Thursday, January 20, 2022

Coming weeks are typically the most active for human West Nile virus infections. Here’s why

An Aedes aegypti mosquito takes a blood meal from her human host. (WYDaily/Courtesy Flickr, Prof. Frank Hadley Collins, Director, Center. for Global Health and Infectious Diseases, University of Notre Dame)
An Aedes aegypti mosquito takes a blood meal from her human host. (WYDaily/Courtesy Flickr, Prof. Frank Hadley Collins, Director, Center. for Global Health and Infectious Diseases, University of Notre Dame)

It’s probably safe to say that everyone knows this summer has been a very active one for mosquitoes across Hampton Roads.

Step outside just about any evening or morning and you’re likely to become a mosquito snack.

Fortunately, though, so far this year in Virginia there have been only two reported human infections of West Nile virus. But the virus tends to peak in late summer, so what might we expect in the next four to six weeks?

“Although the mosquito surveillance programs in the Hampton Roads area are seeing more WNV positive mosquitoes than usual, that does not always translate into human cases,” said David Gaines, public health entomologist with the Virginia Department of Health. “Therefore, I cannot be certain if we will start to see human WNV cases in the coming weeks.”

Gaines said, in general, his experience has been that more WNV cases are seen in dry years than in wet years.

“And this has certainly been a very wet year,” he said.

West Nile virus usually causes a mild illness — so mild some people aren’t even aware they’re infected — but may also cause encephalitis (inflammation of the brain), meningitis (inflammation of the lining of the brain and spinal cord) or polio-like paralysis.

Experts say best way to avoid West Nile is to avoid being bitten: Wear loose-fitting, light colored, pants and long sleeves; be outside at dusk and at dawn as little as possible, since that’s when mosquitoes are most active; use an approved mosquito repellent; and eliminate breeding sites around your home, including anything in which water can pool.

Mosquitoes become infected when they feed on a bird that carries the virus. West Nile doesn’t spread from person to person or directly from birds to humans. However, some cases have resulted from blood transfusions and organ transplants, and there has been one case of an infected mother transmitting the virus to her unborn child.

So far this year across the United States there have been 106 human West Nile infections, with Louisiana recording the most at 21.

Temperatures

Gaines said the West Nile virus doesn’t disseminate quickly in a mosquito’s body at environmental temperatures below 80 degrees Fahrenheit.

“So, if a northern house mosquito bites a WNV infected bird and takes in a WNV infected blood-meal, it may take more than a month for the WNV virus to disseminate to the mosquito’s salivary glands at environmental temperatures of 75 degrees,” he said.

However, when temperatures are 85 degrees or higher, the virus will disseminate within three days.

Gaines said the thinks the reason hot, dry years are more favorable to West Nile may be that since the northern house mosquitoes that transmit the virus spend their days resting in underground culverts, storm sewer pipes and crawl spaces, they may not be exposed to high environmental temperatures in those environments during wet years.

“That is because if the ground is saturated with water from heavy rainfall, causing a lot of evaporative cooling in the underground pipes. So, even if the outdoor air temperature is 95 degrees, the air temperatures in the wet underground sites where these mosquitoes rest may not even reach 80 degrees,” he said.

This story was published in partnership with our sister publication, Southside Daily. 

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Sarah Fearing
Sarah Fearing is the Assistant Editor at WYDaily. Sarah was born in the state of Maine, grew up along the coast, and attended college at the University of Maine at Orono. Sarah left Maine in October 2015 when she was offered a job at a newspaper in West Point, Va. Courts, crime, public safety and civil rights are among Sarah’s favorite topics to cover. She currently covers those topics in Williamsburg, James City County and York County. Sarah has been recognized by other news organizations, state agencies and civic groups for her coverage of a failing fire-rescue system, an aging agriculture industry and lack of oversight in horse rescue groups. In her free time, Sarah enjoys lazing around with her two cats, Salazar and Ruth, drinking copious amounts of coffee and driving places in her white truck.

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