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

Best way to avoid the virus is to avoid being bitten by mosquitoes

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 (knock on wood). 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 Dr. 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.

Jennifer Barritt, a biologist with Virginia Beach’s Mosquito Control program, said they have collected West Nile virus-positive mosquitoes at five locations in the city (including Oxford Drive in the King’s Grant neighborhood, North Witchduck Road in the Kempsville area, and Constitution Drive in the Pembroke area).

“Most of these locations are in the northwest section. There have been a total of 13 samples positive out of almost 200 samples tested,” Barritt said. “This level of virus detection is about average so far this season.”

Barritt said human cases typically start around this time of year and usually continue through September.

Gaines said it’s possible many of the infected mosquitoes that have been trapped and tested have had enough WNV in their bodies to yield a positive test result, but that the WNV infection in these mosquitoes had not yet disseminated to the mosquito’s salivary glands.

“West Nile virus-infected mosquitoes that do not have disseminated infections cannot transmit the virus because the mode of West Nile virus transmission is via mosquito saliva during a bite,” Gaines said.

The 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.

Barritt added the 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 transfusion 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 nation there have been 106 human West Nile infections, with Louisiana recording the most, 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.

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