Equine infectious anemia is a bloodborne infection that affects horses and other equids. It is caused by a virus. In many infected animals, the signs of illness are minimal; however, equine infectious anemia may also infect many animals in a region simultaneously with severe infections and a high death rate. Infection appears to persist for life.

Blood-feeding insects transfer the virus from an infected animal to nearby uninfected animals while feeding. The most efficient transfer of the infection seems to involve horseflies and deer flies. The reason is simple; when these insects bite, the pain causes the horse to switch its tail and take other defensive behavior that interrupts the insect's feeding and the insect flies off to bite yet another animal and transfer the virus.

There are 2 forms of the disease. One form exists without noticeable signs or with subtle signs (such as fever or lack of appetite) that go unrecognized. Often, infection is noted only after routine surveillance testing for the disease or when the horse develops recurring bouts of fever accompanied by anemia, depression, weight loss, general ill health, malnutrition, and swelling (hallmarks of the longterm form of the disease). This most often occurs in horses on pasture. Thus, the virus frequently enters a herd without the knowledge of the owner and spreads until a high percentage of the herd is infected.

No specific treatment or vaccine is available. If a horse tests positive for equine infectious anemia, euthanasia is often recommended as the most prudent option, albeit a difficult one. Lifelong quarantine in a screened stall is another, less acceptable, alternative. Infected horses will always pose a health risk to other horses, whether or not they show signs of illness. Even in the best management situations, blood-feeding insects cannot be totally controlled or eliminated.

Horses testing positive for equine infectious anemia are often required by law to be permanently identified via branding or tattooing and to be quarantined. Transportation and housing are severely restricted. Owners who choose quarantine must post signs clearly stating the housing of a quarantined animal. As equine infectious anemia-positive horses present the only known source of infection, horses should be quarantined at least 200 meters (220 yards) away from all other animals. A screened enclosure is best.

Last full review/revision July 2011 by Otto M. Radostits, CM, DVM, MSc, DACVIM (Deceased); Delores E. Hill, PhD; Barton W. Rohrbach, VMD, MPH, DACVPM; Charles J. Issel, DVM, PhD; Max J. Appel, DMV, PhD; David A. Ashford, DVM, MPH, DS; Daniela Bedenice, DrVetMed, DACVIM, DACVECC; Farouk M. Hamdy, DVM, MSc, PhD, MPA (Deceased); Kenneth R. Harkin, DVM, DACVIM; Johnny D. Hoskins, DVM, PhD; Eugene D. Janzen, DVM, MVS; Jodie Low Choy, BVMS; John E. Madigan, DVM, MS; Dale A. Moore, MS, DVM, MPVM, PhD; J. Glenn Songer, PhD; Joseph Taboada, DVM, DACVIM; Charles O. Thoen, DVM, PhD; John F. Timoney, MVB, PhD, Dsc, MRCVS; Ian Tizard, BVMS, PhD, DACVM; Brian J. McCluskey, DVM, MS, PhD, DACVPM; Bert E. Stromberg, PhD; Peter J. Timoney, MVB, MS, PhD, FRCVS