Equine Infectious Anemia

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Also known as: EIA, Swamp Fever


Equine Infectious Anaemia (EIA) is an exotic viral disease affecting horses, donkeys and mules characterised by viraemia, anaemia and thrombocytopaenia. The disease is caused by Equine Infectious Anaemia Virus (EIAV), an equid-specific lentivirus in the retrovirus family that is closely to related to HIV in humans. Cases of EIA in the UK are extremely rare and have previously occurred in horses that have travelled abroad or been imported from areas of endemic disease. In the UK the disease is notifiable and confirmed cases must be humanely destroyed.


All equids are susceptible to EIA but donkeys and mules appear to be less severely affected. No breed, age or sex predilection has been reported.


EIA occurs worldwide but most commonly in countries with warm climates. The virus is usually transmitted via mechanical inoculation of blood via large biting flies such as horseflies (Tabanid spp) or stable flies (Stomoxys calcitrans). Additionally, transmission of the virus may occur through saliva, milk, body secretions or via contaminated needles and blood products. The virus may also be passed from pregnant mares to their foals across the placenta.

Viral replication occurs primarily in mature tissue macrophages located in the spleen. These macrophages act as the main source of high-titre viraemia during the course of the disease and also as a reservoir for infection during subclinical infection. Infection in subclinically affected horses may be reactivated during times of stress, disease or during treatment with immunosuppressive drugs.

Clinical signs

The incubation period of the disease ranges from 10 to over 45 days and the clinical presentation is highly variable. EIA occurs in acute and subacute forms in susceptible animals but more commonly assumes a chronic course. However, a large number of affected horses do not display any clinical signs and unapparent carriers may be clinically normal.

In the acute stages, clinical signs may include mucosal petechial and ecchymotic haemorrhages, depression, lymphadenopathy, fever, lethargy and inappetance. In rare cases the disease may result in sudden death. Horses that have been infected for thirty days may display the characteristic signs of EIA including ventral and limb oedema, anemia, bloody diarrhoea, icterus, pyrexia and cachexia.


The 'gold standard' for diagnostic confirmation is an agar gel immunodiffusion test (the Coggins test) which detects serum antibodies against the EIA virus. The test is 95% accurate for the diagnosis of EIA but false positives may be obtained from foals that have absorbed colostrum from affected mares and false negatives may occur during the acute stages of EIA. The Coggins test may be performed in combination with an ELISA or PCR test in order to increase the test sensitivity.

Additionally, haematology may reveal a moderate to marked anaemia, leukopaenia and bilirubinaemia. Mild thrombocytopaenia is common in the acute phase of infection.


No specific treatment or vaccine for EIA is available and strict procedures for affected horses are enforced in the UK. Until the presence of disease has been confirmed by a positive Coggins test, supportive treatment may be provided such as NSAIDs to reduce pyrexia and inflammation. Any horse that is suspected to be infected with EIA must be reported to the appropriate divisional veterinary manager of DEFRA. DEFRA policy is that horses with confirmed EIA must be slaughtered under Section 32 of the Animal Health Act 1981. Horses that have been in close contact with diseased animals must be kept in isolation for 90 days and tested on a monthly basis with regular veterinary assessment. After this period of testing, restrictions may be lifted if no disease is found.


Although attenuated live vaccines are available in the United States, these are not in current use in the UK. Acutely affected horses carry high levels of virus in the blood and are a high risk source of infection to other horses so should be isolated immediately if EIA is suspected. All movement of horses on and off the premises must be prevented.


Foals exposed to EIAV have a high fatality rate but generally horses recover from the disease, becoming lifelong inapparent carriers. Relapse of the disease may occur during times of stress or illness. The current UK control measures dictate that confirmed cases of EIA carry a grave prognosis.

Equine Infectious Anemia Learning Resources
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Equine Infectious Anaemia publications since 2000


  • Dwight, C., Hirsh, Y., Zee, Y. C. (1999) Veterinary Microbiology Wiley-Blackwell
  • Higgins, A., Snyder, J. R. (2006) The Equine Manual Elsevier Health Sciences
  • Jain, N. C. (1993) Essentials of Veterinary Haematology Wiley-Blackwell
  • Lavoie, J. P., Hinchcliff, K. W. (2009) Blackwell's Five-Minute Veterinary Consult: Equine John Wiley and Sons

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Equine Infectious Anemia Learning Resources
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