First Case of Rare Virus Detected in SLC Mosquitoes

The Salt Lake County Health Department has confirmed the first detection of St. Louis encephalitis virus (SLEV) in mosquitoes in northern Utah. The virus, which is closely related to West Nile, was found in several mosquito pools during routine monitoring (KSL).

SLEV is not new to Utah. It appeared in southwestern counties in 2022 and 2023, Millard County in 2024, and was detected in birds in Utah County as far back as 2005 (Deseret News). However, this is the first confirmed presence in Salt Lake County mosquitoes. Health officials say the discovery highlights the importance of ongoing surveillance, but stress there is no need for alarm.

Most people infected with SLEV experience no symptoms. When illness does occur, it can include fever, headache, nausea, and fatigue. Rarely, the virus may cause severe complications such as inflammation of the brain, tremors, or paralysis. There is no vaccine or specific treatment.

Like West Nile virus, SLEV spreads when mosquitoes feed on infected birds and then bite humans. It cannot spread directly from person to person. Salt Lake County has also reported West Nile virus activity this summer, including one human case requiring hospitalization (Axios).

The best protection remains prevention. Officials recommend using mosquito repellent, wearing long sleeves and pants outdoors at dusk and dawn, and removing standing water around homes where mosquitoes can breed. Residents are also encouraged to maintain screens on windows and doors and keep yards free of overgrown weeds where mosquitoes rest.

Health authorities will continue monitoring and mosquito abatement efforts until the first hard freeze ends the season.