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CORVALLIS - The horses of Oregon and the Pacific Northwest actually face more imminent risks from the expected arrival of West Nile Virus than the human residents of the region, experts say, and horse owners are now being advised to have their animals vaccinated immediately.

As a sometimes-fatal disease that naturally infects at least 111 species of birds, West Nile Virus causes infections mostly in just two other groups - humans, and to a much greater degree, horses.

"Aside from birds, horses and other equines are the animal species most prone to infection and serious complications from West Nile Virus," said Dr. Donald Mattson, a virologist at the Oregon State University College of Veterinary Medicine.

Mattson and a group of veterinary experts in the college, along with Dr. Andrew Clark, the state veterinarian, are now recommending the immediate vaccination of all horses in the state.

"Fortunately, an effective vaccine also exists for horses," Mattson said. "We advise its use, since we now believe it's possible that West Nile Virus could be in Oregon soon, conceivably within a month or so, and it takes up to five weeks for the two-part horse vaccine to reach full effectiveness."

Even if the virus does not arrive in Oregon until next summer, Mattson said, horse owners could have their animals vaccinated now and then get a simple booster shot next spring to protect them. The vaccine is issued under a "conditional license" by the U.S. Department of Agriculture, which means that it has been proven to be safe, pure, and has a reasonable expectation of being effective. It has been approved for use in Oregon by the state veterinarian at the Oregon Department of Agriculture and is now available to horse owners through local veterinarians.

Of the 150,000 to 200,000 horses in Oregon, only 1-2 percent have been vaccinated against West Nile Virus.

Clinical illness in horses can manifest itself as ataxia, difficulty walking, knuckling over, head tilt, muscle tremors, inability to stand, or death caused by encephalitis.

Horses vaccinated against eastern equine encephalitis, western equine enchephalitis and Venezuelan equine encephalitis are not protected against West Nile Virus.

The closest spread of the virus to Oregon, in fact, was in a documented case in Wyoming this month in which a horse died of enchephalitis. Because many horses may get infected and recover with few signs of illness, it's not clear exactly what the incidence of infection is, or mortality rate, in horses which become infected, Mattson said.

More than 700 horses in the U.S. were officially determined to have been infected with West Nile virus in 2001 and about 30 percent died. Other mammals such as dogs, pigs, llamas, alpacas and rabbits appear to be resistant to infection and show no signs of illness or diseases.

The overall risk to horses appears sufficiently high that vaccination is warranted, Mattson said.