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.
To reduce cost and focus efforts, Issel and his colleagues on the EIA subcommittee propose a program based on the following framework:
Establish risk zones using an equine census and reports of incidents from previous testing records and statistics. A detailed census is not yet available, but "we could use prevalence estimates by looking at the number of positive cases and comparing that to the number of tests done to statistically determine some degree of expectancy," says Steve Halstead, DVM, chair of the EIA subcommittee. With this information, states could be grouped into regions ranked from A to C, with "A" representing the lowest risk of exposure to EIA and "C" the highest.
Create a testing structure according to risk level. Guidelines that use the risk rankings would mandate annual testing of horses in regions where EIA is most prevalent while allowing less frequent tests in areas where the disease is rare.
"There are areas of the country where testing has occurred in a high proportion at regular intervals where the chance of finding a test-positive horse approaches zero," explains Issel. "In those areas, testing every second or third year will not increase the risk of acquiring EIA." For example, he says, New Jersey already has altered regulations to require EIA testing every other year. Despite a high testing rate, the state has been EIA-free for five years.
Adapt testing requirements according to movement among regions. A significant challenge in controlling any equine disease is the testing and tracking of horses transported from one region to another. The committee proposes adapting regulations so that testing is based on the likely EIA risk a horse poses. For example, a horse traveling from a low- to a high-risk region would not need to be tested for EIA. However, when the same horse returns to a low-risk area, he would have to be retested and temporarily quarantined to make sure he did not contract EIA while in the high-risk area.
Members of the subcommittee recognize that a quarantine lasting a few weeks would be difficult to impose, especially on horses who compete, race or otherwise travel frequently. To facilitate ease of transport, the group recommends the creation of a program to certify EIA-free facilities, allowing animals to move to and from them without additional testing.
The ultimate goal of the plan is to ease requirements in areas with low numbers of positive horses while stepping up testing in areas more likely to harbor infected animals. The USDA has estimated collective savings for horse owners of $10 million if states agree to regional regulations, using this "test-by-risk" model, says Issel.
As with the current regulations, the new model would face the challenge of reaching out to the owners of horses who have never been tested for EIA. However, some states have used various incentives to increase testing rates, and test rates generally seem to be rising.
Ten years ago, for example, it was estimated that about 10 percent of the country's domestic horses were tested each year for EIA infection. Today, Issel believes about 30 percent of the country's horses are tested annually.
As appealing as the test-by-risk model is both economically and in terms of disease control, the proposed program faces significant obstacles. For one, it would require unprecedented cooperation among states, which would have to implement a standard set of EIA regulatory measures. In the Northeast, for example, where high levels of testing have yielded only a dozen cases in recent years, a cooperative intrastate agreement could allow horses to undergo EIA tests only once every three years, with no additional tests required for animals transported among those 11 states.
Louisiana State Veterinarian Maxwell Lea, DVM, has participated in the EIA subcommittee's deliberations and believes that the test-by-risk program could work, but not without a true census of the equine population. "It's difficult to assess a risk level to a state or region if you don't know how many horses you're dealing with," he says. "I don't think we know yet."
Support from the horse industry as a whole also is crucial, says Halstead, who notes that concern over EIA has waned as diseases such as West0 Nile encephalitis have become more common. "The industry is pretty satisfied with EIA control," he says. "Eradication is possible, but people don't feel as strongly about it as they used to. They don't see it as much and have learned to live with it."
Nonetheless, Halstead says he has heard from many equine organizations interested in establishing uniform measures for EIA monitoring and testing. The current patchwork of state regulations is often difficult and frustrating for owners trying to comply and protect their horses from EIA.
Eradication of the EIA virus may seem like an unrealistic goal, but Halstead believes it is possible with a new approach. "We've spent billions on this disease, but a lot of money is spent scattered about," he says. "It would be much smarter to precisely focus on areas that need testing."
For inspiration, he suggests looking to the swine industry's handling of pseudorabies. An eradication program facilitating cooperation between government and producers began in 1989, and by 2003 all U.S. commercial swine herds were free of pseudorabies.
"They did lots of testing, got the industry involved and eradicated it," Halstead says. "If we have the will to do the same thing with EIA, we would be relieved of the burden and cost of the disease."
This article originally appeared in the November 2006 issue of EQUUS magazine.