High counts of West Nile virus cases are being reported in the U.S. this year, according to federal health data — and health officials are warning the season for this mosquito-borne disease isn’t over yet.
As of Sept. 9, at least 771 human cases across 39 states have been reported, according to data posted by the Centers for Disease Control and Prevention this week. Of these cases, 490 are neuroinvasive, meaning they are severe cases impacting the brain.
West Nile virus is the leading cause of mosquito-borne disease in the continental U.S. While it primarily infects people during mosquito season in the summer and fall, experts have warned a warming climate means mosquito season is getting longer, prompting greater potential for the spread of disease.
After Massachusetts recently announced two additional human cases, the state’s public health commissioner Dr. Robbie Goldstein encouraged the public to stay vigilant.
“Infections can continue to occur until the first hard frost of the season, which is likely still many weeks away,” Goldstein said in a news release. “We are encouraging everyone to continue to take the necessary steps to protect themselves from mosquito bites, including by using mosquito repellent, wearing long sleeves and pants to reduce exposed skin, and to drain anything that may be collecting water outside your home.”
The CBS News data team is tracking confirmed West Nile cases nationwide as new data is released by ArboNET, a national arboviral surveillance system managed by CDC and state health departments. (The map below is updated bi-weekly on Tuesdays when the latest data is released.)
U.S. map showing the number of reported cases of West Nile so far this year.
West Nile virus symptoms
A West Nile virus infection often leads to flu-like symptoms, including:
NauseaAches and painsVomitingFeverChillsRash (occasionally)
“Less than about 1 in 150 people who get infected get a more serious form, and that involves the central nervous system,” CBS News chief medical correspondent Dr. Jon LaPook said last year. In those more severe cases, “You get inflammation of the brain, and this the the membrane that lines the spinal cord, and that’s a much more serious situation there.”
Most people infected with the virus do not develop symptoms, according to the CDC, but for those who do, symptoms usually start about 2-6 days after being bitten by an infected mosquito, though in some cases, it may be up to 14 days. “It may take longer for symptoms to start for people with a weakened immune system,” the agency notes.
There is no specific treatment available for West Nile virus, so health experts say it’s important to focus on prevention, like the precautions Goldstein shared to reduce the risk of mosquito bites.
Protecting yourself is especially important for people who are more at risk, including the elderly and those immunocompromised, LaPook said.
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