Equine Infectious Anemia

ByPeter J. Timoney, MVB, PhD, FRCVS, Equine Programs, College of Agriculture, Food and Environment, University of Kentucky
Reviewed/Revised May 2019 | Modified Jun 2019

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, but in other instances, many animals in a region can be simultaneously infected and die. Infection appears to persist for life. For these reasons, the disease is highly controlled: whenever horses are transported in the US, a negative Coggins test, which is the test for equine infectious anemia, is required.

Blood-feeding insects transfer the virus from an infected animal to nearby uninfected animals while feeding. The most efficient transfers of the infection involve horseflies, deer flies, and to a lesser extent stable flies. Transfer of the blood-borne virus is enhanced by the insect's bite, which is painful and causes the horse to swish its tail and interrupt the insect’s feeding. The insect flies off to bite yet another animal and transfer the virus.

There are 2 forms of the disease, a chronic (longterm), low-grade illness and an acute, severe illness. The chronic 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, and general ill health. This most often occurs in horses on pasture. The virus frequently enters a herd without the knowledge of the owner and spreads until a high percentage of the herd is infected. The acute, severe form of the disease can cause high fever, severe loss of platelets (needed for blood to clot), depression, and death.

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.

For More Information

Also see our professional content regarding equine infectious anemia.

Test your Knowledge nowTake a Quiz!
Download the free MSD Vet Manual App iOS ANDROID
Download the free MSD Vet Manual App iOS ANDROID
Download the free MSD Vet Manual App iOS ANDROID