PORTLAND, Ore. (AP) - Warm temperatures have made it feel like summer in much of Oregon this month, and state health officials are concerned that a summertime pest might be confused.

"If I think it's summer, I'll bet the mosquitoes think it's summer, too," Emilio DeBess, the state public health veterinarian, told 100 participants in a West Nile virus "summit" Tuesday.

The meeting included county public health workers and hospital officials from around the state. If Oregon follows the pattern of other states, this year could see a large number of human cases. Usually, states experience a few cases one year and then see a sharp increase the following year. A drop-off occurs after the spike.

West Nile made its Oregon debut in 2004. It sickened five people, and killed 19 birds and 22 horses. Most of the dead birds were found in Southern Oregon.

DeBess said he has asked Southern Oregon counties to begin collecting mosquitoes to see if they harbor the virus.

People become infected from mosquito bites, not through contact with birds, horses or other people who have been exposed to the virus. About 80 percent of those who become infected don't realize it and have no symptoms. About 20 percent of infections result in mild illness.

However, in about 1 percent of those infected, the virus can cause neurological disease, which can be fatal, especially in the elderly.

The disease has proven particularly deadly to horses in Oregon, said Don Hansen, Oregon state veterinarian with the Oregon Department of Agriculture. The virus was found in 32 horses last year, and 22 of them died - a far higher death rate than the national average.

Hansen's department is urging all horse owners to vaccinate their animals.

As for humans, health workers were told to tell the public to eliminate mosquito breeding areas around their homes. Backyard pools, old tires and plugged gutters can provide stagnant water, where mosquitoes can reproduce.

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Information from: The Oregonian, http://www.oregonian.com