Skip to main content

St. Louis Encephalitis Virus

May contain: virus particle, pattern, and fractal

Source: California Department of Public Health

What is St. Louis encephalitis virus (SLEV)?

SLEV is a virus spread by mosquitoes that can make people sick. Most people infected with SLEV don’t have symptoms, but in rare cases, SLEV can lead to swelling or inflammation of the brain (encephalitis) and lead to death. The symptoms and transmission of SLEV are similar to West Nile virus (WNV), but SLEV is less common in California than WNV.

How do people get SLEV?

People get SLEV 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.

Who is at risk of being infected with SLEV?

People who spend time outdoors when mosquitoes are active are more likely to get bitten by an infected mosquito and get SLEV. The mosquitoes that spread SLEV are most active during dawn and dusk. Anyone can be infected with SLEV, but not everyone gets sick if they are infected. Older adults are the most likely to get very sick if they are infected.

What are the symptoms of SLEV?

Most people that are bitten by a mosquito that is infected with SLEV will have no symptoms and most likely won’t know they have been infected with SLEV. Some people may develop mild symptoms, including fever, headache, or nausea up to two weeks after being bitten. Rarely, SLEV can cause more serious symptoms that affect the brain or central nervous system. These symptoms include stiff neck, confusion, and dizziness. Paralysis, coma, and sometimes death can result. Older adults, people with diabetes, and people with weakened immune systems are more likely to get very sick from SLEV.

Where can I get more information about SLEV?

CDPH SLEV webpage 

U.S. Centers for Disease Control and Prevention St. Louis Encephalitis webpage