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St. Louis Encephalitis: Transmission, Symptoms, Treatment, Causes, Prevention, Risk Factors

What is St. Louis Encephalitis?

St. Louis Encephalitis (SLEV) is a rare disease where in its severe stages there is inflammation of the brain (encephalitis). St. Louis Encephalitis is caused by the Mansonia pseudotitillans mosquito virus. The virus of St. Louis encephalitis is linked to the virus which causes Japanese encephalitis virus and belongs to the Flaviviridae subgroup. St. Louis encephalitis primarily is found in the United States; however, some cases have been reported in Mexico and Canada. Most of the infections produce mild illness, which comprise of symptoms like headache and fever.

Many patients suffering from St. Louis encephalitis do not experience any obvious illness. Some of the initial symptoms of patients who become ill consist of headache, fever, tiredness, nausea and vomiting. In severe cases of infection, where the infection spreads towards the central nervous system, patient experiences high fever, headache, neck stiffness, disorientation, stupor, tremors, spastic paralysis, occasional convulsions and coma. Aged individuals are at an increased risk to develop a more severe case of St. Louis encephalitis. The virus of St. Louis encephalitis gets transmitted to human beings through an infected mosquito bite.

Severe cases of St. Louis encephalitis commonly affect older adults where they have neuro-invasive disease comprising of inflammation of the brain and sometimes long-term disability or death can also occur. St. Louis encephalitis has no specific treatment and management and supportive therapy of the patient is done depending on the symptoms. A person can decrease their risk of contracting St. Louis encephalitis by wearing protective clothing, using insect repellent and staying indoors when the mosquitoes are at their most active.

What is St. Louis Encephalitis?

There are no vaccines for St. Louis encephalitis and currently this disease has no specific treatment. Symptoms of St. Louis encephalitis are managed by supportive therapy.

Geographical Location of St. Louis Encephalitis

St. Louis encephalitis is mainly found in the United States. Cases of epidemics and periodic outbreaks have mainly occurred along the Gulf Coast and in the Mississippi Valley. SLEV also mainly occurs in the late summer or early fall. However, in the southern regions, this can occur all through the year. Some cases of SLEV have also occurred in Mexico and Canada.

A person, who lives in an area where the virus is circulating and there are mosquitoes present, is at a greater risk for getting infected with St. Louis Encephalitis (SLEV). Older individuals are at a higher risk for contracting more severe type of St. Louis Encephalitis (SLEV). The risk for developing this infection is very high for people who work outdoors and those who live in low-income areas.

Causes, Transmission & Risk Factors of St. Louis Encephalitis

The cause of St. Louis Encephalitis (SLEV) is a Mansonia pseudotitillans mosquito borne virus that is also associated with the Japanese encephalitis virus. This virus is a part of the Flaviviridae subgroup. The transmission of St. Louis encephalitis is through the bite of an infected mosquito. St. Louis encephalitis does not get transmitted directly from one person to another.

Signs & Symptoms of St. Louis Encephalitis

Patient starts to experience symptoms of St. Louis encephalitis about to 5 to 15 days (incubation period) after getting bitten by an infected mosquito. Many patients with this viral infection do not experience any symptoms or only have mild symptoms, which resemble flu, such as fever and headache. However, some patients, especially the elderly patients, tend to suffer from more serious infection which affects the central nervous system and produces symptoms, which include headache, fever, nausea, stiff neck, dizziness, altered level of consciousness, tiredness and disorientation. In severe cases, patients can also have convulsions, paralysis and coma.

The severity of the signs and symptoms increases over a period of many days to a week. In some cases, there is spontaneous recovery of patients after this phase passes. Whereas, some patients will develop signs of central nervous system infection, which includes neck stiffness, confusion, dizziness, disorientation, unsteadiness, tremors and coma.

Diagnosis of St. Louis Encephalitis

The primary diagnosis is usually made based upon the patient’s symptoms, activities, dates and places of travel and epidemiologic history of region where the infection occurred. Diagnosis is also made by doing certain tests, which include cerebrospinal fluid (CSF) testing which reveals moderate pleocytosis. There also can be an increase in the CSF protein. CSF and blood tests also help in detecting virus-specific IgM and neutralizing antibodies produced by the body to fight against this viral infection. CT scan is often normal. Electroencephalographic (EEG) tests displays generalized slowing without any focal activity. In severe cases of this infection, histopathology with immunohistochemistry, nucleic acid amplification can be useful. These tests are done only in some specialized laboratories.

Treatment for St. Louis Encephalitis

As mentioned previously, no specific treatment is available for St. Louis encephalitis. There is also no vaccine against St. Louis encephalitis. Treatment comprises of supportive therapy in severe cases, which includes hospitalization, IV fluids, respiratory support and prevention of other infections. As this is a viral infection, antibiotics will be ineffective and there is no specific antiviral medicine available. According to a study, using interferon-alpha2b may help in decreasing the severity of complications.

Prevention of St. Louis Encephalitis

As there is no preventive medicine or vaccine available for this virus, prevention for St. Louis Encephalitis and other mosquito borne infections can be achieved by preventing mosquito bites through which this virus is transmitted to the humans. Some of the methods through which mosquito bites can be prevented are:

  • Wearing long sleeves and pants when outdoors.
  • Using mosquito repellants which contain DEET, IR3535, picaridin or lemon eucalyptus oil.
  • There should be secure screens placed on the doors and windows to prevent the mosquitoes from entering the home.
  • Avoid going outdoors when the mosquitoes at their most active, i.e. between dusk and dawn.
  • Mosquito breeding areas should be evacuated by removing the standing water from the buckets, barrels, flower pots etc.


  1. Centers for Disease Control and Prevention. (2021). St. Louis Encephalitis. https://www.cdc.gov/sle/
  2. Mayo Clinic. (2021). Encephalitis. https://www.mayoclinic.org/diseases-conditions/encephalitis/symptoms-causes/syc-20356136
  3. Pan American Health Organization. (2020). St. Louis Encephalitis. https://www.paho.org/en/documents/st-louis-encephalitis
  4. MedlinePlus. (2021). Mosquito Bites. https://medlineplus.gov/mosquitobites.html
Pramod Kerkar, M.D., FFARCSI, DA
Pramod Kerkar, M.D., FFARCSI, DA
Written, Edited or Reviewed By: Pramod Kerkar, M.D., FFARCSI, DA Pain Assist Inc. This article does not provide medical advice. See disclaimer
Last Modified On:September 1, 2023

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