Equine Infectious Anemia (EIA)

The incubation period is usually 1-3 weeks but can reach 3 months, and many infections stay clinically inapparent. Acute episodes bring fever, reduced appetite and severe anaemia, sometimes with jaundice, rapid breathing and heart rate, nosebleeds, bloody faeces or sudden death. Recurrent fever, weight loss, low platelet counts and swelling of the lower body and limbs characterise the disease. Episodes become less frequent and milder over about a year, leaving a chronic carrier with no signs. At necropsy of acute cases, lymph nodes, spleen and liver are congested and enlarged.

Equine infectious anaemia (EIA) is caused by equine infectious anaemia virus, a lentivirus in the family Retroviridae, related to the feline and human immunodeficiency viruses. The virus is blood-borne and establishes lifelong infection. Tissue macrophages are its main replication site and reservoir: replication is extensive during acute disease and heavily restricted during subclinical phases, which lets the virus persist. Antigenic variants of the virus are thought to trigger recurrent febrile episodes. Thrombocytopenia during acute disease appears to reflect a primary production deficit rather than simple destruction of cells. In acute cases the lymph nodes, spleen and liver become enlarged and infiltrated with immature lymphocytes and plasma cells, and liver Kupffer cells contain haemosiderin.

Infection is limited to equids and occurs worldwide. Once infected, a horse stays viraemic and infectious for life. Transmission requires transfer of blood or contaminated secretions: in nature mainly through interrupted feeding of horseflies and deerflies moving from a sick horse to a healthy one, and through people via re-used needles and syringes, blood transfusions and contaminated instruments. Infection in the womb can also occur. Horses with clinical signs carry more virus and are a higher transmission risk than inapparent carriers. The virus is not considered a human health risk. In the United States, testing since 1972 cut estimated prevalence from nearly 4% to 0.004%. EIA is WOAH-listed.

Equine viral arteritis and infection with Anaplasma phagocytophilum are named differentials, along with other causes of fever, oedema, anaemia, low platelets or haemorrhages. Clinical signs alone cannot confirm EIA; diagnosis rests on serology, with ELISA screening and a positive result confirmed by the agar gel immunodiffusion (Coggins) test.

Blood (red cells, platelets), macrophages, spleen, liver, lymph nodes, lower limbs and ventral body (oedema)

Regular serological testing (Coggins/ELISA); quarantine or euthanasia of positives, kept at least 200 yards from other equids; fly control; strict hygiene, never re-using needles, syringes or instruments.

No treatment or vaccine. Horses surviving acute attacks become lifelong carriers, usually without signs but infectious to other equids; acute episodes can be fatal.