West Nile making few inroads in Oregon

[Amelia]

West Nile making few inroads in Oregon

June 15, 2006

Associated Press

SALEM, Ore. -- Where are those mosquitoes bearing the West Nile virus?

Despite repeated warnings that the virus could sicken hundreds of Oregonians, it has largely avoided the state and the rest of the Pacific Northwest.

Blood samples from the state's eight sentinel chicken flocks have turned up no trace of the virus this year, said Emilio DeBess, veterinarian with the state Public Health Department. No dead birds have tested positive, and no mosquitoes, horses or llamas have been found to harbor the virus.

Health officials are pleasantly puzzled by the development, but one researcher at Oregon State University says climatic differences between the Northwest and other parts of the country could be a factor.

West Nile arrived in Oregon in 2004, sickening five people, and killing 19 birds and 22 horses. Typically, the virus shows little activity the year it arrives in a region. The following year, however, infections generally soar, and health officials warned that Oregon could see hundreds of cases in 2005.

But Oregon had fewer than 10 cases.

The relentless rains this past winter had health officials concerned that 2006 would be the year that West Nile struck large numbers of Oregonians. Not yet.

West Nile, a viral illness that can cause symptoms similar to influenza, is primarily a disease that birds get from infected mosquitoes. First appearing on the East Coast in 1999, the virus made its way across the continent, carried by birds.

Most people infected with the virus never get sick, but about 20 percent suffer flu-like symptoms. Fewer than 1 percent become severely ill; some people have a potentially fatal inflammation of the brain, membranes of the brain or spinal cord.

Jeffrey Shaman, an assistant professor of atmospheric sciences at OSU, has studied the effect of climate on West Nile transmission in Florida. Shaman found that wet regions with lots of mosquitoes don't necessarily make for the most favorable conditions to spread West Nile.

The best conditions, he said, seem to be those that bring mosquitoes and birds together in heavy concentrations.

In a drought, he said, moisture often dries up everywhere except in isolated, densely vegetated areas. Both birds and mosquitoes are drawn to these relatively humid areas, the only remaining source of water. In those close quarters, infected birds are bitten by far more mosquitoes than if they were dispersed over wide areas.

A newcomer to the state, he has hasn't studied Oregon's situation, but said "it's possible that the hydrology factors into it."