Health officials say West Nile virus activity spikes with mosquitoes in summer months going into fall, and this week, that fact is surely on display.
States across the U.S. — from Wyoming to Colorado, Kansas to Delaware — are sounding alarm bells after detecting the virus among groups of residents this week, with some saying it's their worst season yet.
The virus, shortened as WNV, is the leading cause of mosquito-borne disease in the U.S. according to the CDC. Mosquitoes become infected whey then feed on infected birds, then the infected bugs spread it to humans through bites.
Most people infected with the virus don't feel sick. But about 1 in 5 infected people develop fever symptoms, and about 1 out of 150 infected people develop serious or fatal illness, with no specific medicines or antibiotics able to treat it.
As of Sept. 12, there have been 879 cases across 43 states this year.
Here's everything we know about where the virus is popping up.
Wyoming is one state seeing an overactive season for West Nile virus.
This week, the state's Department of Health stressed the need for residents to try preventing mosquito bites, after 20 cases of WNV were reported in nine different counties there so far this year.
One of those cases ended in death for an older woman, and 12 of the cases involved a more severe, neuroinvasive type of the illness. The CDC says less than 1% of infected people typically develop neuroinvasive disease, which usually manifests as meningitis, encephalitis or acute flaccid paralysis.
"This is clearly Wyoming's most active West Nile virus season in at least a decade and it does not appear to be over yet," said Courtney Tillman, an epidemiologist with the state's health department. "It remains important to avoid mosquito bites to help prevent illness with this virus, which can sometimes be quite serious."
Tillman said there have probably been more cases that haven't been confirmed with testing.
She warns residents to use the "5 Ds" for prevention: Avoid time outside at dawn and dusk, dress in more clothing, drain standing water and use insect repellent containing deet.
Colorado is in the midst of one of its deadliest West Nile virus seasons since the illness first hit the U.S. in 1999, health care system UCHealth said.
A chart, pictured above, from its health department shows the state has seen more than 80 cases of WNV at one time this year, hitting numbers way above last year's rates.
Dr. Michelle Barron, senior medical director of infection prevention and control for UCHealth, said the high rates could be because the state has had a wet spring and summer, which increases mosquito population, and that its residents often are spending time outdoors.
But these harsh numbers have been constant in the state the past few years, with 20 deaths in 2022 and 11 the year before.
Barron echoed similar prevention efforts as Wyoming health experts and told anyone feeling sick with symptoms to seek medical assistance for the virus immediately.
The Kansas health department has issued a high-risk warning for WNV infections in all of its regions other than the southeast, which is at moderate risk.
This comes after the state has seen 22 human cases so far this year, including 17 neuroinvasive cases and three deaths, the health department said. Its agriculture department has also reported seven equine cases this year.
"We're right in the middle of our peak time frame for WNV transmission here in Kansas, and with more widespread virus activity this year than in the previous several years, it's important to take mosquito bite prevention measures to protect yourself, your family and livestock against all mosquito-borne illnesses," said Dr. Erin Petro, state public health veterinarian.
WNV cases in horses can be fatal, but infection is preventable with vaccinations. WNV is not directly contagious between horses and humans.
On Tuesday, Delaware announced it saw its first case of WNV this year, and it came in a package of three.
The Delaware Public Health Laboratory identified the virus in three men aged 50 and older, all living in New Castle County.
All three had to be hospitalized for the infection, meaning their cases were severe according to the CDC's definition.
The state's public health department is investigating the origin of the virus, but it said the cases were likely contracted locally. It also noted the case increase coincides with an increase in WNV activity in the state's sentinel chicken monitoring program, which monitors chickens to track mosquito-borne virus spread in the birds. Delaware is seeing an increase in infected chickens for this time of year.
The state's Department of Natural Resources and Environmental Control is now increasing mosquito population surveillance efforts around where the infected people live. It may also conduct local control operations to try to prevent further spread, it said.
Georgia is also seeing the virus in a three-pack.
A man in his 20s was confirmed to have the virus in DeKalb County, which is within Atlanta. And health officials there are also investigating two additional unrelated cases in other areas of the county.
DeKalb County hasn't seen a confirmed human case of WNV since 2020.
Crews have been treating the affected areas by spraying low-lying land and storm drains with larvicide, which stops young mosquitoes from becoming flying adults.
Maryland also saw its first human WNV case of the year this week.
An adult living in Baltimore's Eastern Shore region is currently recovering after testing positive for the virus.
"We are in the season when the West Nile virus can spread in Maryland," said Dr. Nilesh Kalyanaraman, deputy secretary for public health services in the state. "We urge people to be vigilant and take steps to avoid infection and eliminate standing pools of water where mosquitoes can breed. Our teams are continuing to monitor mosquito activity across the state."
Mosquitoes in Maryland were found to be carrying the virus in July.
Texas' Galveston County Health District confirmed a case of WNV this week in one of its residents.
It was confirmed in a female in her 60s.
Utah saw its fourth confirmed case of the year this week, with the individual still hospitalized with the infection.
It's the first human case of the virus in Salt Lake County, and its become neuroinvasive.
"There are a growing number of mosquitoes carrying the disease, so it is now especially important that people protect themselves from mosquito bites, particularly in the hours from dusk to dawn," said Dr. Angela Dunn, the health department's executive director.
A New Jersey resident died of West Nile this week, prompting alarms from health officials who have detected the virus in three different counties.
The state has reported seven other WNV cases this season, with six of them having to be hospitalized.
The state's health department said WNV activity is significantly higher this year compared to five years ago.
"With continued rainfall and warm weather, we can expect the mosquito season and the potential for disease transmission to extend well into the fall," said Shawn M. LaTourette, New Jersey environmental protection commissioner. "Managing our mosquito population, through our mosquito control agencies and individual efforts to eliminate suitable habitats for the insects, plays an important role in protecting public health. Get rid of standing water in your yard and cover or turn over any empty containers that can hold water for several days."
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