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.
