St. Louis Encephalitis

St. Louis Encephalitis (SLEV) is:


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

Signs & Symptoms


Many persons infected with SLEV have no apparent illness. People with mild illness often have only a headache and fever.

More severe disease is marked by:
  • Coma
  • Disorientation
  • Fever
  • Headache
  • Neck stiffness
  • Occasional convulsions (especially in infants)
  • Spastic (but rarely flaccid) paralysis.
  • Stupor
  • Tremors
The risk of severe disease generally increases with age.