Taking Precautions to Prevent Infection Transmission of EIA Virus Between Horses 

By Leslie Barlow, EDCC Communications Manager 

The recent news of an equine infectious anemia (EIA) outbreak among a string of Quarter Horse racehorses in California, Texas, and New Mexico has highlighted the importance of using sterilized medical equipment and biosecure injection techniques when treating horses and the need for routine testing of the virus. 

Equine infectious anemia (EIA) is a potentially fatal blood-borne infectious viral disease that produces a persistent infection among equids nearly worldwide. It is known to be naturally spread from affected horses by certain species of biting flies which transfer the virus in blood from an infected horse. 

EIA can also be transmitted iatrogenically (caused by humans through medical procedures) via medical equipment such as needles, syringes, IV tubing, or other equipment contaminated with blood or through transfusion of blood or blood products from an infected horse. Transmission from the mare to foal in utero has also been documented and the EIA virus has also been isolated from semen of infected stallions. Infection does not occur with horse-to-horse contact unless there is an exchange of blood. 

The United States Department of Agriculture’s Animal and Plant Health Inspection Services determined the Quarter Horses in the care of one trainer had contracted the virus through iatrogenic transmission. 

While previously naturally transmitted by biting flies in untested or under-tested hers were the main cause of cases in the U.S., in recent years the epidemiology of transmission has shifted and now most EIA cases reported each year are now due to iatrogenic transmission of the virus in Quarter Horse racehorses. Many of these cases have ties to unsanctioned racing with reuse of blood contaminated equipment between horses and some of the cases have involved horses illegally moved from Mexico where the disease is widespread and endemic. Infected horses from these high-risk populations are often transported to various locations where they are in close proximity with other horses where they may serve as a source of continued transmission. 

The incubation period of horses infected with EIA is usually 15 to 45 days and horses that have been infected with the virus are lifelong carriers. Horses showing clinical signs of the disease are more of a threat to healthy populations because of higher levels of virus circulating in the blood.

Horses that survive the initial clinical phase of the disease usually become outwardly inapparent carriers and remain a life-long reservoir for the disease. There is no treatment or cure for EIA virus infection. Horses are usually euthanized if confirmed positive for the disease to prevent ongoing transmission to other horses. If not euthanized, horses must be permanently quarantined with separation from non-infected horses by at least 200 yards to prevent spread of the disease by biting flies. Infected horses cannot be moved from quarantined premises except by approval of state and federal animal health officials. 

Clinical signs of EIA may include fever, depression, low platelet count, anemia, red or purple spots on the mucous membranes, edema, muscle weakness and atrophy, although chronically infected horses often show no obvious clinical signs of disease. 

EIA is diagnosed by testing antibody levels in the blood. The most common test used is the Coggins Test, which is an agar gel immunodusion (AGID) test. ELISA (enzyme-linked immunosorbent assay) test is also available to detect EIA antibodies. Veterinarians must collect and submit blood to certified laboratories for testing. 

There is no vaccine available for EIA. Methods of prevention include vector control (insect control) to reduce the possibility of transmission by biting flies and ensuring that needles, syringes, and intravenous administration sets are not reused unless sterilized. Any other medical equipment which can be contaminated with blood such as dental equipment or stomach tubes should never be shared between potentially affected horses.  Sterile technique must be used when administering intravenous drugs to avoid introducing contaminated blood into a multi-dose vial of equine drugs. Blood transfusion between horses should only be conducted by veterinarians using EIA-negative blood donor horses.   

Routine EIA testing should be performed to determine if horses have contracted the virus and thereby preventing transmission of the virus to other horses. Many equine events and boarding facilities require proof of a negative Coggins Test within the previous 12 months to enter the facility. States require a negative EIA test for interstate movement and some states have an EIA test required for change of ownership. 

The EDCC reports all cases submitted by state animal health officials in the United States and Canada.