CORVALLIS, Ore. - Oregon State University technicians have documented more than a hundred pools of mosquitoes in eastern Oregon that have tested positive for West Nile virus, and though no human cases have yet been reported, the state is urging caution.

"The risk of an individual contracting West Nile virus is low, but we do encourage people to take appropriate precautions to protect themselves against mosquito bites," said Emilio DeBess, the state public health veterinarian with the Oregon Department of Human Services.

Tests for West Nile virus in mosquitoes and animals that can carry the virus are conducted annually at the Oregon State University Veterinary Diagnostic Laboratory. Thus far, OSU diagnosticians have documented 78 mosquito pools with West Nile virus from Morrow County; 24 pools from Umatilla County; six pools from Baker County; and five from Malheur County.

Two birds from Morrow and Umatilla counties tested positive for West Nile as did a horse from Umatilla County, according to Jerry Heidel, director of OSU's Veterinary Diagnostic Laboratory.

"Many of the positive tests have occurred just recently," Heidel said, "so there is a possibility that the prevalence of West Nile virus may be increasing in mosquito populations, as well as the birds and horses that they bite."

OSU diagnosticians tested 2,260 pools of mosquitoes from eastern Oregon and found 70 of those pools to contain mosquitoes with the virus. Two of 70 birds tested came back positive; and one of 12 horses.

The diagnostic laboratory, which is part of OSU's College of Veterinary Medicine, also test 260 pools of mosquitoes from Washington state and found 143 of those pools contained the virus - 140 from Benton County (Washington) and three from Adams County.

Previous studies by researchers at OSU and elsewhere suggest that "amplification" of West Nile virus prevalence comes when mosquito pools are in close proximity to flocks of birds - especially those that migrate.

Testing of mosquitoes is not conducted throughout the entire state, so there likely are pools of mosquitoes that contain West Nile virus that have not yet been detected, Heidel pointed out. Rocky Baker, an OSU diagnostician and supervisor of the Molecular Diagnostics section at the laboratory, has indicated they will be conducting tests throughout the mosquito season, which lasts until the first heavy frosts occur.

DeBess, who works for the Oregon Department of Human Services, said high numbers of mosquito pools with the virus can lead to further human and animal infections. In 2008, Oregon reported 16 human cases to the Centers for Disease Control.

West Nile virus frequently is detected through dead birds, which are sent to OSU for testing. Persons noticing dead birds - especially crows, magpies, jays and robins - should contact local county health departments.

Humans infected with West Nile virus may experience mild flu-like symptoms such as fever, headache, body aches and swollen glands, according to DeBess. In some serious cases, the central nervous system is infected and individuals can contract meningitis or encephalitis. Not all humans infected become ill, however, DeBess added.

DeBess offers several suggestions for reducing risks of contacting West Nile virus:

  • Eliminate all sources of standing water that can be a breeding ground for biting mosquitoes, including water troughs, bird baths, clogged gutters and old tires;
  • Avoid outdoor activities at dusk and dawn when mosquitoes are most active;
  • Wear long-sleeved shirts and long pants when in mosquito-infested areas;
  • Use mosquito repellents containing DEET, Oil of Lemon Eucalyptus and Picardin, making sure to follow directions on the container;
  • Make sure screed doors and windows are in good repair and fit tightly.

More information is available on the Oregon Department of Human Services West Nile Website at:



Jerry Heidel, 541-737-6964;

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