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 known as "Swamp fever", is a potentially fatal viral disease affecting horses as well as other members of the equine family (such as donkeys and mules) worldwide. The disease is caused by a lentivirus, which means it contains RNA material that produces DNA, which becomes incorporated into the genetic material of the infected cells. The virus reproduces in the horse's white blood cells that circulate throughout their body. The immune system, via antibodies, may attack and destroy red blood cells, leading to anemia. Inflammation associated with the viral infection may damage vital organs, such as bone marrow, liver, heart and kidney. Secondary infections (e.g. pneumonia) may occur due to subsequent immunosupression. EIAV-infected horses may die from the direct effects of the virus or from secondary infections.

Clinical Signs of EIA

EIA can often be difficult to diagnose since the associated symptoms are nonspecific and can vary widely from horse to horse. Some horses may not show any signs of being infected (these horses are referred to as inapparent carriers). Affected horses will often develop repeated flare-ups of clinical signs during periods of stress or when administered corticosteriods.

How EIA is Diagnosed

It consists of a simple blood test, performed by a number of veterinary diagnostic laboratories. The test works by identifying antibodies in the horse's 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.

How EIA is Transmitted

EIAV is transmitted to horses by:
  • Bites from blood feeding insects, such as horse flies, and deer flies.
  • Blood transfusions
  • Blood-contaminated needles, syringes, surgical instruments, and teeth floats. The virus can live up to 96 hours on needles.
  • From infected mares to foals through the placenta or through her milk.
  • Through semen.
  • Aerosol transmission upon close contact.
Incidences of EIA tend to occur most frequently in warm, wet, swamp-like areas. Once infected, affected animals remain carriers of the virus for life and can be a source of infection for other animals. The incubation period is generally two to four weeks, but may range from one week to three months.

Country Regulations

EIA is a reportable disease in the United States and to the World Organization for Animal Health (OIE). Approximately 22% of the horse population in the US 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. According to USDA regulations, if a horse tests positive for EIAV, the state veterinarian is required to ensure that the horse is either euthanized or required to live the remainder of their lives under strict lifelong quarantine.


Intermittent fever up to 105°F (41°C)
Small hemorrhages under the tongue and eyes
Decreased appetite
Swelling of legs, lower chest and abdomen (edema)
General weakness
Weight loss


  • 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

General 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.