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St. Louis encephalitis

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St. Louis encephalitis i a mosquito-borne viral encephalitis

St. Louis Encephalitis (SLE) virus
St. Louis Encephalitis (SLE) virus


  • Saint Louis encephalitis (SLE) is a viral disease spread to people by the bite of an infected mosquito.
  • Most people infected with SLE virus have no apparent illness.

Other names, and abbreviations

St. Louis encephalitis virus is abbreviated to SLEV.

Clinical features

Initial symptoms of those who become ill include fever, headache, nausea, vomiting, and tiredness.

Risk factors for severe disease

Severe neuroinvasive disease involving encephalitis, occurs more commonly in older adults.

Culex mosquito life cycle
Culex mosquito life cycle


St. Louis encephalitis virus (SLEV) is maintained in a mosquito-bird-mosquito cycle, with periodic amplification by peridomestic birds and species mosquitoes.

Vertebrate hosts

  • Wild birds are the primary vertebrate hosts.
  • Birds sustain inapparent infections but develop viremias (i.e., virus in their blood) sufficient to infect the mosquito vectors.
  • Birds that are abundant in the urban-suburban environment, such as the house sparrow, pigeon, blue jay, and robin, are principally involved.


  • The principal vectors are Cx pipiens and Cx quinquefasciatus in the east, Cx nigripalpus in Florida, and Cx tarsalis and members of the Cx pipiens complex in western states.
St Louis encephalitis case distribution in the US
St Louis encephalitis case distribution in the US

Human transmission

  • Humans and domestic mammals can acquire SLEV infection, but are dead-end hosts, no human to human transmission reported.


  • Less than 1% of St. Louis encephalitis virus (SLEV) infections are clinically apparent and the vast majority of infections remain undiagnosed.

Incubation period

  • The incubation period for SLEV disease (the time from infected mosquito bite to onset of illness) ranges from 5 to 15 days.


  • Onset of illness is usually abrupt, with fever, headache, dizziness, nausea, and malaise.
  • Signs and symptoms intensify over a period of several days to a week.

Clinical course

  • Some patients spontaneously 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 than in older adults.
  • About 40% of children and young adults with SLEV disease develop only fever and headache or aseptic meningitis; almost 90% of elderly persons with SLEV disease develop encephalitis.

Fatality rate

  • The overall case-fatality ratio is 5 to 15%. The risk of fatal disease also increases with age.


  • Preliminary diagnosis is often based on the patient’s clinical features, places and dates of travel (if patient is from a non-endemic country or area), activities, and epidemiologic history of the location where infection occurred.

Lab diagnosis

  • Laboratory diagnosis of arboviral infections is generally accomplished by testing of serum or cerebrospinal fluid (CSF) to detect virus-specific IgM and neutralizing antibodies.
  • In acute SLEV neuroinvasive disease cases, cerebrospinal fluid (CSF) examination shows a moderate (typically lymphocytic) pleocytosis.
  • CSF protein is elevated in about a half to two-thirds of cases. Computed tomography (CT) brain scans are usually normal; electroencephalographic (EEG) results often show generalized slowing without focal activity.
  • In fatal cases, nucleic acid amplification, histopathology with immunohistochemistry and virus culture of autopsy tissues can also be useful.


  • No vaccine against SLEV infection or specific antiviral treatment for clinical SLEV infections is available. Patients with suspected SLE should be evaluated by a healthcare provider, appropriate serologic and other diagnostic tests ordered, and supportive treatment provided.


  • The most effective way to prevent infection from ­­­Saint Louis Encephalitis virus is to prevent mosquito bites. Mosquitoes bite during the day and night.
  • Use insect repellent, wear long-sleeved shirts and pants, treat clothing and gear, and take steps to control mosquitoes indoors and outdoors.


  • In rare cases, long-term disability or death can result.


  • There are no vaccines to prevent nor medications to treat SLE.

Latest research - St. Louis encephalitis

Clinical trials


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