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Babesiosis test on the fast track.

Babesiosis Test Is on the Fast Track

Horses around the world--from the polo ponies of Argentina to Russian thoroughbreds--may carry a serious parasitic blood disease called equine babesiosis.

In countries where the disease is endemic, such as Brazil, Argentina, Russia, and Poland, many horses maintain low, persistent levels of infection and are thereby immune to a severe illness.

Infection can be fatal, however, in animals not previously exposed to the parasite, including most American horses, says Donald P. Knowles, of the ARS Animal Disease Research Unit in Pullman, Washington.

More safety for U.S. horses may be on the way, though. A new discovery by Knowles will aid veterinarians in detecting babesiosis more accurately and help check this sometimes deadly disease.

USDA's Animal and Plant Health Inspection Service restricts horses that test positive for babesiosis from entering the United States.

But the current testing method occasionally yields an uncertain result, which led officials to seek an improved test, according to Al Strating, director of science and technology at APHIS. The agency funded the work done by Knowles and his colleagues at Washington State University.

Babesiosis resembles malaria in humans because in both diseases, insects transmit the infectious agent, which attacks and destroys red blood cells. Sick animals become feverish and lethargic and refuse to eat.

Microscopic, single-celled parasites, called Babesia equii, enter the horse through infected, blood-sucking ticks. The parasites then multiply in the bloodstream, forming tiny pear- or ring-shaped bodies, called merozoites, that invade the red cells.

Next, the horses' immune system kicks in and forms antibodies against the merozoites in an attempt to fend off the assault.

By studying the blood of infected horses, Knowles identified specific proteins found on the surface of the merozoites. Certain proteins, he found, were the same, regardless of where the horse originated. This information is important in devising an antibody-based test that would work anywhere in the world.

"One merozoite protein produced an antibody response in nearly 200 horses from 20 different countries," says Knowles.

His colleague, Lance E. Perryman, of Washington State University, made a monoclonal antibody that can accurately detect this protein. The antibodies are produced by a laboratory cell culture of fused mouse spleen and myeloma cells called a hybridoma. These antibodies are being studied as a component of a diagnostic test.

Several companies have expressed interest in the test, says Knowles. APHIS' National Veterinary Services Laboratory in Ames, Iowa, which screens all blood samples from imported animals, would be the main users of the test, he adds.

The NVSL often runs a courtesy test, which allows importers to have an animal's antibody level checked before it's shipped, says Wayne M. Frerichs, chief of diagnostics at the bacteriology lab at NVSL.

Horses are then re-tested when they actually arrive at one of the quarantine facilities in the United States, in Miami, New York, Los Angeles, and Honolulu. Last year, over 45,000 horses were tested for babesiosis. Only 2 or 3 percent were denied entry because of a positive test, according to Frerichs.

A very small number of tests give inconclusive results and must be repeated, using a more expensive, more time-consuming test.

The monoclonal antibody test may eliminate those problems, says Knowles.

Because previously exposed horses exhibit a strong antibody response to the protein, Knowles says it may also be a good candidate for producing a vaccine against the disease.

He is currently using recombinant DNA techniques to produce enough of the protein to use in vaccine trials, which may begin this summer.

Race horses make up the bulk of the international horse commerce. Horse fanciers also want to import exotic horses, such as the German Trichanor, for breeding purposes. Other foreign breeds have desirable qualities for varied uses, from barrel racing and trail riding to draft work.

Horses may also travel for international competition, as in polo games or equestrian competition. About 350 horses came to Los Angeles, for example, to compete in the 1984 Games.

PHOTO : Most American horses can become targets of equine babesiosis, a blood disease caused by the parasite Babesia equii. (K-4032-1)

Donald P. Knowles is at the USDA-ARS Animal Disease Research Unit, 337 Bustad Hall, Washington State University, Pullman, WA 99164-7030. Phone (509) 335-6029
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Title Annotation:horse disease
Author:Corliss, Julie
Publication:Agricultural Research
Date:May 1, 1991
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