Equine infectious anemia (EIA) is a retroviral disease caused by a lentivirus in the family Retroviridae, closely related to HIV in its mechanism of persistence. The virus infects macrophages and monocytes, integrates permanently into the host genome, and is never cleared — an infected horse carries EIA for life. The disease is sometimes called swamp fever or horse malaria, names that reflect historical descriptions of the intermittent fever that marks acute episodes.
Transmission occurs primarily via blood transfer: large biting insects (horse flies and deer flies of the family Tabanidae) mechanically carry virus on mouthparts interrupted mid-meal and moved to another horse. Contaminated needles and surgical instruments are the principal iatrogenic route, a particular concern in historical herd vaccination programs where needles were reused. Vertical transmission from mare to foal across the placenta or in milk occurs at low frequency. EIA is not transmitted by casual contact, shared water, or small insects such as mosquitoes.
Clinical presentation falls into three patterns. Acute EIA produces high fever (up to 41 degrees C), thrombocytopenia, dependent edema, and severe anemia within 30 days of infection; untreated acute cases can be fatal. Chronic EIA follows with recurring febrile episodes every few weeks to months, progressive weight loss, and deteriorating body condition. Inapparent carriers show no clinical signs but remain persistently viremic and are the primary reservoir for transmission in managed herds.
The Coggins test (agar gel immunodiffusion, AGID) detects antibodies against EIA p26 antigen and is the regulatory standard for diagnosis. A positive Coggins result is a reportable finding in all US states and most countries; regulations require either lifelong quarantine at least 200 yards from all other equids, euthanasia, or export under USDA permit. Negative Coggins tests are required for interstate transport, competition entry, and most boarding facilities. A current negative test is a standard document in any horse purchase. There is no vaccine and no cure; control is entirely through testing and movement restriction. See also pre-purchase documentation requirements for why a current negative Coggins test is a non-negotiable item in any sale, and body condition scoring for monitoring the progressive deterioration seen in chronic EIA carriers.
Further Reading: The virology, Coggins test protocol, and regulatory requirements for EIA are detailed on Wikipedia’s Equine infectious anemia entry. Utah State University Extension’s equine health program publishes a practical overview of testing requirements at USU Extension: Equine Infectious Anemia.