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
- Asymptomatic in the vast majority (>99%)
- Malaise, fever, headache, and myalgia
- Often presents with respiratory (cough, sore throat) or urinary tract (dysuria, urgency) symptoms
- 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
Case fatality rate of St. Louis encephalitis is higher (20%) in those older than 60 years of age.