Equine Infectious Anemia:
A Review for the Horse Owner

Equine Infectious Anemia (EIA) is a viral infection spread by biting insects, primarily horse-, deer-, and stable-flies. The horse owner also knows the disease as "Swamp Fever."

The virus destroys red blood cells and causes a decrease in their production in the bone marrow--both result in anemia. Infected horses can die acutely, be chronically affected and severely debilitated, or be symptom free for their lifetimes. Horses that become infected remain so for the rest of their lives. In addition to the effects on the horse, the disease is devastating to the owner, because infected animals must be quarantined from other horses or destroyed. There is no treatment, cure, or vaccine for EIA.

The virus that causes EIA is called a retrovirus. Retroviruses known to infect other animals cause bovine leukemia in cattle, feline leukemia in cats, caprine arthritis-encephalitis in goats, Maeda-Visna in sheep, and, more recently discovered, AIDS in humans. It is important to understand none of these viruses normally can infect animals of different species. Infected horses are the only source of infection of EIA to other horses; deer and cattle moving through or near horses do not spread the disease.

EIA can manifest itself in three ways: acute, chronic, and inapparent. The acute form occurs when a horse is first exposed to the virus. There is fever and possibly hemorrhages 7 to 30 days after exposure. During the early stages of acute EIA, the horse can still have a negative Coggins test, because there hasn't been sufficient time for the horse's body to mount a defense to the virus. Anemia isn't usually associated with the disease at this time.

The chronically infected horse has recurring acute clinical bouts of the disease. This is the more classic form of EIA, with weight loss, anemia, and swelling. The recurrent episodes of illness in chronically infected horses can last for only a few days or for months. Commonly there is some history of stress (e.g., hard work, surgery, pregnancy, hot weather, another disease, or a steriod injection) before an acute flare-up of EIA in a chronically infected horse.

In the inapparent form infected horse is not sick and has never shown clinical signs of EIA but is still positive on the Coggins test. These horses also can change to the subacute stage with intermittent episodes of the acute illness at any time. The stresses listed above could influence the inapparent carrier to become a subacute case. Because of the damage caused by the virus in the horse's body, it is unlikely these horses will remain normal for their lives.

Spread of the disease is mechanical transmission by insects, primarily horseflies and deerflies. Mechanical transmission means these spread the virus, but it does not reproduce in them. These large flies cause a painful bite that makes the horse switch its tail, kick, or twitch its skin; this reaction of the horse interrupts the feeding of the flies. If the flies go to another horse to feed, they can spread the virus. Because of their size, these flies transmit a fairly large amount of blood from one horse to another.

Mosquitoes and ticks, which also feed on blood, do not appear to have a role in the transmission of EIA. The horse doens't detect the bite of these insects and, therefore, doesn't interrupt their feeding. Mosquitoes may also have inhibitors (in their secretions) that will inactivate the EIA virus.

Another way the virus can be spread is the use of any instrument that causes bleeding in a horse. Any needle, syringe, or instrument used on an infected horse and not disinfected before use on an uninfected horse can spread the virus.

The EIA virus reproduces in the liver, spleen, kidneys, heart, cardiovascular system, and other organs in the horse's body. When an infected horse dies or is destroyed, lesions can also be found in the eyes, lymph nodes, and even in the nervous system.

Infected horses typically develop clinical signs anywhere from 7 to 45 days after exposure to the virus. Experimentally infected horses develop antibody responses that are detectable 16 to 42 days following inoculation with the virus. Thus, it is possible that a horse can be infected with the virus, yet still test negative for a period of time.

Clinical signs associated with EIA are not characteristic for only E IA. Symptoms include intermittent fever, anemia, depression, and chronic weight loss. Horses can exhibit a number of additional nonspecific signs, e.g., unthriftiness, rough hair coat, colic, nervous system disorders, and abortion. The above signs can be associated with other diseases and conditions such as liver disease, abdominal abscesses, chronic pneumonia, tumors, equine viral arteritis, and parasites. In fact, because of the many tissues the virus damages, the initial presenting signs can be primarily those of the tissue most affected at that time.

Foals born to EIA-infected mares may or may not be infected with the virus. Transmission to the foal can occur while the foal is in the uterus or the virus can be in the milk of the mare even though the mare shows no symptoms. Transmission during pregnancy is probably more likely if the mare has an acute case of EIA.

Foals from positive mares can be positive, but not infected with the virus because of the maternal antibodies they absorb in the colostrum when they nurse for the first time. Positive foals are isolated from other horses and rechecked when they are 6 to 9 months old. This gives sufficient time for the maternal antibodies to be replaced with the foal's own antibodies and, thus, a previously positive foal that is negative at the nine-month recheck will not have EIA. A previously positive foal that is positive on recheck will have EIA. The maternal antibodies passed to the foal in the colostrum will not protect the foal from infection with the EIA virus.

It is possible to obtain negative foals from positive mares and stallions; however, the virus is present in the semen of infected stallions. There is a report of a mare with a vaginal injury that is believed to have been infected by the semen of an infected stallion. The virus is present in all body fluids so, be careful to avoid contamination with any body fluid of a horse.

One current test for EIA is the Coggins test. It is an agar gel immunodiffusion test (abbreviated AGID) created in the early 1970's by Dr. Leroy Coggins, a professor at North Carolina State University College of Veterinary Medicine. The Coggins test checks for the presence of antibodies to the EIA virus in the blood of horses. Until this test was developed, the only way to determine if a horse were infected was by inoculating a known healthy horse with blood from a suspect horse and waiting to see if the healthy horse developed signs compatible with those of EIA. This method is called the horse inoculation test.

Another test for EIA is an ELISA test -- Enzyme Linked Immunosorbant Assay. This test, however, may not be recognized by all states. Horse owners should check with their veterinarians for current test regulations before moving a horse from one state to another.

Should a horse be identified as positive, the owner has three options. First, the horse can be humanely destroyed; second, it can be sold for slaughter; or third, the horse can be quarantined. The horse is quarantined by confining it to either a double screened stall or by keeping the horse a minimum of 200 yards from any other horses that do not already have the disease.

Control of EIA is possible by frequent testing of horses and restricting the movements of those that are positive on either the Coggins or ELISA test. There is no Federal program of control, although Federal regulations prohibit interstate transfer of positive horses. Primary control efforts are dictated by each state, and there is some variation in state regulations regarding EIA. The disease is reportable to the State Veterinarian. In Mississippi all positive reactors are freeze-branded in large letters and numbers on the left side of the neck.

Owners whose horses are reported to be positive can request a retest, but the retest must be com pleted within 15 days after notification of the results of the initial test. Errors are possible when the test is performed, but they are due to misreading the test or mishandling the blood sample.

Testing followed by elimination of those horses that are positive could eradicate the disease; EIA has been eradicated in Hong Kong by this method. This approach, however, lacks support by the horse-owning public and much of the horse industry in this country.

EIA is more prevalent in southeast United States because of the warm climate that results in a longer period of activity of the insect vectors. Horse owners should test their horses at least once a year.

Prepared by J. Patrick Gillis, D.V.M. MS, Assistant Professor, Equine Medicine and Surgery, College of Veterinary Medicine, Mississippi State University, and Clyde E. Taylor, D.V.M., M.P.H., A.C.V.P.M., Professor, College of Veterinary Medicine and Leader, Extension Veterinary Medicine.

Mississippi State University does not discriminate on the basis of race, color, religion, national origin, sex, age, handicap, or veteran status.

Information Sheet 1422
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

Copyright by Mississippi State University. All rights reserved.

This document may be copied and distributed for nonprofit educational purposes provided that credit is given to the Mississippi State University Extension Service.

Visit: DAFVM || USDA
Search our Site || Need more information about this subject?
Last Modified: Thursday, 12-Dec-02 09:54:34
URL: http://msucares.com/pubs/infosheets/is1422.htm
Mississippi State University is an equal opportunity institution.
Recommendations on this web site do not endorse any commercial products or trade names.