West Nile virus nearing Oregon, may threaten horses


CORVALLIS - The West Nile Virus that has caused both animal and human deaths across much of the East, Midwest and South is moving across the western U.S. with surprising speed and could arrive in Oregon next year, or possibly even this fall, say veterinary experts at Oregon State University.

Accordingly, officials in the OSU College of Veterinary Medicine are now recommending that all horse owners in Oregon have their horses or other equines immediately vaccinated for the disease - it poses a special risk to horses and that is the only animal species for which an effective vaccine exists.

The OSU college and its Veterinary Diagnostic Laboratory are also gearing up their efforts to deal with a new viral threat that they say will be in Oregon indefinitely, posing new challenges for diagnosis, treatment, public education and disease monitoring.

A new laboratory has already been set up at OSU for testing and procedures will be available to the public to help identify or test for the disease in infected animals. "The first case of West Nile Virus in Wyoming was reported a few days ago, and it now appears that it could be in Oregon within a month or two, and almost certainly by next summer," said Dr. Donald Mattson, a virologist with the OSU College of Veterinary Medicine. "It is moving east to west more rapidly than we had thought it would, and it's time now for Oregonians to learn about this emerging virus and get ready to deal with it."

Birds are the natural host of the virus, but it can also infect horses and, to a much lesser extent, humans. It is transmitted by many species of mosquitoes, including some of those found in Oregon. Most people who become infected do not even become ill, but it can cause fever, headache, other aches and pains, and in severe cases encephalitis, meningitis and death. The most severe cases are usually found in people with a weakened immune system. There is no specific treatment or vaccine for human illness.

"People should be prepared but not be overly concerned about the arrival of this virus," said Rocky Baker, a microbiologist in the OSU College of Veterinary Medicine. "Even when it arrives, it appears that less than 1 percent of mosquitoes may carry the virus and less than 1 percent of the people infected will become seriously ill. And there are preventive steps that people can take to avoid infection."

Much of that information can be found in web sites that have been set up to provide the latest information about the virus; two of the best are at the U.S. Center for Disease Control - www.cdc.gov - and the Oregon Public Health Services - www.ohd.hr.state.or.us/acd/wnile/home.htm.

Information about mosquito control, repellants and other issues can also be found at the web site of the National Pesticide Information Center - www.npic.orst.edu - which is a cooperative effort of OSU and the EPA.

In simple terms, the OSU experts say, the best way to deal with the virus is not to get it, by avoiding mosquito bites via repellants, or preventing mosquito breeding in stagnant water.

Traditionally, the peak incidence of the virus is in late August or early September, but this year the virus seems to have appeared earlier than usual and is moving more rapidly to new areas - part of the reason the OSU veterinary officials believe it's time for people in the state to prepare. For one thing, the horse vaccine is a two-part vaccination that can take up to five weeks to offer full protection. In terms of monitoring, the public can assist health and veterinary experts by staying alert for signs the virus may have arrived. It may first be found in mosquitoes being tested by the Oregon Public Health Service, but could also manifest itself in dying birds.

"Birds are the natural host of the virus, and it is known to infect at least 111 species of birds," said Baker. "Highest mortalities are seen in the Corvidae family of birds, which includes jays and crows, and that makes them important sentinels for the virus. Suspicious signs of the neurologic problems caused by this virus might be birds that were unable to fly, flopping, uncoordinated movements, or, of course, an unusual number of dead birds in a small area."

Anyone who observes such symptoms or animals should feel free to contact Dr. Emilio DeBess, the Oregon public health veterinarian, at the Oregon State Health Division, at 503-731-4024. DeBess is in charge of the Oregon West Nile Virus Surveillance Project, and as part of that project he can authorize testing of dead animals at OSU facilities. Or, by paying a small fee themselves, people can send animal remains directly to the OSU Veterinary Diagnostic Laboratory. The public can call the OSU laboratory at 541-737-3261 for more detail on that procedure.

Simply picking up a dead bird or other animal, officials say, should not pose a risk of infection. The virus is transmitted primarily by bites from mosquitoes that have become carriers by biting birds with high levels of viral infection. It also cannot be transmitted from humans or horses, which tend to develop lower viral loads than some bird species and are considered "dead end" hosts from which the virus will not spread any further.

West Nile Virus first appeared in New York in 1999 and has since spread to 41 states, three provinces and Washington, D.C. Aside from the viral impact on horses and humans, it's unclear what long-term effect the virus will have on bird populations, experts say.

"This is another disease problem we're going to be dealing with here in Oregon for a long time," said Dr. Jerry Heidel, director of the Veterinary Diagnostic Laboratory at OSU. "We're fortunate that our College of Veterinary Medicine will be able to help diagnose and monitor this disease, and help the people of Oregon and their animals manage it. "We'll stay on the cutting edge of research, prevention and treatment," Heidel added.

OSU is already able to perform sophisticated diagnostic tests on necropsied animals to diagnose the virus with "polymerase chain reaction" tests, Heidel said. Its new laboratory in the OSU Veterinary Medical Animal Isolation Laboratory is ready to work with the live virus, which is a Biosafety Level Three virus, in more stringently controlled conditions.