St. Louis encephalitis is a virus that spreads through infected mosquito bites. Symptoms are usually mild — you may not even have any. But it’s possible to develop severe symptoms, including serious brain inflammation. There’s no cure for the virus. But medications can help ease your mild symptoms. Severe symptoms may require hospitalization.
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St. Louis encephalitis virus (seynt loo-es en-suf-uh-LI-tis VAHR-ruhs) is a virus that spreads through mosquito bites — specifically the Culex species of mosquitoes. It can cause inflammation in your brain (encephalitis).
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Most people get St. Louis encephalitis virus (SLEV) from an infected mosquito bite. The virus doesn’t spread from person to person through bodily contact (hugging, shaking hands, sharing drinking glasses, kissing and so on). But it’s possible for it to spread through blood transfusion.
SLEV isn’t common. Most cases occur in the U.S., particularly in the Eastern, Central and Southwest states. But the following areas also have reported cases:
According to the U.S. Centers for Disease Control and Prevention (CDC), there were a little over 300 reported cases of SLEV between 2003 and 2024. In the U.S., cases usually occur during warmer months — June through October — but they may occur throughout the year in warmer climates.
SLEV symptoms vary. In mild cases, you may not have any symptoms, or you may have flu-like symptoms.
St. Louis encephalitis virus symptoms are usually mild, if you even have any symptoms at all. Mild symptoms include:
Severe SLEV causes brain inflammation. Symptoms of brain inflammation include:
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If you have any confusion or mental function changes, get to an emergency room as soon as possible.
Severe SLEV can also cause coma and death.
St. Louis encephalitis virus symptoms usually develop between four and 14 days after a bite from an infected mosquito (incubation period).
Mosquitoes get SLEV when they feed on infected birds. Infected mosquitoes then transfer SLEV when they bite your skin. Humans are a “dead-end host” of the virus. That means you can’t transfer the infection to another person because the virus doesn’t exist in high enough levels in your bloodstream. If someone has SLEV, they don’t have to self-isolate at home, wear a mask or clean and disinfect surfaces.
In rare cases, the infection may spread through blood transfusion.
St. Louis encephalitis virus is rare. But you may be more at risk of an SLEV infection if you:
You have a greater chance of developing severe SLEV symptoms if you:
Even with successful treatment, SLEV may cause long-term complications, including:
SLEV may also cause an intellectual disability in children.
Healthcare providers will first review your medical history and symptoms and conduct a physical exam. They may ask questions such as:
If a provider suspects you have SLEV, they’ll recommend a serological test. A serological test is a type of blood test that looks for antibodies, specifically immunoglobulin M (IgM). Your immune system makes antibodies to help protect you from infections, including SLEV. Providers may also recommend a spinal tap (lumbar puncture) to test your cerebrospinal fluid (CSF) for IgM.
There isn’t a cure for St. Louis encephalitis virus. But healthcare providers may recommend the following to treat mild symptoms:
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If you have severe symptoms, you need to stay at a hospital for monitoring. If SLEV causes a dangerous amount of inflammation in your brain, a surgeon may need to remove part of your skull (craniotomy) to relieve pressure.
No, antiviral medications can’t treat SLEV. Over-the-counter medications can treat mild symptoms.
No, there aren’t any vaccines to help protect you from SLEV.
Your recovery depends on the severity of the infection. In mild cases, your symptoms may go away within a few weeks. But some symptoms may take a few months to go away completely.
Reach out to a healthcare provider if you develop symptoms of St. Louis encephalitis virus, especially if you have recent mosquito bites. A provider can evaluate your symptoms, order tests and recommend treatment.
You may wish to ask a provider the following questions:
There isn’t a vaccine to prevent St. Louis encephalitis virus. The best way to prevent SLEV is to protect yourself from mosquito bites. You can avoid mosquito bites by:
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Many people who get St. Louis encephalitis virus don’t have any symptoms or their symptoms are mild. You’ll likely recover before the virus causes inflammation in your brain. But it can be life-threatening, especially if you’re over 50 or have diabetes or a compromised immune system. If you have symptoms, schedule an appointment with a healthcare provider. They can tell you what to expect.
The mortality (death) rate for people who have SLEV is between 5 and 20 out of every 100 people. The rate is higher if you’re 50 or older or if you have high blood pressure (hypertension).
St. Louis encephalitis got its name because the virus was first identified in St. Louis, Missouri, U.S.A., in 1933. Healthcare professionals reported over 1,000 cases during the outbreak.
No, you can’t donate blood if you have an active SLEV infection. But you should be able to donate blood about four months after the infection goes away. Be sure to let a healthcare provider know if you’ve recently had an SLEV infection before donating blood.
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Most of the time, mosquitoes are just pests. But sometimes, they carry diseases that they transfer to you when they feed on your blood. St. Louis encephalitis virus is one of those diseases. For most people, your risk of getting very sick is low. But it’s still possible to have severe or even dangerous symptoms. The best way to protect yourself is to avoid mosquito bites, especially in areas where SLEV outbreaks are more likely to occur. Reach out to a healthcare provider if you have bites and develop symptoms of SLEV. They can help treat the symptoms and, if necessary, monitor you to make sure they don’t get worse.
Last reviewed on 02/28/2025.
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