And the mosquitoes still biting likely are not Culex, Gary said. They are probably Asian tiger mosquitoes or other nuisance species that do not carry West Nile.
“Once we have a good, hard freeze, it’ll kill a lot of the mosquitoes that are still out there, and drive the rest of the Culex mosquitoes into hibernation,” he said.
Culex mosquitoes are the biggest culprit in spreading West Nile virus. The virus, first identified in the U.S. in 1999, made a resurgence this year, causing more than 4,200 reported human infections and 168 deaths nationally. In Ohio, five deaths were reported, and 110 human infections were logged.
An 84-year-old Clark County man died last month according to Springfield Regional Medical Center, where the man had been hospitalized. It is believed the man was outside gardening when he was bitten by a mosquito infected with the virus, Clark County Combined Health District said. The man sought medical treatment after suffering from a high fever but succumbed to complications of viral encephalitis, a common side effect of West Nile, health officials said.
The other deaths this year included an 87-year-old man in Cuyahoga County and two men from Hamilton County, ages 76 and 80.
Though the season is winding down, public health officials say, new cases and additional deaths could still be reported. It can take two weeks or more for symptoms to develop after a person is infected. About 80 percent of people who are infected with West Nile never develop symptoms. About 1 in 150 people with West Nile will develop severe illness, including meningitis and encephalitis, which can be fatal.
Every summer, health departments trap mosquitoes and send them to the state health department’s lab for testing. Though West Nile virus is now considered endemic in Ohio, knowing the virus is active can help direct mosquito control programs.
The last batch of mosquitoes went to the state lab on Oct. 9. The most recent human infection recorded reported symptoms starting Sept. 26, Gary said.
“We’ve seen infection rates in mosquitoes drop steadily over the last several weeks,” he said.
Culex mosquitoes only feed on blood while they are getting ready to lay eggs, he said. Those that reach adulthood late in the season feed mostly on nectar to build up fat stores that will carry them through the winter, Gary said.
Those that survive to overwinter will seek out shelter underground — often in storm sewers and other cave-like environments, Gary said. “They like to go down in areas like that and rest,” he said. “So we’ve created a lot of good habitats for them, not only for breeding, but for overwintering.”
It is unclear why this year’s West Nile outbreak was so severe, but high temperatures and drought, which create perfect breeding conditions for the Culex mosquito, played a role, Gary said.
And he cannot say what next summer will be like. “It’s dependent on weather to a large extent. If we have a hot, dry year like we did this year, I would expect it to be a pretty bad year. That’s the pattern we’ve seen so far,” he said.
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