Snohomish County has escaped another year with no reports of either humans or animals being infected with West Nile virus, a mosquito-borne illness that sickened more than 3,300 people and caused 93 deaths nationally this year.

The Snohomish Health District budgeted $100,000 this year to look for the virus this summer through steps such as trapping mosquitoes to see if they carried the virus and sending off dead birds for testing to see if they were infected.

Although mosquito traps were set up in Marysville, Everett, Snohomish and Lake Stevens, all the tests came back negative for the virus, said Mike Young, who oversees West Nile virus surveillance for the Snohomish Health District.

Tests on 49 nine dead birds, including crows, jays, an owl and a hawk, also found none tested positive for the virus.

“That’s the way we like it,” Young said.

Last year, two of 81 dead birds from Snohomish County tested positive for the virus but there were no human cases.

The disease is spread when an infected bird is bitten by a mosquito, which then infects humans or other animals.

State health officials said no human cases were reported anywhere in Washington this year, a surprising trend since three people tested positive for the virus in 2006, two in Pierce County and one in Clark County.

Generally, when a handful of cases are diagnosed in a state one year, the number of cases snowballs the following year.

But in Washington, the next step didn’t happen, said Donn Moyer, a spokesman with the state Department of Health.

“It’s a mystery,” Moyer said.

No one knows why Washington has had so few cases while other surrounding states, such as Idaho, have been hit hard in the past, he said.

“There’s probably a wide variety of factors, from climate to the mosquito populations that came out this year,” Moyer said.

The only cases reported this year were in the Yakima area, he said, where eight horses, one bird and one dog were infected.

While the virus can affect a number of different kinds of mammals, “horses and people seem to be more susceptible,” Moyer said.

“Fortunately, it’s not spread person-to-person,” he said.

Many people infected with the virus may not even know it. But about one in 150 people develops severe symptoms, including high fever, headache, neck stiffness, tremors, muscle weakness and, in some cases, numbness and paralysis.

The effects can last for several weeks or they may be permanent, according to the Centers for Disease Control and Prevention.

Herald reporter Sharon Salyer at 425-339-3486 or salyer@heraldnet.com.