West Nile virus season arrives in Erie County. What is the risk for humans?

West Nile virus has returned to Erie County, as five batches of infected mosquitoes have been trapped in the past five weeks.

County health officials have found the virus in mosquitoes or birds nearly every summer since 2000, a year after West Nile was first detected in the United States. The only years without a documented West Nile case in Erie County were 2010 and 2017, according to the Pennsylvania Department of Environmental Protection.

But the feared epidemic of human West Nile cases has not materialized. Only nine Erie County residents have tested positive for the virus during that time, and four of those cases were found in 2002.

"I don't think that's unusual. (West Nile) primarily affects birds and mosquitoes. Humans are only incidentally infected," said Dr. Howard Nadworny, a Saint Vincent Hospital infectious diseases specialist and Erie County Department of Health adviser. "And when most people get West Nile, they develop a mild fever, nausea and diarrhea. Few people get severely ill and require treatment."

West Nile remains a health concern for humans, however, because the virus can cause encephalitis — a life-threatening swelling of the brain. Less than 1 percent of West Nile infections lead to encephalitis, but the death rate for West Nile encephalitis is about 9 percent, according to Penn State Extension.

And the number of human West Nile infections is rising in the United States.

"In 2022, there were 1,126 (West Nile) infections and 90 deaths in the U.S. This year there have been 126 cases in the U.S. already as of (Aug. 8) and we expect many more," said Dr. Nancy Weissbach, a Saint Vincent infectious diseases specialist. "With warming temperatures, it is predicted that mosquito populations will continue to increase."

Where have West Nile-infected mosquitoes been found?

This summer's batches of infected mosquitoes have been trapped in Millcreek, Harborcreek and Fairview townships.

Additional trapping has been done at those sites, along with larviciding, and barrier treatments of insecticide to surrounding vegetation, said Breanna Adams, director of environmental health services for the County Health Department.

"The good news is that when we have gone back to check the traps a second time at those sites, the number of mosquitoes we find is low," Adams said.

Mosquitoes trapped July 27 in Millcreek tested positive for Jamestown Canyon virus, another mosquito-borne illness that can, in rare cases, cause encephalitis.

"It's not that this virus has just arrived in northwestern Pennsylvania," Adams said. "It's that the state is now able to test for that particular virus."

No human cases of Jamestown Canyon virus have been reported in the county.

Some people have a higher risk of West Nile encephalitis

Certain people are more at risk of West Nile and Jamestown Canyon complications than others. They include those older than 60, people with weakened or compromised immune systems, and those with chronic diseases like diabetes and high blood pressure.

If you fall into one or more of those categories, Nadworny recommended taking precautions to reduce your risk of mosquito bites when heading outdoors, especially between dusk and dawn. These include using insect repellent, and wearing long pants and a long-sleeved shirt.

Weissbach also encouraged residents to remove any standing water from their property, because that is where mosquitoes gather and breed.

"This would entail removing items around the yard that hold standing water such as old tires, empty flower pots, birdbaths, etc.," Weissbach said. "All open windows should have screening. Screens should be checked for any holes that would allow mosquitoes into the home."

More: Warmer weather means longer mosquito seasons. List of US cities with the biggest increase

Though mosquitoes can be found from spring through fall in the county, most West Nile-infected batches are trapped in late summer. The species that tend to transmit West Nile are most active during that time of the year.

"We tend to see West Nile in August and September, so the next eight to 10 weeks are the prime season," Nadworny said.

Contact David Bruce at dbruce@timesnews.com. Follow him on Twitter @ETNBruce.

This article originally appeared on Erie Times-News: West Nile virus returns to Erie County as mosquitoes test positive