Equine infectious anemia (EIA)

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Equine Infectious Anemia (EIA)

Swamp Fever, Mountain Fever, Coggins Disease, Equine Malarial Fever, Slow Fever

Equine infectious anemia (EIA), also referred to as "swamp fever", is a potentially fatal, infectious disease of horses, caused by the EIA virus (EIAV). EIAV is characterized by intermittent fever, progressive anemia, emaciation and death in severe cases. EIA is a reportable disease in the United States and to the World Organization for Animal Health (OIE). It is considered to be a serious threat to horses, donkeys, mules, zebras and ponies worldwide. There is no cure for EIA.

EIA manifests as three different clinical forms---acute, chronic and inapparent.
  • Acute form: Horses develop the acute form of EIA within 1-2 weeks following initial exposure to the virus. This form is responsible for causing the most damage to the horse, and is often difficult to diagnose since antibodies are not yet produced and anemia is not present at this stage.
  • Chronic form: Horses that survive the acute form of EIA will often proceed to a chronic or subacute form. Clinical signs include fever, depression, weight loss, anemia, and petechial (pinpoint sized) hemorrhages on the mucous membranes (gums). Affected horses will often develop repeated flare-ups of clinical signs during periods of stress or when administered corticosteriods.
  • Inapparent form: This is the most frequent form of EIA in horses. Affected horses remain inapparent carriers of EIAV for life, and serve as a source of infection for other horses, donkeys, and mules.
In the United States, approximately 22% of the horse population gets tested annually for EIA through a coggins test. A Coggins test is required for all horses competing in events, moving interstate or overseas, changing ownership, entering auctions or sales markets, or attending equine events. A Coggins test refers to two available methods of determining whether a horse is infected with EIAV, 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 to be the "gold standard", however the c-ELISA test offers 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.

In the United States (US), the US Department of Agriculture (USDA, www.usda.gov ) and state animal health regulatory agencies require that if horses test positive for EIAV that they are either euthanized or live the remainder of their lives under strict quarantine.

EIAV is transmitted to horses by blood or by in-utero passage from mare to foal. Blood transmission can occur through bites from blood-sucking insects (such as horse flies, deer flies and mosquitoes), blood transfusions, blood-contaminated needles and instruments, semen and milk. Incidences of EIA tend to occur most frequently in warm, wet, swamp-like areas. In the United States, the highest infection rates of EIAV have been documented in Texas, Oklahoma, Arkansas, Louisiana, South Dakota, Mississippi, Minnesota, Michigan, Alabama and Florida.

Incubation period
The incubation period is usually 1–3 weeks but can be as long as three months.


Fever (often may exceed 105°F (40.5°C)
Petechial hemorrhages on gums
Decreased appetite
Swelling of legs, lower chest and abdomen (edema)
Rapid breathing
Nasal bleeding (epistaxis)
Fatigue, reduced stamina or weakness
Rapid weight loss
Pale or yellowish (icteric) mucous membranes
Weak pulse
Colic signs
Abortion in mares
Decreased platelet numbers (thrombocytopenia)


  • Agar gel immunodiffusion (AGID)
  • Competitive enzyme linked immunoadsorbent assay (C-ELISA) test


Report diseaseEIA is a reportable disease, meaning that if you suspect that your horse has this disease, by law you need to report it to your veterinarian, or a state or federal veterinarian.
Supportive care


  • Practice good insect control methods to reduce populations of blood sucking insects such as deer flies, horse flies, and mosquitoes
  • Eliminate mosquito breeding areas
  • Protect horses from biting insects through the use of fly sheets and masks and insect repellents

Scientific Research References

Good Overviews

Risk Factors

  • Horses that live in warm, wet regions containing swamps
  • Poor insect control methods to reduce blood-sucking insect presence
  • Horses that receive blood transfusions
  • Horses that live in close proximity to areas where EIA outbreaks have previously occurred
  • Keeping horses at boarding facilities that frequently get new horses, especially if negative Coggins certificates are not required.
  • Attending horse shows, sales or other events which do not strictly enforce and verify whether horses participating have a negative Coggins test.