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St. Louis encephalitis virus


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 (WNV), with similar symptoms and transmission, but is less common in California. SLE virus is one of a group of mosquito-transmitted viruses that can cause inflammation of the brain (encephalitis). 

Press Release: 
Long Beach Health Department Confirms Human Case of St. Louis Encephalitis (11.9.23)


If you have any questions about SLEV, please call our Public Health Information hotline:
Telephone with solid fill (562) 570-7907

Frequently Asked Questions

  • What are the signs and symptoms of St. Louis encephalitis virus?

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

    Although everyone is at risk for SLEV, people over 50 years of age and those with underlying health conditions are at greater risk of more serious symptoms. Severe SLEV affects the brain and nervous system and can cause stiff neck, confusion, dizziness and sometimes death. 

  • How do people get St. Louis encephalitis virus?

    SLEV is spread to people by the bites of infected culex mosquitoes.

    It is not spread from person to person.

  • How is St. Louis encephalitis virus treated?

    Currently, there is no specific medicine to treat St. Louis Encephalitis Virus (SLEV).

    The key to managing SLEV is by working with your doctor to address the symptoms and receive appropriate care. Your doctor may recommend medications to help alleviate symptoms, and getting plenty of rest and staying well-hydrated can be beneficial.

  • How can I protect myself and others from St. Louis encephalitis virus?

    The best way to protect oneself from SLEV and other diseases spread by mosquitoes is to avoid mosquito bites. Take these steps to reduce mosquito populations and protect yourself from mosquito bites, including: 

    • Use a mosquito repellent with DEET, IR3535, picaridin or products containing oil of lemon eucalyptus (do not use oil of lemon eucalyptus on children younger than 3 years old). 
    • Wear loosely fitted, long-sleeved shirts and long pants. 
    • Eliminate standing water around clogged rain gutters, rain barrels, discarded tires, buckets, watering troughs, and anything that holds water for over a week; aedes mosquitos can breed in a very small amount of water, including a bottle cap. 
    • Keep weeds, vines, hedges and grass trimmed; adult mosquitos like to rest in vegetation. 
    • Change water in pet dishes, birdbaths and other small containers weekly. 
    • Ensure that swimming pools, spas and ponds are properly maintained. 
    • Report neglected swimming pools in your neighborhood to the Health Department’s Vector Control Program.