West Nile Infections Rising in the US

Marcia Frellick

August 11, 2023

Several signs are pointing to an impending surge in the number of human cases of West Nile virus in several regions of the United States.

West Nile virus is spread by infected mosquitoes and currently there is no cure or virus-specific treatment. In rare cases, it can be deadly. It can infect humans, birds, horses, and other mammals.

West Nile Virus is the leading cause of mosquito-borne disease in the continental US. And as of August 8, 126 human cases had been identified across 22 states, according to the Centers for Disease Control and Prevention (CDC).

Vicki Kramer, PhD, chief of vector-borne diseases in the California Department of Public Health, said, "Particularly here in California, it's peak risk right now."

She said scientists there are seeing higher mosquito and infected mosquito numbers.

"Peak Risk Right Now"

Dead birds are tested for the virus and by August 4, 181 of the 913 birds tested in California have been positive, three times the total testing positive by this time in 2022.

"Last year at this time, we had 60 positive dead birds out of 817 tested," Kramer said.

Severe flooding and high heat can contribute to the rise in mosquito populations and many parts of the country have seen plenty of both.

One of the ways scientists track infected mosquito patterns in California is by using flocks of strategically placed sentinel chickens.

"Chickens are a mosquito magnet," Kramer said.

Chickens don't get sick with the virus, but they do build antibodies to it. Surveillance teams check their blood every other week to track the virus.

Daniel Pastula, MD, MHS, chief of neuroinfectious diseases and global neurology at the University of Colorado School of Medicine and the Colorado School of Public Health, says the state is watching troubling signs as well.

"The concern this year," Pastula said, "particularly along the Front Range in Colorado, is we've found many more mosquitoes [that are] positive for West Nile earlier in the season compared with other years."

"We're bracing for higher-than-baseline human cases," he said.

Asked about this year's first human case reported in Toronto, Canada, a region with a long winter and low incidence of the virus, he said that provides a further example that people need to be prepared even in climates not known to be mosquito-dense.

He added, however, that climate is only one factor in the severity of the season. Others include birds' immunity and migratory patterns.

Pastula said that fluctuations in temperature and rainfall are rising with climate change and are disrupting normal baseline levels of West Nile.

"That shows we need to be prepared for West Nile virus and other mosquito-borne diseases in any place in North America or really the world. We recently saw malaria cases in the southern United States. It just shows you how dangerous mosquitoes can be."

Avoid Mosquito Bites

Pastula and Kramer list the precautions people can take to protect themselves from West Nile virus:

  • Limit outdoor exposure particularly at dusk and dawn

  • Wear protective clothing

  • Use EPA-approved insect repellent

  • Repair window screens so mosquitoes can't fly through

  • Dump and drain standing water on your property and maintain swimming pool

Pastula notes that summer is the time human cases start to mount — typically from July and August to the first hard freeze.

"We have been warning people here up and down the Front Range of Colorado to take prevention very seriously," Pastula said.

He pointed out that 80% who are infected with West Nile will have no symptoms.

About 20% will have flu-like illness — high fever, body and joint aches, rash, diarrhea, or headaches. Symptoms may last for weeks. About 1% of the time, he says, people can get neuroinvasive West Nile.

Pastula explains that the virus can infect the covering of the brain and spinal cord causing meningitis with very high fever, severe headaches, stiff neck, and sensitivity to light.

So far this year, there have been 89 neuroinvasive cases reported nationally, according to the CDC.

With West Nile encephalitis, the virus "can infect the brain itself causing altered mental status, movement disorders, or weakness," Pastula said.

Sometimes it can infect the gray matter of the spinal cord causing a West Nile virus poliomyelitis, which brings polio-like symptoms.

"The West Nile encephalitis and poliomyelitis can cause permanent deficits or even death," he said. "It's uncommon but it's not trivial."

Several vaccine candidates are in development, Pastula says, but none has reached clinical trials. Part of the reason for that, he says, is that scientists must be able to predict the timing of an outbreak.

"We're not really great at predicting outbreaks," he said.

Although the risk for neuroinvasive disease is small, it can be higher in certain groups, he says — those who are over age 60 years or are immunocompromised; those who have diabetes, cancer, or kidney disease; or those who have undergone organ transplants.

Those infected should see a healthcare professional and may be able to get relief with the usual medications for flu-like illness.

Some with severe infection may need to go to the hospital, Pastula said.

Marcia Frellick is a freelance journalist based in Chicago. She has previously written for the Chicago Tribune, Science News and Nurse.com and was an editor at the Chicago Sun-Times, the Cincinnati Enquirer, and the St. Cloud (Minnesota) Times. Follow her on Twitter at @mfrellick

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