St. Louis Encephalitis Virus

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).

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:

• Argentina
• Canada
• The Caribbean
• Mexico

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.

What are the symptoms of SLEV?

St. Louis encephalitis virus symptoms are usually mild, if you even have any symptoms at all. Mild symptoms include:

• Feeling very tired (fatigue)
• Fever
• Headache
• Muscle weakness
• Nausea

Severe SLEV causes brain inflammation. Symptoms of brain inflammation include:

• Confusion, disorientation or other changes to your mental function
• Stiff neck
• Uncontrollable movements (tremors)

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.

How long does it take St. Louis encephalitis virus symptoms to develop?

St. Louis encephalitis virus symptoms usually develop between four and 14 days after a bite from an infected mosquito (incubation period).

What is St. Louis encephalitis caused by?

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.

Risk factors

St. Louis encephalitis virus is rare. But you may be more at risk of an SLEV infection if you:

• Live or work in areas where there’s an SLEV outbreak, particularly in areas where mosquitos live and breed, like bodies of water or wooded areas
• Spend a lot of time outside in areas where there’s an outbreak, especially between dawn and dusk when mosquitoes are more active

You have a greater chance of developing severe SLEV symptoms if you:

• Are 50 or older
• Have a weakened immune system
• Have diabetes

Complications of St. Louis encephalitis virus

Even with successful treatment, SLEV may cause long-term complications, including:

• Changes in your behavior, including anxiety, depression, and irritability
• Difficulty walking
• Problems remembering things (memory loss)

SLEV may also cause an intellectual disability in children.

How doctors diagnose St. Louis encephalitis virus

Healthcare providers will first review your medical history and symptoms and conduct a physical exam. They may ask questions such as:

• Have you traveled recently?
• Have you had any recent mosquito bites?
• When did the bites happen?
• When did you first notice symptoms?

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.

How is SLEV treated?

There isn’t a cure for St. Louis encephalitis virus. But healthcare providers may recommend the following to treat mild symptoms:

• Acetaminophen (Tylenol^®), aspirin (Bayer^®, Bufferin^®), ibuprofen (Advil®, Motrin IB^®) or naproxen sodium (Aleve^®) to treat headaches
• Drinking fluids, especially water, to help prevent dehydration
• Medications to treat nausea and vomiting, like bismuth subsalicylate (Pepto-Bismol^®) and phosphorated carbohydrate solution (Emetrol^®)

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.

Can antivirals treat St. Louis encephalitis virus?

No, antiviral medications can’t treat SLEV. Over-the-counter medications can treat mild symptoms.

Is there a vaccine for SLEV?

No, there aren’t any vaccines to help protect you from SLEV.

Recovery time

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.

When should I see my healthcare provider?

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:

• Are my symptoms serious?
• Will I require hospitalization?
• When should I start to feel better?
• How do I manage my symptoms at home?
• Should I be aware of any serious symptoms?
• Should I schedule a follow-up appointment?

Can St. Louis encephalitis virus be prevented?

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:

• Avoiding areas in which there’s an SLEV outbreak
• Avoiding standing water, such as ponds and swamps, and emptying standing water in birdbaths, buckets, watering cans, trash cans and so on
• Making sure your windows and doors have tight-fitting screens
• Using insect repellant when you go outside
• Wearing thick, long-sleeved shirts, pants and socks when you spend time outside

What can I expect if I have this condition?

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.

What is the mortality rate for St. Louis encephalitis?

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).

Why is it called St. Louis encephalitis?

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.

Can I donate blood if I have a St. Louis encephalitis virus infection?

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.