Two Oregonians, one in Coos County and one in Malheur County, tested positive for the West Nile virus this week, according to state health officials. The man and the woman, both 50 or older, are recovering.

Statewide, West Nile virus cases in mosquitoes are considerably higher than in recent years, although no cases have been found in Central Oregon.

West Nile virus is a potentially serious illness spread by mosquitoes and birds. Most infections in humans are mild, with fever and flu-like symptoms. Severe infections may cause encephalitis — an inflammation of the brain. On rare occasion, it can cause death.

Oregon’s state and county public health departments, along with local vector control districts, have been testing mosquitoes and tracking West Nile virus cases since 1999, when the virus appeared in the United States. It was first diagnosed in Oregon in 2004, in humans, birds and horses.

Crows, ravens, jays and other members of the corvid family that are infected carry high loads of West Nile virus, said Donna Mulrooney, supervisor of the molecular diagnostics section of Oregon State University’s Veterinary Diagnostic Laboratory, which conducts West Nile testing. If a mosquito bites an infected bird, the mosquito invariably becomes infected with the virus, she said.

“Horses, on the other hand, are known as ‘dead-end’ hosts,” Mulrooney said. “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.”

An uptick in West Nile

This year has seen the highest levels of West Nile virus activity in mosquitoes since 2009, and it’s hitting Malheur County the hardest, according to the OSU diagnostic laboratory.

“Things had died down for a couple of years, but this year (West Nile) came back to being around,” said Chad Stubblefield, director of the Four Rivers Vector Control, which does West Nile surveillance and monitoring and mosquito control between Sunriver and La Pine, where there are lots of rivers and a “horrendous” amount of mosquitoes.

Until the two recent cases in humans, the virus had only been found in animals this year: a horse in Klamath County; a mosquito pool in Jackson County; two mosquito pools in Morrow County; and 55 mosquito pools and a bird in Malheur County. A mosquito pool is a sample of up to 50 female mosquitoes of the same species collected at one site.

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

The virus peaked in Oregon in 2006, when it was found in some 1,100 pools of mosquitoes.

That year, a record number of 73 humans in Oregon were known to be infected with West Nile, according to the Oregon Health Authority. Seventy of them contracted it in-state. Three contracted it elsewhere. Three-quarters were Malheur County residents. Thirteen, or 18 percent, of those were diagnosed with encephalitis or meningitis from the virus. One died. Another Oregonian died in 2007 of West Nile.

Central Oregon’s scene

Central Oregon’s tri-county region has largely been spared from the West Nile virus, with no confirmed cases in animals or humans since 2006, when West Nile was confirmed in one Deschutes County resident, one Deschutes County bird and one Jefferson County horse.

Cliff Kiser, the Crook County Vector Control district manager, said there’s no evidence to explain why Central Oregon has been lucky, but there are some pretty good theories.

The most logical is that this is high country, and it’s a little cooler here than in the state’s eastern and southern counties, where the disease is more prevalent. Those regions have a longer warm season. The species of mosquitoes that best transmits the virus, called Culex, need time to build their numbers large enough to create a widespread problem. The eggs of the Culex mosquitoes are not sturdy enough to survive all winter, Kiser said. Some adults hide in houses and survive the winter, come out when spring warms up, and start laying eggs.

“That means we start with almost a zero population of Culex,” Kiser said.

Some other species of mosquitoes can hatch right out of the ice, but those types are less likely to transmit the virus than the Culex.

So, it’s a numbers game. The odds of getting bitten by a virus-carrying Culex in Central Oregon are lower than in other regions of the state.

That is, right up until about Labor Day, when the Culex have rebuilt their populations, he said. A couple of hard frosts could knock their populations down any time, though, he said.

“Temperatures are what really drives most of these mosquitoes that we have,” Kiser said.

Stubblefield said the Culex thrives in floodwater areas, such as a flood-irrigated pasture. They prefer warm water and develop quickly in it. They can be found in east Bend, Alfalfa, and northeast and northwest Redmond, he said. They can also be found, in small numbers, in south Deschutes County.

Prevention

State health officials are warning people who plan to camp or play outside this late-summer weekend to take preventive action against getting bitten by mosquitoes.

“Having tracked West Nile cases for many years now, we know that the number of cases typically peaks by Labor Day weekend,” said Emilio DeBess, a Oregon Health Authority veterinarian. “There are simple things people can do to protect themselves.”

Namely, avoid getting bitten. Cover your skin during the dawn and dusk hours when mosquitoes are most active, and wear repellent.

Take precautions

• Make sure screen doors and windows are in good repair and fit tightly.

• Eliminate sources of standing water, including watering troughs, bird baths, clogged gutters and old tires, which are a breeding ground for mosquitoes.

• When outdoors at dusk or dawn, when mosquitoes are most active, protect yourself by using mosquito repellents containing DEET, oil of lemon, eucalyptus or Picaridin.

• Wear long-sleeved shirts and long pants in mosquito-infested areas.

Source: Emilio DeBess, Oregon Health Authority veterinarian.

For more about West Nile in Oregon, including symptoms in humans:

http://public.health.oregon.gov/DiseasesConditions/DiseasesAZ/WestNile Virus/Pages/survey.aspx