Long Beach confirms 1st human case of mosquito-borne St. Louis Encephalitis since 1984


The Long Beach Department of Health and Human Services confirmed the city’s first case of St. Louis Encephalitis — a mosquito-borne virus with symptoms akin to West Nile virus — on Thursday, Nov. 9.

It’s the first documented case of St. Louis Encephalitis, or SLEV, in Long Beach since 1984, the health department said Thursday. The city’s announcement came a little more than a week after it confirmed a case of dengue fever.

The person who was infected is recovering after being hospitalized, the announcement said, and no other cases have been identified in the city to date.

SLEV is rarely seen in Los Angeles or in California generally. The state has identified 12 human cases of the disease in 2023 as of early November, according to the health department.

SLEV is a disease that is caused by the St. Louis Encephalitis virus and is spread to people by the bites of infected culex mosquitoes. It is in the same virus family as West Nile virus, with similar symptoms and transmission, but is less common in California. It is not spread from person to person.

Most people who are infected will not experience symptoms, but those who do typically have mild symptoms that may include fever, headache and nausea.

People older than 50 and those with underlying health conditions are at higher risk of severe SLEV, according to the health department’s website, which can affect the brain, cause stiff neck, confusion, dizziness and sometimes death.

The disease is spread by culex mosquitos, and isn’t transmissible between people.

“We are working diligently with health care providers to educate the community to prevent more cases of SLEV,” Mayor Rex Richardson said in the announcement. “Mosquito control is a shared responsibility and residents must take an active role in reducing the threat of mosquito-borne illnesses in their neighborhoods.”

SLEV-carrying mosquitos are most active at dusk and dawn.

Residents are encouraged to take protective measures to prevent mosquito bites, such as using adequate mosquito repellent, wearing long-sleeved shirts and long pants, and getting rid of standing water.

“The first confirmation of SLEV in Long Beach should serve as a reminder that we need to protect ourselves against mosquitoes,” said City Health Officer Dr. Anissa Davis said. “The health department encourages everyone to continue reporting issues regarding mosquito control in their area.”

More information about the disease and mosquito prevention measures is available longbeach.gov/SLEV.


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