CORVALLIS, Ore. - Although summer may be winding down, the threat of West Nile virus in Oregon appears to be increasing, putting humans and some animals at risk.

The Oregon Health Authority has confirmed two human cases of West Nile virus in Oregon this week. And the Veterinary Diagnostic Laboratory at Oregon State University, which provides testing for the state, is reporting the highest levels of West Nile virus activity in mosquitoes since 2009. The lab has confirmed the virus in 58 different "pools" of mosquitoes - of which 55 were located in Malheur County.

In contrast, only three pools of West Nile virus were confirmed last year in Oregon, and four in 2010.

"It is still far less than in past years, such as 2006, when we confirmed the virus in some 1,100 pools of mosquitoes," said Donna Mulrooney, supervisor of the molecular diagnostics section of OSU's Veterinary Diagnostic Laboratory. "However, activity has been increasing over the past couple of weeks and it's possible that it could continue moving west across the state."

The virology section of the diagnostic lab has also confirmed West Nile virus in one horse in Klamath County, and in a crow from Malheur County. Certain birds are known carriers of West Nile virus, Mulrooney said.

"Infected crows, ravens, jays and other members of the corvid family are considered 'reservoirs' and will carry very high viral loads of West Nile," she pointed out. "If they are bitten by mosquitoes, those mosquitoes invariably will become infected with the virus.

"Horses, on the other hand, are known as 'dead-end' hosts," Mulrooney added. "They don't carry enough of a viral load to infect mosquitoes if bitten. However, the danger to horses is real. Anywhere from 30 to 50 percent of horses infected with West Nile virus will die."

In Oregon, West Nile virus surveillance efforts are coordinated and funded by the Oregon Health Authority, with testing of animals and mosquitoes performed at the Veterinary Diagnostic Lab in OSU's College of Veterinary Medicine. Emilio DeBess, Oregon Health Authority veterinarian, said West Nile virus typically peaks around Labor Day Weekend. He recommends the following precautions for Oregonians:

  • Eliminate sources of standing water that are a breeding ground for mosquitoes, including watering troughs, bird baths, clogged gutters and old tires;
  • Protect yourself when outside, especially at dusk or dawn when mosquitoes are most active, by using repellants containing DEET, oil of lemon eucalyptus or Picardin, and follow directions;
  • Wear long-sleeved shirts and long pants in mosquito-infested areas;
  • Make sure screen doors and windows fit tightly and are in good repair.

More information on West Nile virus, including symptoms in humans, is available from the Oregon Health Authority at:

Seventeen Oregon counties maintain mosquito control programs, and many of these vector control agencies conduct preliminary testing for West Nile virus then send the mosquitoes to the OSU Veterinary Diagnostic Laboratory for confirmation. Mulrooney and her colleagues use a polymerase chain reaction test to confirm the virus in mosquitoes, and serology (blood) tests to confirm the disease in horses.

The vector control agencies capture mosquitoes in traps at different strategic locations throughout the county, thus the 50 pools of West Nile virus means infected mosquitoes were found at multiple locations in Malheur County.

"Fifty to 60 pools of West Nile virus isn't necessarily unusual, but that kind of widespread activity through Malheur County suggests that it might be ready to jump county lines," said Jerry Heidel, director of the Veterinary Diagnostic Lab.

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Donna Mulrooney, 541-737-6615