Equine Infectious Anemia

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Also Known As

Anemia, Low red blood cells, Red blood cell deficiency


When horseflies or deerflies bite a horse infected with equine infectious anemia, the flies get blood from the infected horse on their mouth parts and then carry the infection on to other horses. In some cases, the initial clinical signs of equine infectious anemia are mild and go unnoticed, but other horses may become seriously ill within one to three weeks.

The initial clinical sign is usually a high fever, up to 105 degrees or more. As the disease develops, the immune system, via antibodies, attacks and destroys the horse's red blood cells leading to anemia.

Vital organs such as bone marrow, liver, heart and kidney may be damaged by accompanying inflammation and secondary infections such as pneumonia may occur because of suppression of the immune system.

Three distinct types of illness make up the progression of equine infectious anemia, but symptoms overlap: The acute illness is characterized by high fever, severe anemia, weakness, swelling of abdomen and legs, a weak pulse and an irregular heart beat. The rapid breakdown and destruction of red blood cells results in severe anemia and death may be the first indication of the disease.

Less serious cases of the illness progress more slowly and are less severe. In the less severe form of the disease similar symptoms occur, but they are not nearly as serious as in the acute form of the  illness. The horse may become jaundiced and an enlarged spleen may be felt with a rectal examination.

The chronic illness is characterized by recurrent fever and anemia. The horse tires easily and is lethargic and depressed.

After the first bout with the disease, most horses suffer recurrent episodes of illness accompanied by fever, loss of appetite and lethargy. A blood test will usually reveal decreased red blood cells and decreased platelets. With recurring bouts, weight loss and edema in the chest and legs will become apparent. EIA infected horses may die from direct effects of the virus or from secondary infections.

Some infected horses never show any signs of the illness. Although a horse can live with the virus, that horse can be a source of infection for other horses. This fact makes testing very important. The Coggins test that checks for EIA antibodies in the horse's blood is often used. Blood samples are sent to a state-approved laboratory. Some states require the test before a horse can be transported or sold.


  • Fever
  • Anemia
  • Depression
  • Swollen legs and belly
  • Lack of appetite
  • Loss of coordination
  • Weakness
  • Pregnant mares may abort their foals


Bites by horse flies and deer flies are the main source of equine infectious anemia. Transmission of the disease from the mare to the foal may also occur in-utero. Blood transfusions, contaminated needles and instruments may spread the disease. The virus may also be found in semen and milk.


The current testing program which was started in the 1970's has helped keep the disease in check for a number of years. Unfortunately, asymptomatic carriers in the horse population continue to perpetuate the problem.

Without a vaccine for EIA, it is important that each horse owner take responsibility to make sure all pasture mates and other horses in the area undergo the Coggins test or the EIA ELISA test with a negative result before allowing any contact with other horses.

Although the virus cannot live very long on the horse or deer fly, maybe as little as 15 to 30 minutes, all horses in close proximity need to be tested to make sure they are healthy and are not asymptomatic carriers of the disease.

Other preventative measures include routine health exams and yearly EIA tests, never reusing blood contaminated materials between horses and controlling horse and deer flies during the summer months.


Treatment includes supporting the horse with blood transfusions and good nursing care. The horse should immediately be isolated from other horses and care taken that no cross contamination of equipment occurs. The advice of a veterinarian should be followed regarding feeding, activity level, and general nursing care.

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EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.