St. Louis encephalitis virus detected in Salt Lake County mosquitoes for first time
Aug 20, 2025, 12:52 PM
FILE: Mosquito. (Bear River Health Department)
(Bear River Health Department)
SALT LAKE CITY – The Salt Lake County Health Department announced Wednesday that a virus has been found in Salt Lake County mosquitoes. The department urged residents to take precautions to stay healthy.
St. Louis encephalitis virus was detected in mosquitoes “from multiple locations throughout Salt Lake County,” Nicholas Rupp with the Salt Lake County Health Department said in a release.
The department said 2025 is the first time the virus has been detected in mosquitoes in northern Utah.
“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 department said in a release.
The health department said there are no vaccines or medicines to prevent or treat St. Louis encephalitis virus.
“The only way to reduce your risk of infection is by preventing mosquito bites,” the department said.
Signs and symptoms of the virus
The department said symptoms and transmission of the virus are “similar” to West Nile virus, but less common.
While the department said most people infected with the virus do not have symptoms, there are some symptoms to look out for. These include:
- fever
- headache
- nausea
- vomiting
- tiredness
Some people can develop neuroinvasive disease, “which causes high fever, neck stiffness and neurological symptoms like tremors, disorientation and paralysis,” the department said. “In rare cases, it may (cause) long-term disability or death.”
How the virus circulates
The department said this virus mainly moves between mosquitoes and birds, but people can be transferred to people when mosquitoes feed off an infected bird, then bite humans.
“The virus is not transmitted from person-to-person, except rarely by blood transfusion,” the department said.
The department said the Salt Lake County’s mosquito abatement districts will work to provide “mosquito surveillance and control countywide to help minimize the threat of mosquito-borne illnesses to county residents and visitors.”
In order to prevent bites, the department recommends using an EPA-registered mosquito repellent, wear long clothing after dusk, drain standing water in yards, ensure screen windows and doors are in good condition, keep weeds and grass cut short, and keep roof gutters clear of debris.
The department noted the virus was found in mosquitoes inhabiting southwestern Utah in 2022 and 2023, as well as Millard County in 2024.
