West Nile virus activity is on the rise in some northern California counties, according to the Sacramento-Yolo Mosquito and Vector Control District.
Sacramento-Yolo counties
Three dead birds and 13 mosquito samples tested positive for the virus on Aug. 13, “the highest number of positive results reported in a single day this year,” the agency said in a news release.
While the birds and mosquito samples were found in different areas, most of the day’s activity was in the town of Davis “and other rural areas in Yolo County,” officials said in the release.
Routine monitoring helped the district detect the “increase in virus activity and overall mosquito populations due to the very warm temperatures of recent days,” officials said. Hot weather speeds up the mosquito lifecycle and can increase the risk of the virus to residents.
As of Aug. 13, 29 mosquito samples and 17 dead birds have tested positive for the virus in Sacramento County, officials said. Sixteen mosquito samples have tested positive in Yolo County.
The virus first cropped up in the capital region in June, when mosquitoes collected near Gibson Ranch in Elverta tested positive for it, the Sacramento Bee previously reported.
“Mosquito and West Nile virus season is definitely underway, and it’s important for residents to be vigilant in preventing mosquitoes from breeding in their backyards,” District Manager Gary Goodman said in a statement. “Mosquito control is a collaborative effort, and we encourage everyone to do their part.”
Residents had to ramp up mitigation efforts in 2024 after record-breaking storms drenched California in 2023 — and West Nile cases rose 980% in a one-year period, the Sacramento Bee previously reported.
Two people died and 54 people contracted the virus in Sacramento County in 2023, the Bee reported.
Placer-El Dorado counties
Nearby areas are also facing the surge. One week after the virus was confirmed in Sacramento mosquitoes, the Placer Mosquito and Vector Control District confirmed its first positive case from mosquito samples collected from a rural area in the western part of the county, the Sacramento Bee previously reported.
Like the first positive sample in Sacramento, the Placer County mosquito sample also came from an agricultural area, one west of the city of Roseville near Phillip Road.
“With the ongoing warm temperatures this season, we anticipate we will see more West Nile virus activity after this initial detection,” District Manager Joel Buettner previously said in a statement. “We urge residents to take protective measures, including wearing an EPA-registered repellent, to prevent mosquito bites. Our team is actively conducting surveillance and treatments to help reduce the risk of mosquito-borne disease transmission to our community.”
The virus has not been detected in El Dorado County this year. In 2023, a bird sample tested positive in early August, then a person contracted it about two weeks later, the Sacramento Bee reported.
A person infected with the virus died in November 2023, and the virus killed seven people across the four-county region of Sacramento, El Dorado, Placer and Yolo in 2023 — making it the deadliest year for the disease in the past decade prior, the Bee reported.
First human West Nile virus infection confirmed in Northern California
How does West Nile virus spread? What are the symptoms?
West Nile virus is a mosquito-borne illness that can affect people, horses and birds. Mosquitoes become infected after feeding on birds that carry the virus, then spread it.
A vaccine is available for horses, but there’s no vaccine — or cure — for people.
Human cases are typically mild, but about 1 in 150 can lead to serious illness or death.
Most people who are infected don’t experience symptoms, according to the U.S. Centers for Disease Control and Prevention.
About 1 in 5 people infected with West Nile will have symptoms “like fever, headache, body aches,” including joint pains, as well as nausea and vomiting, or rash days after they’re bitten by an infected mosquito, according to the California Department of Health.
In severe cases, people might experience encephalitis or meningitis, which can be deadly.
One bite is all it takes for the virus to spread, so effective prevention is the best way to protect yourself.
How to stay safe
Here are ways to stay safe, according to the Placer Mosquito and Vector Control District:
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Use insect repellent with EPA-registered ingredients like DEET, Picaridin, IR3535 or Oil of Lemon Eucalyptus.
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Cover your skin with long sleeves and pants.
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Get rid of standing water on your property.
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Avoid going outdoors at dawn and dusk when mosquitoes are most active.
The mosquito and vector control districts have also ramped up surveillance and treatments targeting both larvae and adults, including ground fogging in areas where positive mosquitoes and birds were found, officials said.
“While it has been a relatively mild season for WNV so far, we are in the middle of summer and conditions can change quickly,” Goodman, the Mosquito and Vector Control district manager in Sacramento, said in a statement. “We expect to see continued activity over the next few weeks. Protecting yourself is key—always apply an effective mosquito repellent when spending time outdoors.”
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