By Joanne Meszoly

It's easy to forget today the long shadow that equine infectious anemia (EIA) once cast over the horse world.

A potentially fatal viral disease, EIA was initially identified in France in 1843, and in 1888 the first U.S. case was diagnosed in Wisconsin. An EIA epidemic was reported in Wyoming 13 years later, and in 1947 more than 70 horses at a New Hampshire racetrack died or had to be euthanatized because of EIA. Less publicized but equally tragic were countless cases in which horses died of EIA or had to be put down to stop the spread of the disease.

Once known as equine relapsing fever, EIA is characterized by high body temperature, depression, progressive weakness, weight loss, edema and anemia. Horses with the acute form become extremely ill and sometimes die as a result of massive depletion of red blood cells and the resulting lack of oxygen.

Those with chronic EIA cycle in and out of ill health, repeatedly losing weight and condition for weeks at a time, only to recover and appear normal. Inapparent carriers show no signs of illness but have the virus in their bloodstream. There is no vaccine against EIA and no specific treatment beyond supportive care.

Perhaps the most sinister aspect of EIA is its stealthy mode of transmission. If large bloodsucking insects, such as horseflies and deerflies, are interrupted while feeding on an infected horse, they can transmit the EIA virus when continuing their meal on another animal. The disease also can be transmitted via equipment contaminated with blood or by contact with contaminated surfaces. Horses remain infected for life.

The hard facts about EIA have never changed, but significant progress has been made in reducing the impact of the disease on America's horse population. The greatest boost came in 1970 when Leroy Coggins, DVM, PhD, of Cornell University developed a method for testing blood for EIA antibodies.

The Coggins test, as it came to be called, made it possible to identify infected horses even if they showed no signs of illness. It was quickly embraced by states and municipalities across the country to verify the health of horses crossing state lines, coming into the country, and/or participating in shows, races, rodeos and other events. Specific regulations vary among states, but in most areas horses who test positive for EIA infection must be euthanatized or placed in permanent quarantine at approved facilities.

These measures have been strikingly effective. Each year, according to the U.S. Department of Agriculture, the number of horses tested for EIA increases, yet the number of horses found to be infected declines. In 1986, for example, more than 770,000 equine blood samples were tested and 3,368 EIA-positive horses identified. By comparison, in 2004 more than two million horses were tested and just 333 EIA-positive horses found.

As heartening as these statistics sound, experts emphasize that they don't tell the whole story. One is the geographic reality of EIA: The disease is increasingly rare in northern states, where chilly autumn and cold winter weather restrict fly activity, but it continues to be a problem farther south, where mild weather allows flies to feed and breed year-round.

In addition, the statistics are skewed by the fact that equine athletes, breeding animals and the like travel often and are tested repeatedly, always coming up negative, while other horses are never tested because they do not go to shows or venture far from home. These untested animals, experts say, probably serve as reservoirs of the EIA virus, providing the base from which epidemics can emerge.

All of this suggests that EIA will always lurk as a potential health threat for America's horses. Now, however, a group of veterinarians, researchers and government officials is calling for more aggressive efforts to seek out and eliminate EIA's safe harbors in the equine population. The EIA subcommittee of the U.S. Animal Health Association's Committee on Infectious Diseases of Horses has proposed a program to identify risk according to region and target testing for maximal impact. If implemented, they say, the program could reduce the number of infected horses, save millions of dollars and ultimately bring the United States a step closer to eliminating EIA.

Plan of Action
"If you look at the testing numbers, incidence rates and money spent, we could be doing this in a much more sensible way," says Charles J. Issel, DVM, PhD, of the University of Kentucky's Gluck Equine Research Center. Given that each Coggins test costs about $25, he estimates that horse owners spend $50 million each year on EIA testing. "That averages out to $150,000 to identify each positive horse," says Issel, who has studied EIA for 30 years.