County reviews human case of St. Louis Encephalitis Virus


The San Joaquin County Public Health Service received a report of a human case of St. Louis Encephalitis Virus (SLEV) in the county. This is the first reported human case in San Joaquin County since 1973. The person with SLEV lives in Stockton and is recovering at home.

In August 2020, the San Joaquin County’s Mosquito-borne Disease Monitoring Program SLEV discovered the Lodi area in a group of collected mosquitos at ZIP Code 95240. This also marked the first environmental SLEV certification in San Joaquin County since 1973.

Stanislaus County also has collections of mosquitos that tested positive for SLEV this year. (For national SLEV data see:

The symptoms and transmission of SLEV are similar to those of West Nile Virus (WNV), but SLEV is less common in California than West Nile. WNV and SLEV both belong to the flavivirus family and are transmitted by mosquito bites. In 2020, there was only one report of WNV in San Joaquin County from an asymptomatic blood donor.

People can become infected with St. Louis encephalitis virus from the bite of an infected Culex mosquito. A mosquito becomes infected with SLEV after it has bitten a bird that is infected with the virus. SLEV is not spread from person to person.

Most people infected with SLEV have no symptoms or develop mild symptoms such as a fever, headache, or nausea that occur for up to two weeks after the sting. In a small number of people, SLEV can cause swelling or inflammation of the brain (encephalitis) that can lead to death. Older adults are most likely to get very sick if they are infected.

To reduce your risk of mosquito-borne diseases like St. Louis Encephalitis Virus and West Nile Virus, follow these recommendations:

Get rid of any stagnant water on your property where mosquitoes can breed.

When outdoors, apply insect repellants that contain EPA-registered agents such as DEET, picaridin, lemon eucalyptus oil, or IR3535. Follow the directions on the label.

Avoid spending time outdoors when the mosquitoes are most active, at dusk and especially in the first two hours after sunset.

Wear long pants, loose fitting / long sleeved shirts, and other protective clothing outdoors when mosquitoes are active.

Keep mosquitos out of your home with tight-fitting screens on doors and windows.

Report biting mosquitoes or significant mosquito infestations during the day in the Mosquito & Vector Control District of San Joaquin County at, (209) 982-4675 or 1-800-300-4675.

For more information, visit the following websites:

Centers for Disease Control and Prevention,;

California Department of Public Health,;

San Joaquin County Mosquito and Vector Control District,

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