Salt Lake County mosquitoes test positive for St. Louis encephalitis virus

SALT LAKE COUNTY (ABC4) — The Salt Lake County Health Department has detected St. Louis Encephalitis virus in Salt Lake County mosquitoes, marking the first confirmed incident for the county

According to the Salt Lake County Health Department (SLCoHD), St. Louis encephalitis virus (SLEV) has been detected in mosquitoes from multiple locations in Salt Lake County. While a Salt Lake County resident contracted SLEV from an unknown source in 2016, this is the first time that the virus has been found in Northern Utah mosquitoes or animals in recorded history.

St. Louis encephalitis virus is similar to West Nile virus, which has been found in Utah residents this year, but is less common. In addition to the 2016 human case, it was previously identified in mosquitoes in southwestern Utah in 2022 and 2023 and in Millard County in 2024, and in Utah County birds in 2005.

SLEV Symptoms

Mary Hill, SLCoHD epidemiology bureau manager, says that the positive identification of St. Louis encephalitis virus in local mosquito populations has some implications for health care providers.

Hill says, “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.”

Not everyone infected with SLEV will be symptomatic. However, those who are infected may experience fever, headache, nausea, vomiting, and tiredness, according to SLCoHD. Additionally, some people may contract neuroinvasive disease, which can cause high fever, neck stiffness and neurological symptoms.

In severe cases, people may experience tremors, disorientation and paralysis. In rare incidents, it can cause long-term disability or death.

Unfortunately, there is no vaccine or medicine to prevent and treat St. Louis encephalitis, according to SLCoHD. The only way to reduce infection risk is to prevent mosquito bites.

SLEV Prevention

SLCoHD’s three mosquito abatement districts regularly test local mosquito populations to minimize the spread of St. Louis encephalitis virus. However, they are also urging people to prevent mosquito bites however possible until it is colder.

They are also recommending individuals follow several tips to prevent mosquito bites:

• 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 is rarely transmitted person-to-person, and individuals are most likely to become infected from being bitten by an infected mosquito. Preventing mosquito bites can be essential to avoid contracting SLEV and other viruses, like West Nile virus.