FieldDxEquine Infectious Anemia (EIA)Clinical signs, differential diagnosis, treatment and prevention in horses.
Overview
A bloodborne viral disease of equids, ranging from a subclinical carrier state to severe illness and death; no treatment or vaccine exists. Equine infectious anemia virus (renamed Lentivirus equinfane), a retrovirus of the Lentivirus genus.
Clinical signs
- Acute phase (1-3 days): fever, depression and thrombocytopenia.
- Later: recurrent fever, depression, anaemia, jaundice, petechiae on the mucous membranes, nosebleed, dependent oedema, muscle weakness and wasting, raised heart and respiratory rates.
- Peracute cases: very high fever, severe thrombocytopenia, sudden depression, nosebleed and death.
Predisposing factors
- Spread by blood-feeding insects, mainly horse flies and deer flies; stable flies are less efficient. Risk is higher in hot, humid areas with many biting flies.
- Lifelong carriers are the natural reservoir, and febrile horses transmit more readily than inapparent carriers.
- Iatrogenic spread through reused needles, syringes, surgical or dental instruments, IV sets and transfusions.
- Transmission from dam to foal in utero; body fluids and tissues may be infectious, especially during fever.
Look-alikes to rule out
Treatment
- No antiviral treatment or cure exists; confirmed cases are lifelong viral carriers and are usually euthanized.
- The alternative is permanent isolation and quarantine at least 200 yards from other equids, combined with strict insect control and biosecurity (sterile needles, disinfected equipment, licensed blood products).
Prevention
1. Reactor horses must be removed from the herd by euthanasia, slaughter, or quarantine at the premises of origin. 2. They may move interstate only under official permit to a federally inspected slaughter facility or a federally approved diagnostic or research facility, or to return to the premises of origin. 3.
Source
MSD Veterinary Manual — Equine Infectious Anemia (accessed 2026-08-11)
MSD Veterinary Manual — Equine Infectious Anemia (accessed 2026-10-29)
The Merck Veterinary Manual 11th Edition (Page 222)
Condensed and attributed, never reproduced whole. Open the full entry in the app.