Although it has become relatively rare, EIA is an insect-borne disease, so it’s hard to protect horses who are constant targets of biting flies. There is no vaccine or treatment.

What if you knew of an equine disease that had no cure, no treatment, and in many cases would require you to destroy your horse if he became infected? You’d probably find that pretty scary, especially these days, when medical science has come so far and eradicated so many diseases.

Well, such a disease does exist, but luckily it’s rare enough that we tend to forget about it. Because the Coggins test has proved so effective, equine infectious anemia (EIA) receives very little attention these days. But it’s still around, and it’s still a killer.

Also known as “swamp fever,” EIA has more recently been called “equine AIDS.” It is caused by infection from a lentivirus, the same family of virus that causes acquired immune deficiency syndrome (AIDS) in people and a variety of chronic diseases in other species. However, the AIDS virus and the EIA virus are not the same. People cannot get AIDS from the horse virus, nor can horses get EIA from the human virus.

When most people hear “virus,” they think of upper respiratory symptoms—at worst pneumonia. This virus is different. It hides inside a type of white blood cell (macrophages) that carries it throughout the horse’s body. Wherever there are macrophages, the virus can be found. Highest concentrations are usually in the lymph nodes and lymphatic system, liver, spleen, kidney and bone marrow, but it can go to any organ, even causing encephalitis in the brain.

Why EIA Is So Serious

There is no vaccine for equine infectious anemia. No effective treatment exists for EIA, so if a horse gets the disease, it is considered infected for life. Large biting flies spread EIA from horse to horse, though passage of the virus in saliva and manure are also possibilities. Horses testing positive for EIA must be destroyed or quarantined for life. An infected horse may or may not show obvious signs of the disease. A Coggins test, followed by two other tests, are used to confirm the diagnosis.

Flies Are the Culprits
Like AIDS, the virus cannot be spread by casual contact. It is usually transmitted via large biting flies, which carry virus-packed blood from an infected horse (see sidebar on page 12) to a neighboring horse. Although transmission through sexual contact has never been documented for EIA, the virus has been known to show up in semen. Passage of virus in saliva and manure is also a possibility. Using the same needle, or dental instruments with blood on them, on different horses is one way that people can spread the infection between horses.

Symptoms appear from one to three weeks after the horse is first infected, but vary greatly between horses. Some may have only a very short period of fever (about 24 hours) and be lethargic, maybe off feed, while others may die from it in as little as two to three weeks (30% or fewer of the cases).

The horse may then have normal periods that alternate with symptomatic periods in approximately two-week cycles. Or, a horse may be symptom-free for a long period of time, until some stress (another infection, shipping, hard exercise, etc.) weakens his immune system and the virus becomes activated. Other horses may never show they have the infection and are called “inapparent carriers.”

The symptoms of EIA infection are only evident when the virus is active. Fever is the first sign that virus is circulating in the body, but is easily missed. The next symptom is usually anemia, which occurs because chemical-signaling molecules become attached to the red cells and trigger the immune cells to engulf, or “eat,” the cells. This causes weakness, depression, poor oxygen delivery, and possible organ damage.

As the disease progresses, problems with clotting appear because the platelets are destroyed. The white cell numbers also start to decline. Eventually, the liver and other organs become damaged. The horse slowly but steadily loses weight, showing swelling (edema) of the belly and legs. If EIA isn’t suspected, the horse will probably be treated with dewormings, dietary changes and antibiotics, but none of these things help. The horse eventually dies, is euthanized, or ends up at a killer auction.

Protecting Your Horse

While researchers are working hard to develop a vaccine for EIA, this virus’ ability to change its appearance and hide from the immune system makes it a difficult task. With no treatment available either, protecting your horse boils down to minimizing his chances of exposure: Board your horse at a faciilty that requires a negative Coggins test on all new horses and isolates new horses without a Coggins until they get one. Insist on a negative Coggins test when you buy a horse. Choose shows and rides that require participants’ horses to have a negative Coggins. Insist on a negative Coggins before you breed your mare to a stallion. If you take your horse to ride in areas where he may come in contact with horses whose EIA status is unknown, be sure to use an effective fly repellant.

Coggins Test Crucial
The first step in diagnosis is the Coggins test, a blood test that detects antibodies to the EIA virus. When this is positive, two more specialized and more sensitive tests are done to make sure the diagnosis is correct. Since Dr. Leroy Coggins developed the test in 1970, cases of EIA have dropped dramatically because of the drastic measures governmental agencies have taken with positive horses, generally requiring euthanasia, though sometimes quarantine is allowed.

Spread between horses occurs gradually because the major flies that carry it from horse to horse do not fly over great distances. Therefore, outbreaks on farms rarely turn into a statewide problem, or even spread to neighboring farms, as long as the horses aren’t moved. Movement of infected horses to new areas is how EIA can travel long distances.

There is no treatment for EIA. Once horses become symptomatic, they show a slow but steady wasting away until the disease itself kills them or they are killed. Although there may be exceptions we don’t know about, once a horse is infected with EIA, it is considered infected for life.

This virus, like the AIDS virus and others in the same family, has the ability to constantly change its outer proteins, its “clothes,” and that is how it escapes destruction by the body’s immune system. The serious nature of the infection, the constant threat that horses with active infection pose to other horses, and the inability to treat it are why it is considered so important to try to identify infected horses and remove them from contact with healthy ones.

Controversial Control Measures
Every state has laws regarding mandatory Coggins testing. While the laws vary a bit from state to state, they will usually require testing of horses being shipped and competing in shows or races, as well as horses being sold at public auction. Any horse that tests positive will have to either be destroyed or kept quarantined for the rest of its life because as yet there is no effective vaccine for EIA.

Some people object to inapparent and outwardly healthy horses being treated in this manner. They contend that these horses do not pose a threat and may never pose a threat for their entire lives. The problem is, there’s no way to tell which horses might relapse and which won’t.

It’s understandable that owners of positive horses would feel this way about their animal being destroyed or quarantined. But the bigger picture is about the good of all horses. We have to ask ourselves if we would be willing to have our own horse pastured or stabled with a horse that is carrying EIA. The answer has to be no. It’s simply a risk that makes no sense to take.