The disease behind the Coggins test

Farm Forum

Have you ever taken note of how many horses are traveling these days? During a recent road trip to northern Minnesota I observed a whole lot of horse trailers going up and down the road: some big, some small, some old, and some more expensive than my first house.

This got me thinking back to the horses on our farm growing up. I don’t remember our horses doing much traveling at all. About the only times I remember horses getting on a trailer were when Grandpa would take his team the 20 miles into Groton for a parade, or an even shorter trip into Columbia. Otherwise our ponies, cattle horses, and others pretty much were homebodies.

When I started veterinary practice, my horse clients introduced me to a new world. They were trucking their animals all over the place: trail rides, horse shows, rodeos, cutting horse competitions, you name it. These horses were jet-setters compared to the pasture-bound equines I grew up with. I would write health papers for them and if the horses were crossing state lines, I would draw blood for a Coggins test.

The “Coggins test” is a familiar piece of horse owner jargon. But what exactly does it accomplish?

The Coggins test detects antibodies against a virus called Equine Infectious Anemia Virus (EIA) in the horse’s bloodstream.

(Side note: In the old days, if you invented a test or discovered a new disease, you might get it named after you, like Dr. Coggins! Nowadays the names are more scientifically understandable but more boring [e.g. porcine epidemic diarrhea virus] compared to “Johne’s Disease,” or the “Bang’s vaccine.” Alas, the possibility of there ever being a “Daly test” is probably past…)

A horse catches this virus by a bite from a horsefly or other insect. Any object that could transfer blood from an infected horse to another could spread it too, like needles. Infected horses are the only source for EIA – the insects just move it from one animal to the other.

It’s a very tricky virus once it gets inside the horse. It infects blood cells that move it all over the body, to places such as the spleen, liver, and bone marrow. There it messes with the body’s ability to make new blood cells, henceforth the anemia. EIA is a persistent, lifelong disease once a horse catches it. The signs of disease are not very specific. The horse will have episodes of fever, listlessness, and going off feed. As time goes on, signs like fluid retention, jaundice, and weight loss may appear.

There are no vaccines or treatments for EIA, so it’s understandably a concern for horses and their owners. On the positive side, there is a good test for the disease – the Coggins test. A simple blood sample run at the lab will quickly detect antibodies against the disease, usually before the horse starts showing illness.

The other positive aspect of EIA is that there is not too much of it out there. In 2013, only 38 infected horses were found in the US. While most cases tend to be in the southern states, cases were found in Minnesota and Nebraska, too. South Dakota has not had Coggins positive horses in the past 10 years.

The incidence of EIA has significantly declined over the past decade. Chalk that up to the efforts of horse owners testing their horses and to state officials for quickly dealing with any positive cases. Since horses are the source of the disease and the insects that spread them are everywhere, early detection of infected horses is important in limiting the spread of the disease. The incidence of EIA has dropped to the point where new regulations considering more of a risk-based approach to testing and movement requirements are being proposed.

The fact that cases of EIA are declining doesn’t mean that we can become complacent about the disease, however. You can do your part by knowing the requirements when moving your horse across state lines. Make sure your horses’ Coggins tests are up to date (within the past 6 months for most states). And as always, check with your veterinarian when questions about this and other horse illnesses come up.

Russ Daly, DVM, is the Extension Veterinarian at South Dakota State University. He can be reached via e-mail at or at 605-688-5171.