It's a safe bet West Nile will arrive next summer.

Byline: The Health Files by Tim Christie The Register-Guard

MOST HUMANS DON'T have much to fear from the West Nile virus. Horses, however, are another story.

Aside from birds, horses and other equines are most prone to infection and serious complication from the virus, according to Dr. Donald Mattson, a virologist with the Oregon State University College of Veterinary Medicine.

The virus could get to Oregon as early as this fall and almost certainly by next summer, disease experts say. Veterinarians are urging horse owners to vaccinate their animals.

"It is moving east to west more rapidly than we had thought it would, and it's time now for Oregonians to learn more about this emerging virus and get ready to deal with it," Mattson said.

The virus first appeared in the United States in New York in 1999 and has since spread relentlessly west and south. It's been found in 42 states and the District of Columbia. In the continental United States, the only states the virus has yet to reach are Oregon, Washington, Idaho, Nevada, Utah and Arizona.

In a recent edition of CD Summary, a biweekly newsletter for Oregon doctors and nurses, state epidemiologists wrote: "Having observed the Patton-like dash of WNV across the country, we think it inevitable that WNV will arrive into God's country. ... The betting in our office pool suggests that it will get here within a year."

To date, the virus has infected 1,295 humans and killed 54 in the United States, according to the Centers for Disease Control and Prevention.

On Thursday, California public health officials announced that lab tests confirmed a Los Angeles woman was infected with the virus, marking its first appearance on the West Coast. The woman was hospitalized but has since recovered. Although she reported no mosquito bites, blood transfusions or organ transplants, or travel to areas where West Nile virus has been detected, laboratory tests confirm she was exposed, California health officials said.

Most humans who get infected with the West Nile virus don't even know it.

Less than 1 percent of mosquitos may carry the virus, and less than 1 percent of humans bitten by an infected mosquito will become seriously ill, said Rocky Baker, an OSU veterinary microbiologist.

Some get flu-like symptoms: fever, headache, body aches and swollen lymph glands. More severe infections can lead to encephalitis, or inflammation of the brain, marked by headache, high fever, neck stiffness, disorientation, coma and seizures. Those most at risk of severe disease are more than 50 years old and people with compromised immune systems.

But for horses, West Nile virus poses a far more serious threat: 40 percent of horses who exhibit symptoms of the virus will develop a fatal case of encephalitis, Baker said. Symptoms include stumbling, lack of coordination, weakness in the legs, depression and muscle twitching.

During the late summer and fall of 2000 in southern France, 76 horses were infected with West Nile virus and 21 died, according to a study in the journal Emerging Infectious Diseases.

Other animals, such as dogs, pigs, llamas, alpacas and rabbits, appear resistant to infection and have shown no signs of illness or disease.

Though no human vaccine exists for West Nile virus, there is an effective vaccine for horses. It's a two-part vaccine that takes up to five weeks to reach full effectiveness.

West Nile virus is "a hot topic" among horse owners in Eugene and Springfield, said Katie Rose, boarding manager at the Oregon Horse Center.

"It's the big topic on everyone's minds," she said.

The horse center is having a "shot day" next week for owners of the 30 horses boarded there. Rose said she definitely plans to vaccinate Kermit, her 8-month-old Appaloosa.

"An ounce of prevention is worth its weight in gold to get this taken care of now," she said.

About 150,000 to 200,000 horses are in Oregon, and only about 2 percent have been vaccinated, state veterinarians say.

They add it's a good idea to vaccinate now so that a basic immunity is created this fall, followed by a booster shot next spring to strengthen immunity.

Reducing mosquito breeding sites also is a critical factor in controlling the spread of the disease, said state veterinarian Andrew Clark.

Any source of stagnant water - tires, wading pools, wheelbarrows, birdbaths - is a potential breeding site for mosquitos.

Birds are the "reservoirs" of the virus, carrying the disease as they migrate. Mosquitos transmit the disease from birds to humans and other animals.

The virus is known to infect at least 111 species of birds, Baker said. Highest mortalities are seen in the corvid family, which include jays, magpies and crows.

The virus isn't spread human-to-human, or from birds or animals to humans, though health officials caution against handling dead birds or animals.

Tim Christie covers health and medical issues. Call 338-2572 or e-mail [e-mail].

WEST NILE VIRUS

For more information on the Web go to:

The Centers for Disease Control: www.cdc.gov.

Oregon Public Health Services:

www.ohd.hr.state.or.us/acd/wnile/home.htm

National Pesticide Information Center: www.npic.orst.edu.

To report dead birds or animals exhibiting West Nile symptoms, contact Dr. Emilio DeBess, state public health veterinarian, at [phone].