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West Nile infections rising in the U.S.


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 United States. As of Aug. 8, 126 human cases had been identified across 22 states, according to the Centers for Disease Control and Prevention.

“Particularly here in California, it’s peak risk right now,” said Vicki Kramer, PhD, chief of vector-borne diseases in the California Department of Public Health. 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 Aug. 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,” Dr. 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,” Dr. 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, said the state is watching troubling signs as well.

“The concern this year,” Dr. 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, 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.

Dr. 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

Dr. Pastula and Dr. 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 .
  • Repair window screens so mosquitoes cannot fly through.
  • Dump and drain standing water on your property and maintain swimming pools.


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