Most horse owners have heard of a Coggins test. Many have them performed as part of their horse’s annual wellness programs. What disease is this test looking for, and how important is it for your horse?

EIAV is a highly contagious, potentially fatal disease of horses. Dr. Leroy Coggins was a veterinary researcher who developed the test for Equine Infectious Anemia virus (EIA) in the 1970s. Although it is relatively uncommon in the United States, more than 500 new cases are identified each year. There is no vaccine or cure for EIA and infected animals remain contagious for life. Due to the danger to other horses, this is a reportable disease with federal and state regulations in place for infected horses. As a part of these regulations, a negative Coggins test is required for movement of horses off their home farms – especially to areas where other horses will be congregating (i.e. shows, competitions, sales, etc.). In most cases this test is only required on an annual basis, but there may be special circumstances, such as interstate travel, that require more frequent testing.

Equine Infectious Anemia is a virus that reproduces in white blood cells, which are one of the body’s main defense mechanisms against disease. This virus can cause the immune system to attack the body’s red blood cells, destroying them and causing a potentially fatal anemia. In addition, inflammation caused by the virus can lead to damage of the bone marrow, liver, heart, and kidney. Suppression of the immune system can often lead to secondary bacterial infections such as pneumonia. These secondary infections can also be fatal due to immunosupression caused by EIA.

There are three distinct forms of EIA in horses:

  1. The Acute Form – The most serious form of EIA for an individual horse, this can be difficult to diagnose. Occurring within 1-4 weeks after exposure to the virus, antibodies may not be measurable in the blood stream and anemia is often not yet present. The virus is very active in the body, however, and is aggressively multiplying and having devastating effects within the body.
  2. The Chronic Form – This form of EIA is seen in those horses that survive the acute form. Classic clinical signs of this form include fever, depression, weight loss, anemia, and tiny (petechial) hemorrhages on the mucous membranes. During periods of stress, or with the administration of steroids, it is common to see intermittent relapses.
  3. The Inapparent Form – After a period of time (usually within a year of initial infection), it is possible for a horse’s system to control an EIA infection. These horses will show no clinical signs of illness, however they remain carriers of the disease, and continue to pose a threat to previously uninfected animals. It is for this reason that current requirements for known positive horses are either euthanasia or lifelong quarantine.

Transmission of EIA is via blood or placental transfer. Bloodsucking insects (horse flies, deer flies, mosquitoes) are the most common mode of transmission. Blood transfusions, as well as contaminated needles and instruments, are also possible means of EIA spread. The virus also exists in both semen and milk. Also known as “swamp fever,” the states with the confirmed cases in 2012 included: California, Texas, Louisiana, Mississippi, Alabama, Florida, Tennessee, Illinois, Indiana, Ohio, and Michigan.1


  1. USDA, Animal & Plant Health Inspection Service, Equine Infectious Anemia Distribution Maps. http://www.aphis.usda.gov/vs/nahss/equine/eia/eia_distribution_maps.htm