St. Louis encephalitis (SLE) virus is spread to people by the bite of an infected mosquito. Most people infected with SLE virus do not have symptoms. Those people who do become ill may experience fever, headache, nausea, vomiting, and tiredness. Some people may develop neuroinvasive disease, such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes that surround the brain and spinal cord). In rare cases, long-term disability or death can occur.

There are no vaccines to prevent or medicines to treat SLE. You can reduce your risk of infection with SLE virus by using insect repellent, wearing long-sleeved shirts and long pants, and taking steps to control mosquitoes indoors and outdoors.

St. Louis encephalitis virus (SLEV) is a member of the family Flaviviridae, genus Flavivirus. Other medically important flaviviruses found in the Americas include West Nile virus and Powassan virus. SLEV has a single-stranded, positive-sense RNA genome. The virus particles are spherical and have a diameter of 40 nm.

St. Louis encephalitis (SLE) virus is spread to people through the bite of an infected mosquito. Mosquitoes become infected when they feed on birds that have the virus in their blood.

Birds that live in urban-suburban areas, such as the house sparrow, pigeon, blue jay, and robin, are common SLE virus hosts. The principal vectors are Culex species mosquitoes, including Cx. pipiens and Cx. quinquefasciatus in Eastern states, Cx. nigripalpus in Florida, and Cx. tarsalis and members of the Cx. pipiens complex in Western states.

People do not develop high enough levels of the virus in their blood to infect mosquitoes. As a result, people are considered “dead-end” hosts for SLE virus. However, the levels of SLE virus in people’s blood is enough to spread the infection through blood transfusions, though this very rarely occurs.

St. Louis Encephalitis Transmission

In nature, St. Louis encephalitis virus cycles between mosquitioes (primarily Culex species) and birds. Some infected birds can develop high levels of the virus in their bloodstream and mosquitoes can become infected by biting these birds.

People become infected with the virus when mosquitoes feed on infected birds and then bite people. People are considered dead-end hosts because unlike birds, they do not develop high enough levels of virus in their bloodstream and cannot pass the virus on to other biting mosquitoes.

Most people infected with St. Louis Encephalitis (SLE) virus do not have symptoms.

For people with symptoms, the time from infected mosquito bite to feeling sick ranges from 4 to 14 days. Symptoms usually start abruptly, with fever, headache, dizziness, nausea, and generalized weakness. They typically get worse over a period of several days to a week. Some patients recover after this period. Others develop signs of central nervous system infections, including stiff neck, confusion, disorientation, dizziness, tremors, and unsteadiness. Coma can develop in severe cases.

The disease is generally milder in children and young adults than in older adults. Among patients diagnosed with SLE, 5 to 20% die. The risk of dying increases with age.

SLE virus is difficult to isolate from clinical samples and almost all isolates have come from brain tissue or cerebrospinal fluid (CSF). In the absence of a sensitive and non-invasive virus detection method, serologic testing is the primary method for diagnosing SLEV infection. All SLEV disease cases should be reported to local public health authorities.

Antibody Testing

Laboratory diagnosis is generally accomplished by testing of serum or CSF to detect SLE virus-specific IgM and neutralizing antibodies. Immunoassays for SLE virus IgM antibodies are available commercially and through state public health laboratories and CDC. Initial serological testing is performed using IgM capture enzyme-linked immunosorbent assay (ELISA). Because of potential cross-reactivity to other flavivirus infections (e.g., West Nile virus, Powassan virus) or non-specific reactivity, a positive SLE virus IgM antibody test result should be confirmed by neutralizing antibody testing (plaque-reduction neutralization test [PRNT]) of serum specimens at a state public health laboratory or CDC.

Other Tests

Viral cultures and tests to detect viral RNA (e.g., reverse transcriptase-polymerase chain reaction [RT-PCR]) can be performed on serum, CSF, and tissue specimens that are collected early in the course of illness and, if results are positive, can confirm an infection. Immunohistochemistry (IHC) can detect SLE virus antigen in formalin-fixed tissue. Negative results of these tests do not rule out SLE virus infection. Viral culture, RT-PCR, and IHC can be requested through state public health laboratories or CDC.

There are no medications to prevent or treat SLE. Antibiotics are not effective against viruses, including SLE virus.

Rest, fluids, and over-the-counter pain medications may relieve some symptoms.

In severe cases, patients often need to be hospitalized to receive supportive treatment, such as intravenous fluids, pain medication, and nursing care.

St. Louis encephalitis (SLE) virus is spread to people primarily through the bite of an infected mosquito. Mosquitoes bite during the day and night. There is no vaccine to prevent SLE virus infection. The best way to prevent ­­­SLE is to protect yourself from mosquito bites. Use insect repellent, wear long-sleeved shirts and pants, treat clothing and gear, and take steps to control mosquitoes indoors and outdoors.

Protect Yourself and Your Family from Mosquito Bites

Use insect repellent

Use Environmental Protection Agency (EPA)-registered insect repellents with one of the active ingredients below. When used as directed, EPA-registered insect repellents are proven safe and effective, even for pregnant and breastfeeding women.

• DEET
• Picaridin (known as KBR 3023 and icaridin outside the US)
• IR3535
• Oil of lemon eucalyptus (OLE)
• Para-menthane-diol (PMD)
• 2-undecanone

Tips for babies and children

• Dress your child in clothing that covers arms and legs.
• Cover strollers and baby carriers with mosquito netting.
• When using insect repellent on your child:
  • Always follow label instructions.
  • Do not use products containing oil of lemon eucalyptus (OLE) or para-menthane-diol (PMD) on children under 3 years old.
  • Do not apply insect repellent to a child’s hands, eyes, mouth, cuts, or irritated skin.
    • Adults: Spray insect repellent onto your hands and then apply to a child’s face.

Tips for everyone

• Always follow the product label instructions.
• Reapply insect repellent as directed.
  • Do not spray repellent on the skin under clothing.
  • If you are also using sunscreen, apply sunscreen first and insect repellent second.

Natural insect repellents (repellents not registered with EPA)

• We do not know the effectiveness of non-EPA registered insect repellents, including some natural repellents.
• To protect yourself against diseases spread by mosquitoes, CDC and EPA recommend using an EPA-registered insect repellent.
• Choosing an EPA-registered repellent ensures the EPA has evaluated the product for effectiveness.

Wear long-sleeved shirts and long pants

Treat clothing and gear

• Use 0.5% permethrin to treat clothing and gear (such as boots, pants, socks, and tents) or buy permethrin-treated clothing and gear.
  • Permethrin is an insecticide that kills or repels mosquitoes.
  • Permethrin-treated clothing provides protection after multiple washings.
  • Read product information to find out how long the protection will last.
• If treating items yourself, follow the product instructions.
• Do not use permethrin products directly on skin.
• Watch the video, What You Need to Know About Permethrin.

Take steps to control mosquitoes indoors and outdoors

• Use screens on windows and doors. Repair holes in screens to keep mosquitoes outdoors.
• Use air conditioning, if available.
• Stop mosquitoes from laying eggs in or near water.
  • Once a week, empty and scrub, turn over, cover, or throw out items that hold water, such as tires, buckets, planters, toys, pools, birdbaths, flowerpots, or trash containers.
  • Check for water-holding containers both indoors and outdoors.

Prevent Mosquito Bites When Traveling Overseas

Remember to pack the following items

• EPA-registered insect repellents
• Long-sleeved shirts and long pants. Mosquitoes may bite through thin clothing.
• Clothing and gear (such as boots, pants, socks, tents) treated with 0.5% permethrin. You can treat your own items or purchase permethrin-treated clothing and gear.
• Mosquito net if you will be sleeping outside or when screened rooms are not available.

Keep mosquitoes out of your hotel room or lodging

• Choose a hotel or lodging with air conditioning or window and door screens.
• Use a mosquito net if you are unable to stay in a place with air conditioning or window and door screens or if you are sleeping outside.

Sleep under a mosquito net

Sleep under a mosquito net if you are outside or when screened rooms are not available. Mosquitoes can live indoors and bite during the day and night.

• Buy a mosquito net at your local outdoor store or online before traveling overseas.
• Choose a mosquito net that is compact, white, rectangular, with 156 holes per square inch, and long enough to tuck under the mattress.
• Permethrin-treated mosquito nets provide more protection than untreated nets.
  • Permethrin is an insecticide that kills mosquitoes and other insects.
  • To determine if you can wash a treated mosquito net, follow the label instructions.

Use mosquito nets correctly

Mosquito nets can be hung above a bed or crib to protect against mosquito bites. Adults and children should sleep under a mosquito net if air conditioned and screened rooms are not available.

When using a mosquito net:

• Tuck the net under the mattress to keep the mosquitoes out.
• Tuck netting under the crib mattress or select a mosquito net long enough to touch the floor.
• Pull the net tightly to avoid choking hazards for young children. Check label instructions for additional information.
• Hook or tie the sides of the net to other objects if they are sagging in towards the sleeping area.
• Check for holes or tears in the net where mosquitoes can enter.
• DO NOT hang the net near any candles, cigarettes, or open fires, as it can catch on fire.
• DO NOT sleep directly against the net, as mosquitoes can still bite through holes in the net.

What is St. Louis encephalitis?

St. Louis encephalitis (SLE) is a rare disease that is caused by a virus spread by infected mosquitoes. SLE virus is one of a group of mosquito-transmitted viruses that can cause inflammation of the brain (encephalitis).

How do people get infected with SLE virus?

SLE virus is transmitted by the bite of an infected mosquito. SLE virus is not transmitted directly from person to person.

Where and when have most cases of SLE occurred?

Cases have been reported throughout the country, but periodic outbreaks and epidemics have primarily occurred in the Mississippi Valley and along the Gulf Coast and more recently in the Southwest. In temperate areas of the United States, SLE cases occur primarily in the late summer or early fall. In southern states, cases can occur year-round.

Who is at risk for infection with SLE virus?

Anyone bitten by a mosquito in an area where the virus is circulating can get infected with SLE virus. The risk is highest for persons who engage in outdoor work and recreational activities and those living in low-income areas. Elderly persons are at increased risk of severe disease if they are infected.

How soon do people get sick after getting bitten by an infected mosquito?

It takes 4 to 14 days after the bite of an infected mosquito to develop symptoms of SLE.

What are the symptoms of SLE?

Most people who are infected with SLE virus have no symptoms or only mild non-specific flu-like illness. However, in some individuals, especially the elderly, SLE virus can cause serious illness that affects the central nervous system. Symptoms often include fever, headache, stiff neck, disorientation, and altered level of consciousness. Coma, convulsions, and paralysis may also occur.

How is SLE diagnosed?

Diagnosis is based on tests of blood or spinal fluid. These tests typically look for antibodies that the body makes against the viral infection.

What is the treatment for SLE?

There is no specific treatment for SLE. Antibiotics are not effective against viruses. Severe illnesses are treated by supportive therapy which may include hospitalization, respiratory support, IV fluids, and prevention of other infections.

How can people reduce the chance of getting infected with SLE virus?

Prevent mosquito bites. There is no vaccine or preventive drug.

• Use insect repellent containing DEET, picaridin, IR3535 or oil of lemon eucalyptus on exposed skin and/or clothing. The repellent/insecticide permethrin can be used on clothing to protect through several washes. Always follow the directions on the package.
• Wear long sleeves and pants when weather permits.
• Have secure, intact screens on windows and doors to keep mosquitoes out.
• Eliminate mosquito breeding sites by emptying standing water from flowerpots, buckets, barrels, and other containers. Drill holes in tire swings so water drains out. Empty children’s wading pools and store on their side after use.

What should I do if I think a family member might have SLE?

If you or anyone in your household has symptoms that are causing you concern, consult a healthcare provider for proper diagnosis.