Equine infectious anemia (EIA) is a serious, sometimes fatal, viral disease of horses. It affects all equids, including horses, ponies, donkeys, mules, and zebras. Because there is currently no cure and infected horses remain carriers for life, the United States and other countries have enacted regulations and surveillance measures to prevent the spread of EIA.
Dr. Leyi Wang, a veterinary virologist at the University of Illinois Veterinary Diagnostic Laboratory, explains how horse owners can protect their own animals and the horse population at large through routine testing for EIA—the Coggins test—and good management practices.
How Does EIA Spread?
EIA primarily spreads through blood transfer. The most common route is through large biting flies, such as horseflies or deerflies, that carry infected blood from one horse to another. Other routes of transmission include reuse of contaminated needles or surgical equipment, blood transfusions from infected horses, and transmission from mare to foal.
“EIA is not spread through casual contact, like sharing water buckets, grooming, or being in the same pasture,” says Dr. Wang.
Clinical Signs
EIA cannot necessarily be detected based on clinical signs because it presents in three forms: acute, chronic, and asymptomatic.
Horses with the acute form show the most severe signs and are at highest risk of dying. “Clinical signs in this form include fever, decreased appetite, weakness, rapid breathing and heart rate, swollen limbs, bleeding from the nose or bruising on gums, blood in the feces, and sudden death,” notes Dr. Wang.
Horses with the chronic form may alternate between recurrent episodes of acute illness and periods when they appear normal or healthy.
Some horses infected with EIA exhibit no symptoms at all but still carry the virus, meaning they can become a source of infection for others. Testing to detect these horses plays a crucial role in stopping the spread of EIA.
Testing for EIA
The standard test for EIA in the United States, the Coggins test, can identify the presence of antibodies for EIA virus in the blood. Documentation of a negative Coggins test is a yearly requirement for all horses before they are sold, enter boarding facilities, travel between states, or enter competitions.
A Coggins test requires a veterinarian to collect the blood sample and send it to an accredited diagnostic laboratory, such as the University of Illinois Veterinary Diagnostic Laboratory. Dr. Wang’s lab has performed more than 13,000 Coggins tests in the past two years.
A negative Coggins test indicates that there are no detectable antibodies at the time of testing. A positive Coggins test indicates the horse is infected and is a carrier of the virus. Following a positive Coggins test, the sample undergoes a second test to confirm that the result was not a false positive.
Next Steps for a Positive Coggins Test
EIA is a reportable animal disease in all states. This means that positive results must be reported to state and federal animal health authorities. Because EIA has no cure or approved treatment, a positive horse is considered permanently infected.
Regulations are in place to prevent disease spread and require either permanent quarantine of the infected horse (at least 200 yards away from other equids) or humane euthanasia by a veterinarian. These strategies and ongoing testing for the disease have effectively reduced the number of new cases of EIA in the United States.
What Horse Owners Can Do
Prevention is the best way to protect horses. Routine Coggins testing is recommended even for horses that do not engage in travel, boarding, or other activities requiring the test.
“Routine screening for EIA is critical for managing transmission,” says Dr. Wang, “because asymptomatic carriers remain lifelong reservoirs for the virus.”
Fly control is another important preventive measure, since biting flies spread the virus. Effective fly control methods include using fly sheets, applying fly repellents, removing standing water, using fans in barns, and practicing good manure management.
(Fly masks, like the one worn by the horse at the top of the page, can reduce the risk of EIA transmission by limiting access of biting flies to a horse’s face.)
Additionally, never reusing needles between horses, ensuring that surgical and dental equipment are properly sterilized, and purchasing only horses with a recent negative Coggins test are good biosecurity measures to promote horse health.
Thanks to routine testing and responsible horse ownership, EIA is now rare, but continued vigilance is essential. The Illinois Department of Agriculture offers resources for veterinarians and horse owners about testing and current recommendations regarding this serious but preventable disease.
If you have questions about your horse’s testing status or biosecurity practices, please speak with your veterinarian.
By Sara Streit
