Cooler weather and a seasonal change in mosquito life cycles have created a dearth of Jackson County mosquitoes that could carry the West Nile virus.

Despite a recent outbreak of predatory — but non-virus-carrying — mosquitoes in Medford, populations of the West Nile-carrying culex family of mosquitoes have crashed, according to Jackson County Vector Control officials.

Mosquito traps set this week around Ashland, Talent and the Sams Valley area have caught just a handful of bugs, and none have tested positive for the virus since Sept. 7, said Eugene Papineau, manager of the county’s vector control district.

Previous traps netted bugs by the thousands, suggesting that fewer of the mosquitoes are present to pass on the virus.

"We’re not done for the year, but those mosquitoes are on their way out," Papineau said.

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"By Oct. 1, we’ll probably be done with the season," he said.

The drop in mosquitoes, however, does not mean the West Nile threat is over, said Dr. Emilio DeBess, the state’s public health veterinarian.

Four human cases of West Nile have been confirmed in Oregon, including a Eugene man who believed he was infected by mosquitoes along the Rogue River in Josephine County, DeBess said.

Two of those people likely were infected in California, while the fourth was bitten in Malheur County.

Two Jackson County horses have contracted the virus, one of which died.

People infected with bites often take up to 10 days to show any symptoms, such as high fever, aches, pain behind the eyes and overall lethargy. Many cases go unreported to doctors or the victims are not tested by their physicians.

About one in five infected people show no signs of the disease, while the virus in rare cases can infect the brain or spinal column.

"I think we’re going to have two or three more (human) cases in the state, maybe more," DeBess said.

Four possible human cases from throughout Southern Oregon, including Jackson County, are awaiting confirmation from a California lab, DeBess said. Results could be available next week, he said.

"The key is we don’t want to cry wolf and be wrong about it," DeBess said. "We need confirmation."

Mosquitoes become carriers of the disease by biting infected birds and can pass it on to other birds, humans and animals.

The drop in culex mosquitoes generally happens in mid-September as cooler weather slows larvae growth rates or prevents hatching, Papineau said. Also, adult culex mosquitoes shift from blood meals to plant juices in preparation for their sub-hibernation through winter, he said.

"This time of year it’s a phenomenon that happens like clockwork," Papineau said.

But it certainly doesn’t feel that way throughout east Medford, which is suffering from an outbreak of pasture mosquitoes.

The majority of these mosquitoes likely were hatched in an irrigated pasture near Coker Butte and Foothill roads, then descended upon Medford, Papineau said.

"Those mosquitoes don’t carry West Nile," Papineau said. "They’re just a nuisance mosquito."

Reach reporter Mark Freeman at 776-4470, or e-mail mfreeman@mailtribune.com.