Scott Orrock, 80, a resident of Springville in Utah County, died Sunday morning at Utah Valley Regional Medical Center of severe complications caused by the mosquito-borne illness.

"Severe encephalitis and meningitis complications, such as paralysis, confusion and death, occur more often in older patients but can occur at any age," said Joseph Miner, director of the Utah County Health Department.

While most people experience no symptoms, about 20 percent of cases show severe flulike symptoms. Fever, nausea and rashes are common.

"Less common are the severe neurological symptoms such as this case, which affect about one in every 150 people infected," Miner said.

To help stop the spread of the disease, mosquito-abatement teams have been conducting aerial spraying over 50,000 acres, including weekly sprayings over some of the worst mosquito areas next to Utah Lake. They also have land vehicles going out nightly fogging areas to create a buffer zone between the known mosquito breeding habitat and residential and recreational areas.

"Our efforts haven't been in vain," said Bob Mower, Utah County mosquito-abatement director. "When compared to last year, our mosquito population is down by over 50 percent. Last year, our overnight mosquito trapping numbers peaked at over 9,000 mosquitos. This year our highest over-night peak was just over 4,000."

However, while the number of mosquitos may be down, the infection rate is up.

"We collect and test mosquitos in batches or 'pools' of 50 insects," Mower said. "At our peak in the last week of July, about one in every five pools were coming back positive. That means at least one mosquito in that pool of 50 insects had the virus."