SALT LAKE CITY —

The Salt Lake County Health Department announced on Wednesday that the St. Louis encephalitis virus, which is related to West Nile virus, has been found in mosquitoes in Utah.

This year marks the first time the U.S. Centers for Disease Control and Prevention has confirmed the virus’s presence in northern Utah.

Previously, it had been detected in southwestern Utah in 2022 and 2023, and in Millard County in 2024, with reports of infected birds in Utah County dating back to 2005.

A Salt Lake County resident was infected with St. Louis encephalitis virus, or SLEV, in 2016, though the source of that infection remains unknown.

The health department noted that mosquitoes from various locations within the county have tested positive for the virus. As mosquitoes can travel several miles, officials are advising residents to take precautions against bites.

Symptoms of St. Louis encephalitis are akin to those of West Nile virus. Most people infected may not show any symptoms, but those who do can experience fever, headache, vomiting, and fatigue. In more severe cases, infection can lead to neuroinvasive symptoms such as tremors, disorientation, or paralysis.

Like West Nile virus, there is a potential for long-term disability or even death from St. Louis encephalitis, though such cases are rare.

Mary Hill, the epidemiology bureau manager for the county health department, emphasized the importance of considering SLEV as a potential diagnosis for patients exhibiting relevant symptoms.

Health officials clarified that the virus isn’t transmitted directly between people, except through blood transfusions. It is primarily spread by mosquitoes that feed on infected birds. Currently, there are no vaccines or treatments available, so the best prevention is to avoid mosquito bites.

To minimize the risk of bites, the health department recommends:

• Using EPA-registered repellent.
• Wearing long sleeves and pants in the evening hours.
• Draining standing water around your property and clearing debris from gutters.
• Cutting back tall grass and weeds, as adult mosquitoes tend to rest in these areas during the day.

Residents are urged to remain cautious until the first frost arrives.

Recently, the Utah Department of Health and Human Services reported that St. Louis encephalitis virus was found in three mosquito pools, which is about 0.36% of those tested statewide, while West Nile virus was identified in 64 pools, or 7.55% of the tested pools.

This statewide report also indicated three human cases of West Nile virus in Utah this year, with two occurring in the Bear River health district and one in Salt Lake County. Additionally, three cases have been reported in horses, with two in Utah County and one in the Bear River health district.