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CORVALLIS, Ore. - The number of cases of a rare neurologic equine disease appears to have surged dramatically in the past two years, and it now may pose a significant threat to the health of Oregon's 110,000 horses.

Problems with equine protozoal myeloencephalitis, or EPM, are rapidly increasing around the nation, experts say, and a new survey at Oregon State University showed that 45 percent of Oregon's horses have been exposed.

The actual rate of horses that become ill or die from this disease is still less than 1 percent. But the seriousness of the disease, uncertainty about its continued spread, rising number of crippled animals and huge potential for economic loss has alar med veterinary researchers.

"We don't know for sure how bad this may get," said Linda Blythe, professor and assistant dean of the OSU College of Veterinary Medicine. "But as a neurologist for 18 years I'm amazed at the rapid increase in cases."

In Oregon, the first such case of this difficult-to-diagnose disease was confirmed in 1986. After that, the College of Veterinary Medicine saw about two cases a year. Now they're seeing about one a week, and it's getting worse.

In a surprisingly short time - often just a matter of days - the disease can cause loss of appetite, seizures, muscle atrophy, lameness, partial paralysis, "wobbling" and even blindness. Treatment often must begin even before definitive laboratory test s, done via a spinal tap, are complete.

Without treatment, death or euthanasia is often the result.

Once diagnosed, Blythe said, the disease can be treated with special antibiotics and immune boosters. But it's not cheap or easy, costing at least $600 and sometimes $2,000 or more, and takes three to six months.

"Just in the past few years this has become the number one neurologic disease of horses in the Western Hemisphere," Blythe said. "And Oregon is one of the hot spots, probably because of our moist climate that the parasite thrives in, and our increasing opossum populations."

EPM is caused by a single-celled parasite called Sarcocystis neurona, but only infects horses when they eat pasture grass, hay, grain or water that's contaminated by opossum feces.

Opossums can spread the parasite in their feces. Birds that incidentally eat opossum feces also help to spread the disease. The parasite enters their muscles and, when dead birds are eaten by scavenging opossums, the opossums become infected, thus the cycle continues. Horses are referred to as a "dead end" host - a victim, but not part of the infectious cycle. A horse cannot spread the disease to other horses.

"People can try to keep their hay and animal feed as clean as possible, and avoid contamination by opossums or birds," Blythe said. "But that's very difficult to do and we have no way to test animal feed for the parasite. And, of course, there's no vac cine available to prevent this problem."

The emergence of actual disease following infection, Blythe said, may depend on how much contaminated feces is eaten, environmental stresses, the horse's general immune reaction or other factors not yet fully understood.

And the impacts are quite variable - the EPM parasite gravitates towards central nervous system tissue, such as the brain or spinal cord, and symptoms depend on which part of the nervous system gets infected first. Older horses are more susceptible, as are thoroughbreds, standardbreds, and quarter horses.

Elsewhere in the nation EPM is already causing large economic loss.

"EPM is all they talk about on some Kentucky horse farms," she said, "and in Florida, with any horse that shows abnormal neurologic signs it's now assumed EPM is the problem until proven otherwise."

In past months the College of Veterinary Medicine at OSU has been working aggressively to educate the state's horse owners and veterinarians about the disease, set up treatment protocols, monitor the disease spread, conduct workshops, distribute litera ture and do public presentations.

Blythe was one of 14 neurologists in the nation invited to participate last year in a national task force on problems created by this disease, and Terry Gerros, an OSU assistant professor of veterinary medicine, is participating in a national drug tria l, testing the use of new medications.

The College of Veterinary Medicine recently conducted a sampling of 334 horses in Oregon, the findings of which were just published in the Journal of the American Veterinary Medical Association. It detected an immune response to the EPM parasite in the blood of 45 percent of the animals on a statewide average, and 60 percent or more west of the Cascade Range where the wetter climate leads to large opossum populations.

Treatments are often successful if begun early, although there's a 10 percent relapse rate and some animals may need medication for a year or more.

This disease has never been identified in humans.