically widespread disease that is highly infectious and has serious economic implications. It has become rare over the past few years due to strict government policy regulating testing and management. Many events, exhibitions, competitions, sales, and auctions require that all animals in attendance are shown to be free of EIA by providing a recent negative Coggins test.

Horses that participate in events, or other gatherings where negative Coggins tests are not required are at risk of contracting equine infectious anemia.

Horses that live in conditions where biting insects are common, that receive medical attention without proper sterilization of equipment, and that receive untested blood products are most at risk of contracting EIA.

Once a horse has contracted EIA, they are able to transmit the disease to other horses, whether or not they are showing symptoms. Horses showing symptoms of EIA require emergency veterinary examination, to begin the testing process and confirm their EIA status. Horses suspected of having or diagnosed with EIA must be quarantined immediately and kept at a distance of 200 meters from other horses at all times. Extended insect control protocols must be put in place to prevent contact with biting insects.

Risk factors

Equine infectious anemia has three forms: acute, chronic and non-apparent. In all three forms the disease is transmissible to other horses.

In the acute stage, the symptoms are mild, and last between 1 and 3 days. The severity of the symptoms depends on the amount of virus that was originally transmitted to the horse, the specific strain of virus involved, and the overall health and susceptibility of the horse. This stage often goes undiagnosed as it is short-lived and the symptoms can be very mild or unnoticeable. The main symptoms of this stage include fever, lethargy, pale gums, and swelling of the lower limbs.

In the chronic stage, the symptoms develop further. They flare up and then disappear. Intervals between symptomatic periods vary from a few days to months. Flare ups are often associated with periods of stress or immune suppression. Horses experience pale gums, swelling of the lower legs and lower abdomen, and weight loss during a flare up. At this stage, the virus infects the horse’s tissues, becoming a permanent resident. Horses with symptoms have higher concentrations of the virus in their blood and are particularly capable of transmitting the disease.

In the non-apparent form, infected horses are carriers without showing any symptoms. In this state, they are still capable of transmitting the disease. Most horses become inapparent carriers around a year after their initial infection with the virus.

Possible causes

EIA is caused by the equine infectious anemia virus. This virus is blood borne, which means that transmission occurs when the blood of an infected horse makes contact with the blood of an unaffected horse.

In the past, the most common route for blood contamination was through biting insects such as horseflies, deer flies, and to a lesser degree, stable flies. Once these insects feed on an infected animal, the virus remains on the mouth parts for approximately 30 minutes. When the insect lands on the next horse and bites into the flesh, the virus is transmitted from the insect's mouthparts into the blood of the horse. Horseflies, deer flies and stable flies are all particularly dangerous because their bite irritates the horse, triggering the horse to shoo them away before the insect has completed its blood meal. It moves on to the next horse to complete the meal, bringing the virus with it.

Other transmission routes include

  • From the mother to the foal before or after birth
  • From stallion to mare during mating

The transmission of EIA also occurs through infected blood products or improperly sterilized medical equipment, including:

  • Needles
  • Syringes
  • Surgical instruments
  • Intravenous administrations sets
  • Lip tattooing equipment
  • Multi-use vials of medication or vaccines

Main symptoms

The main symptoms of equine infectious anemia vary from horse to horse.

Testing and diagnosis

Horses suspected of having equine infectious anemia require immediate quarantining until a diagnosis is made. A suspicion of EIA is based on symptoms identified on a physical examination. Other diagnostic tests include:

  • Blood testing for the virus
  • Bloodwork

In some countries, only government approved laboratories are allowed to perform the blood tests for the virus. The most common and well-recognized test is called the Coggins test. Recently, ELISA tests have also been used. A combination of the two is sometimes recommended.

There are certain circumstances prior to which testing is required even when no infection is suspected, including:

  • Entering into exhibitions, competitions, or events
  • Being moved from one state or province to another
  • Changing ownership
  • Entering auctions or sales

Steps to Recovery

There is no cure or vaccine for equine infectious anemia. Horses that are diagnosed with EIA must be euthanized or kept in lifelong quarantine to prevent transmission to other horses. In some countries, the government requires euthanasia of these animals in order to control this disease.

EIA is a lifelong disease. The prognosis for horses infected with it is poor. Even in cases where the horse could survive, the threat of transmission to healthy horses is too severe to risk, and euthanasia or lifelong quarantine are required.


Prevention of equine infectious anemia is vital. Strategies to prevent infection include:

  • Euthanizing or permanently isolating horses and other animals of the equidae family (donkeys and asses) that are diagnosed with the disease
  • Quarantining horses at the first sign of fever
  • Only participating in events, competitions, exhibitions, sales, or auctions that require evidence of a recent negative Coggins test
  • Administering a Coggins test to every member of the herd on a yearly basis
  • Requiring proof of a recent negative Coggins test when purchasing new horses
  • Testing new horses before introducing them to the herd and quarantining them until the results are confirmed
  • Managing insect populations effectively
  • Using strict hygiene practices and careful disinfection protocols when using reusable medical equipment including surgical instruments, dental equipment, and tattoo needles
  • Using disposable medical equipment where possible
  • Using a sterile needles on multi-dose medication vials
  • Following all additional protocols required by the government and other appropriate governing bodies

Is Equine Infectious Anemia common?

EIA has been becoming less common since its peak in the 1970s. It is now considered rare in most areas.

Typical Treatment

Euthanasia or permanent quarantine


Peter J. Timoney , MVB, PhD, FRCVS - Writing for Merck Veterinary Manual
Tim Cordes, DVM & Charles Issel, DVM, Ph.D. - Writing for American Association of Equine Practitioners
- Writing for University of Saskatchewan Western College of Veterinary Medicine
- Writing for Canadian Food Inspection Agency

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