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County Reports Human Case Of St. Louis Encephalitis Virus

San Joaquin County Public Health Services has 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 Mosquito and Vector Control District’s mosquito-borne disease surveillance program discovered SLEV in a group of collected mosquitoes in zip code 95240; the Lodi area. This was also the first environmental detection of SLEV in San Joaquin County since 1973.

Stanislaus County has also had mosquito collections that tested positive for SLEV this year. (For statewide SLEV data see:

The symptoms and transmission of SLEV are similar to that of the West Nile virus (WNV), but SLEV is less common in California than West Nile. WNV and SLEV are both in the flavivirus family and transmitted through mosquito bites. In 2020, there has only been one report of WNV in San Joaquin County, in an asymptomatic blood donor.

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

Most people infected with SLEV do not have any symptoms or develop mild symptoms including fever, headache, or nausea, which occur up to two weeks after being bitten. In a small number of people, SLEV can cause swelling or inflammation of the brain (encephalitis) which may lead to death. Older adults are the most likely to get very sick if they are infected.

To help reduce the risk of getting mosquito-transmitted diseases such as St. Louis encephalitis virus and West Nile virus, follow these recommendations:

Eliminate all standing water on your property where mosquitoes can breed.

When outdoors, apply insect repellent that contains EPA registered active ingredients including DEET, Picaridin, oil of lemon eucalyptus, or IR3535. Follow the label’s instructions.

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

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

Exclude mosquitoes from your home with tight-fitting screens on doors and windows.

Report daytime biting mosquitoes or significant mosquito infestations to San Joaquin County Mosquito & Vector Control District 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,