St. Louis Encephalitis Virus Detected in Mosquitoes in Salt Lake County

SALT LAKE COUNTY—The Salt Lake County Health Department (SLCoHD) announced today that St. Louis encephalitis virus (SLEV) has been detected in mosquitoes in Salt Lake County.

Mosquitoes from multiple locations throughout Salt Lake County have tested positive for SLEV. Because mosquitoes can travel several miles, everyone in the region should take precautions against mosquito bites to avoid exposure to this and other mosquito-borne viruses.

The symptoms and transmission of SLEV are similar to West Nile virus (WNV), but SLEV is less common in the United States than WNV.

Most people infected with SLEV do not have symptoms. Infected people who do become ill may experience fever, headache, nausea, vomiting and tiredness. Some people who contract the virus may develop neuroinvasive disease, which causes high fever, neck stiffness and neurological symptoms like tremors, disorientation and paralysis. In rare cases, it may case long-term disability or death.

"Now that we've confirmed St. Louis encephalitis virus is in our local mosquito population, Salt Lake County health care providers should consider it in the differential diagnosis for patients presenting with compatible symptoms," said Mary Hill, SLCoHD epidemiology bureau manager.

SLEV circulates in the environment between mosquitoes (primarily the Culex species) and birds. People become infected with the virus when mosquitoes feed on infected birds and then bite people. The virus is not transmitted from person-to-person, except rarely by blood transfusion.

There are no vaccines to prevent or medicines to treat St. Louis encephalitis. The only way to reduce your risk of infection is by preventing mosquito bites.

Salt Lake County's three mosquito abatement districts will continue providing mosquito surveillance and control countywide to help minimize the threat of mosquito-borne illnesses to county residents and visitors, but it remains essential for people to protect themselves from mosquito bites until the season's first hard freeze:

• Use an EPA-registered mosquito repellent with DEET, picaridin or other active ingredient recommended by EPA; follow package directions about application.
• After dusk, wear long sleeves and pants
• Drain standing water in yards (old tires, potted plant trays, pet dishes, toys, buckets, etc.).
• Keep roof gutters clear of debris.
• Clean and stock garden ponds with mosquito-eating fish or mosquito dunks; contact your local mosquito abatement district for assistance.
• Ensure door and window screens are in good condition so mosquitoes cannot get inside.
• Keep weeds and tall grass cut short; adult mosquitoes look for these shady places to rest during the hot daylight hours.

SLEV was detected in mosquitoes in southwestern Utah in 2022 and 2023, in mosquitoes in Millard County in 2024 and in birds in Utah County in 2005. 2025 is the first time SLEV has been confirmed to be in northern Utah mosquitoes or animals since recordkeeping of mosquito-borne viruses began in 2003. A Salt Lake County resident was infected with SLEV in 2016, but the source of that infection was never determined.