Equine Infectious Anemia (EIA) or Swamp Fever is caused by a lentivirus of the family Retroviridae. This virus is transmitted primarily by blood-sucking insects, especially horseflies and deerflies. Transmission may also occur via contaminated syringes, surgical instruments or blood transfusions. Vertical transmission (transmission from mother to offspring) may occur transplacentally or via colostrum and nursing. The only known reservoirs of infection are members of the Equine Family. Virus replication does not occur in the insect vector.

The incubation period of EIA ranges from 10 to 45 days. Disease may be present in one of three clinical syndromes in the horse. The disease may be acute, chronic, or inapparent. An inapparent carrier would be a horse that does not show any clinical signs of disease but is a reservoir of infection from which biting insects may affect other horses. Clinical signs of acute disease include a fever, often up to 106 degrees, anorexia, thrombocytopenia (decrease in platelets numbers resulting in hemorrhaging), and anemia. The horse will act weak, ataxic (off balance) and lethargic. Occasionally you will see epistaxis (nose bleeds), petechia (small hemorrhages on the mucous membranes and skin), ventral edema and even death. Chronic disease is exhibited by less severe clinical signs but usually results in weight loss and a generally unthrifty animal during episodes of high virus replication. A horse with a chronic infection may appear clinically normal between attacks of the virus. Once a horse is infected with the virus they will be infected for life. Horses having the disease one year or longer tend to mount an immune response until they develop the inapparent carrier state.

This virus is worldwide in distribution. Areas plagued with blood-sucking insects will have a higher rate of infection. Transmission is more prevalent in the warmer seasons due to the prevalence of insects.

Infection is detected through the use of serologic testing. The USDA has approved two different serologic tests, the AGID (agar gel immunodiffusion test) or Coggins test, and the C-ELISA (competitive enzyme-linked immunoadsorbent assay). The Coggins test is the test most widely used and is considered to be 95% accurate, occasionally producing false negative results. The C-ELISA test is considered to be more sensitive which may occasionally lead to false positive results. Animals in the viral incubation period will often test negative up to day 45 of infection. Due to the dire consequences of positive testing all horses with positive results should be retested for confirmation of the results. In the United States, ten to twenty percent of the horse population is tested yearly for this virus. It is estimated that less than ½ % of the United States horse population is affected.

There is no effective treatment for this virus. Treatment tends to be supportive. Horses with severe anemia or thrombocytopenia may require transfusions. Cold-water hosing and NSAIDs may be used to decrease temperature. Seropositive horses should be immediately isolated from other equidae or euthanized. Each individual state has regulations regarding interstate travel and control measures in place regarding quarantine of seropositive horses.

Chances of infection with EIA may be decreased by requiring Coggins testing as a part of every prepurchase examination, or before admitting a new horse to a farm. Recent negative EIA test results should be required before sale of an animal and before admission to shows and race tracks. Boarding stables should require yearly testing. Rigorous fly control and sterility of any instruments used in treating horses should be maintained in order to prevent viral transmission.

References:

Brown, Christopher and Bertone, Joseph. The 5-minute Veterinary Consult Equine. 2002. pp. 562-563.
Blood, Henderson, Radostits. Veterinary Medicine. 5th Edition. pp. 598-600.
Kahn, Cynthia. Editor: The Merck Veterinary Manual. 9th Edition. pp. 559-560.