EIA: What's the Big Deal?

Infected horses, like this one at FRIENDS Horse Rescue & Sanctuary, often show no outward signs of disease, but they must be branded as EIA-positive and quarantined for life if not euthanized.

Photo: Courtesy Debbie Beye-Barwick/FRIENDS

Find out where veterinarians are seeing an uptick in equine infectious anemia cases

Each year, like clockwork, our veterinarians take blood samples from our horses during spring health exams. We sign the forms, and off the tubes go to the lab for the Coggins test. We do this year in and year out, with no positives and no questions asked. So why do we keep up with this annual horse care ritual? 

The Coggins test checks for antibodies against the equine infectious anemia virus (EIAV). This virus is significant because much like the human immunodeficiency virus (HIV), its lentiviral cousin, there is no vaccine and no cure. A horse diagnosed positive for equine infectious anemia (EIA) dies, is euthanized, or gets placed under extremely strict quarantine conditions (at least 200 yards away from other equids) for the rest of his life.

How Do Horses Get It?

The EIA virus passes from one horse to another via blood. Biting flies, such as horseflies, deerflies, and stable flies, can transmit it after feeding on an infected horse. 

“Unlike flaviviruses such as West Nile virus or Zika virus, EIAV does not replicate in insect tissues, and so the infective dose is limited by the amount of blood that can be carried on the fly’s large mouthparts,” says Frank Cook, PhD, a professor at the University of Kentucky, in Lexington. “EIAV survives for less than four hours on fly mouthparts.

If something interrupts a horsefly’s meal, then the probability of it biting another host is inversely proportional to its distance from the next target. “The further away a potential new host is, the more likely the fly is to return to its original victim,” he says. “It was discovered that if a new horse host was tethered at distances of 50 yards or more, it was virtually (although not entirely) immune from attack, as the fly would almost invariably return to the original host.”

This article continues in the November 2017 issue of The Horse: Your Guide To Equine Health Care. Subscribe now and get an immediate download of this issue including this in-depth feature on what the EIA virus does and where veterinarians are seeing an uptick in cases.

Already a magazine subscriber? Digital subscribers can access their November issue here. Domestic print subscribers who have not received their copy should email circulation@thehorse.com.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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