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CORVALLIS - It's very difficult to predict what impact West Nile Virus will have when it arrives in Oregon, one expert says, but if it follows the pattern of other "arboviruses" it may not amount to much, and in any case there's not a great deal anyone can do to deal with this new health threat.

Use of mosquito repellents that contain DEET may be of some value, and avoidance of mosquito bites is the only sure preventive step, said Phil Rossignol, a professor of medical entomology at Oregon State University who has studied insect-transmitted diseases all over the world.

Viewed objectively, the chance of developing a serious illness from West Nile Virus is extremely small, he said, and most people will probably soon learn to co-exist with this virus, as they have others.

But no vaccine exists for humans, and there's no reason to believe one will be available anytime soon. And local or regional efforts to control mosquito populations have not been proven to effectively reduce mosquito-borne disease transmission - with some diseases such as malaria they have largely been abandoned. Programs to control mosquito populations may not hurt, Rossignol said, but there's not much scientific evidence they will help - and they could pose other environmental risks.

"We simply can't predict with any accuracy how big of a problem West Nile Virus will be as it arrives in each new location," Rossignol said. "The human infection process of a mosquito-transmitted disease such as this is fairly complex, and can be greatly affected by a number of local factors, such as weather, insect ecology, and other variables."

The West Nile Virus itself has been something of an enigma since it surged out of Africa a few years ago after being largely quiescent in its native regions for decades, Rossignol said. It's unknown whether the virus itself has mutated or an environment that favored it has somehow changed, but what is clear is that it has spread rapidly across the United States in just the past three years and now is poised at Oregon's doorstep. Most experts expect the first cases in the state this spring or summer.

A research effort at OSU has just been completed studying the mode of transmission and disease "vectors" of West Nile Virus on the East Coast, and one of the few situations linked to a higher number of human disease cases was a rubber tire dump, which provides a major breeding ground for mosquitoes.

According to Rossignol, West Nile is part of a family of "arboviruses" that include Western equine encephalitis, St. Louis equine encephalitis and others.

"There was a huge national epidemic of Western equine encephalitis back in the 1970s, and then it just kind of died off," Rossignol said. "Typically, some of the viruses in this group may have a wave of infection and then drop back, and it would not be surprising if West Nile Virus followed that pattern.

"And it's worth noting some differences in local ecology," he said. "Western equine encephalitis is still routinely found in California, but I haven't heard of an infection in Oregon in 15 years. We may not have the type of local environment that is preferred by arboviruses."

West Nile Virus is transmitted only by mosquitoes and birds. Humans and horses are the two other groups most prone to this infection, but they are dead end hosts.

The virus cycle in birds, Rossignol said, is perpetuated mostly by a genus of "Culex" mosquitoes that feed primarily on birds. The most common species, Culex pipiens, is common in urban areas, including those of Oregon, but they less often bite humans.

The genus of "Aedes" mosquitoes, however, will bite pretty much anything and anyone, Rossignol said, and may form an important bridge in the infection process, even though they are less efficient at carrying the virus. The West Nile Virus may be largely transmitted from bird to bird by Culex mosquitoes, but from bird to human by Aedes mosquitoes. Weather events or climatic conditions that cause surges in certain mosquito populations may also play a role in causing human health impacts, researchers say. There are many variables and unknowns.

"That's another reason we aren't exactly sure what to expect with West Nile Virus in Oregon," Rossignol said. "Right now we don't have good baseline information on exactly what mosquito species we have, where the populations could lend themselves to disease transmission, what their biting habits and host preferences are, and all the other information we need to evaluate this risk scientifically."

A research project is under way at OSU, he said, to obtain some of that data, but may not be completed for some time.

A wide body of research on mosquito control efforts has not shown that reducing mosquito populations has a significant impact on reducing cases of mosquito-borne human disease, Rossignol said. It's actually more important to reduce the life expectancy of mosquitoes, he said, since the viruses tend to be carried by older insects - but that is extremely difficult to do.

Simply reducing the number of mosquito bites you get is not synonymous with reducing your chance of being infected, Rossignol said.

As the state learns to live and deal with this emerging virus, it will be important to conduct monitoring efforts to determine what actions, if any, tend to reduce disease impacts, Rossignol said.

"If you follow the medical dictate of 'first, do no harm,' then we will need to weigh the potential gains and losses before beginning any major control measures," Rossignol said. "And when we do take any steps, it's essential we monitor the results so we know if it worked."

One silver lining, Rossignol said, is that it appears infection with West Nile Virus - which most commonly causes a mild illness that many people may not even know was an infection - provides immunity from future infections. But human vaccines with arboviruses have an unacceptable level of risk, he said, and may not be developed anytime soon.