West Nile virus cases running higher than normal, prompting health
warnings
[September 11, 2025]
By MIKE STOBBE
NEW YORK (AP) — West Nile virus infections are intense so far this year,
with case counts running 40% higher than normal, health officials say.
More than 770 cases, including about 490 severe cases, were reported as
of early September, according to Centers for Disease Control and
Prevention data posted this week. About 550 cases — 350 of them severe —
are usually reported by this time of year.
Health officials are ramping up warnings to the public, because most
cases of the mosquito-borne disease are reported in August and
September.
“West Nile virus can be a very serious disease and its presence in
mosquitoes remains high right now in Massachusetts,” said the state’s
public health commissioner, Dr. Robbie Goldstein, in a statement last
week.
People can protect themselves by wearing long-sleeved shirts and pants
when possible, and using an EPA-registered insect repellent when
spending time outdoors, health officials say.
West Nile virus was first reported in the United States in 1999 in New
York, and then gradually spread across the country. It peaked in 2003,
when nearly 10,000 cases were reported.
Scientists say many people — perhaps tens of thousands each year — are
infected but don’t know it because they have no symptoms, or only mild
ones such as headaches, body aches, joint pain, vomiting, diarrhea and
rashes.
In severe cases, damage to the central nervous system causes
inflammation of the brain or spinal cord, and even death.
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In this photo provided by the U.S. Centers for Disease Control and
Prevention, a female Culex quinquefasciatus mosquito, also known as
the southern house mosquito, sits on a person’s skin before taking a
blood meal in 2022. (Lauren Bishop/CDC via AP)
 In the last decade, health officials
have fielded reports of 2,000 cases annually on average, including
1,200 life-threatening neurological illnesses and approximately 120
deaths. Deaths are on pace to be higher this year, CDC officials
said, but they declined to discuss specifics, saying mortality
statistics are too preliminary.
The problem is not that there are more mosquitoes this year, but
rather that a higher proportion of the bugs are carrying the virus,
CDC officials said. Mosquito infection rates can be affected by such
factors as temperature, rainfall, the amount of insect control going
on, and how many nearby birds are infected.
Colorado, which tends to see more West Nile virus, has reported
about 150 of the nation’s cases — more than double what other states
are reporting.
Fort Collins is a hot spot. Monitoring last month in an area in the
southwest part of the city found that 35 out of every 1,000 female
mosquitoes were infected — far higher than the 8 per 1,000 that
would be expected for that time of year — said Roxanne Connelly, a
CDC entomologist who lives there.
It's not clear why, but she noted it's been a wet and warm year.
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