Equine infectious anaemia (EIA) is a persistent viral infection of equids. The causative agent, EIA virus (EIAV) is a lentivirus in the family Retroviridae, subfamily Orthoretrovirinae. EIA occurs world-wide. The infection, formerly known as swamp fever, is limited to equids. Many cases remain clinically unapparent. The disease is characterised by recurrent febrile episodes, thrombocytopenia, anaemia, rapid loss of weight and oedema of the lower parts of the body. If death does not result from one of the acute clinical attacks, a chronic stage develops and the infection tends to become inapparent. Infected horses remain viraemic carriers for life and, with very rare exceptions, yield a positive serological test result. Although antibody levels fluctuate, EIAV infection generates a persistent antibody response. All equids older than 12 months that test seropositive are identified as virus carriers. As virus reservoirs, infected equids are a transmission risk to other equids. The virus is primarily blood-borne. Biting flies are mechanical vectors for the virus in nature and infection is often spread via iatrogenic routes. There are no vaccines currently available. Further, the strategy for EIA control has shifted from vaccination to quarantine to avoid the interference of vaccinal antibodies with diagnostic tests. EIAV is not considered a risk for human health.