St. Louis encephalitis virus (SLEV) belongs to the genus Flavivirus and, prior to the recent increase in West Nile virus, was the most important cause of viral encephalitis in the United States, with more than 10 000 cases in more than 50 outbreaks through 1990.

Flaviviruses are icosahedral single-stranded positive-sense RNA viruses. Birds serve as the amplifying host of SLEV, with Culex species of mosquitoes as the enzootic vectors and humans as dead-end hosts. Human infection typically occurs as outbreaks in late summer and fall. Illness is more common among elderly individuals and those with human immunodeficiency virus (HIV) infection. Cases have been documented in every US state, Canada, Mexico, and South America; however, infection is more frequently seen in the southern United States.

Illness is classified into 3 syndromes:

• Constitutional symptoms, fever, headache, myalgia, cough, sore throat, dysuria, urinary urgency
• Aseptic meningitis
• Fatal encephalitis

Symptoms:

• Asymptomatic in the vast majority (>99%)
• Malaise, fever, headache, and myalgia
• Often presents with respiratory (cough, sore throat) or urinary tract (dysuria, urgency) symptoms

Signs:

• Progressive lethargy, confusion, tremors, clumsiness, and ataxia
• Confusion, delirium or somnolence with weakness, seizures
• Occasionally, tremulousness with cerebellar and cranial nerve deficits
• Neurologic and psychologic sequelae are not uncommon
• Hematuria

Incubation period is 4-21 days with brief viremia before central nervous system penetration.

Case fatality rate of St. Louis encephalitis is higher (20%) in those older than 60 years of age.