Mosquito-borne arbovirus human diseases in North America cause a spectrum of disease from a mild viral syndrome to encephalitis.

Epidemiology

• Incidence
  • Seasonal occurrence - April to October
• Ages - extremes of age (old, young children) more likely to acquire severe disease
• Transmission - bite of an insect

Organism

• Bunyavirus - California encephalitis (CE)
  • La Crosse virus
    • Primarily in upper Mississippi River Valley
  • Jamestown Canyon virus
    • Upper midwestern states, including New York
  • Snowshoe Hare virus
    • Southern Canada
  • Mosquito species
    • Aedes triseriatus
• Togavirus - Eastern equine encephalitis (EEE)
  • Present in the eastern half of U.S.
  • Mosquito species
    • Aedes, Coquillettidia and Culex
• Flavivirus - St. Louis encephalitis
  • Present in nearly all of the U.S.
  • Mosquito species
    • Culex pipiens and Culex quinquefasciatus
      • Gulf Coast, Ohio and Mississippi Valley
    • Culex nigripalpus
      • Florida
    • Culex tarsalis
      • Western states
• Togavirus - Western equine encephalitis (WEE)
  • Present in western and midwestern U.S.
  • Mosquito species
    • Culex tarsalis
• Flavivirus - West Nile encephalitis (WNV)
  • Endemic in U.S. 
  • Mosquito species
    • Culex tarsalis

Clinical Presentation

• Nonspecific illness consisting of:
  • Fever
  • Headache
  • Myalgias
  • Nausea, anorexia
  • Respiratory effects
  • Sore throat
• Acute encephalitis
  • Lasts from a few days to months, with slow and sometimes incomplete recovery
  • Central nervous system involvement by arboviruses is very similar, with the exception of a more abrupt onset and shorter, more severe course found with Eastern equine encephalitis (EEE)
• WNV
  • Acute flaccid paralysis can also occur in infections from West Nile virus
    • Attributed to peripheral demyelinization process or anterior myelitis
    • Meningoencephalitis occasionally complicates disease
    • Mortality ranges between 5-20% in affected patients
      • Up to 70% in affected patients older than 75 years

Treatment

• Supportive
• Recovery may be prolonged

Diagnosis

• Laboratory testing
  • Serum antibody testing
  • Viral isolation is seldom productive; the majority of human cases are diagnosed by serologic means
  • Significant change in sequentially timed specimens (acute and convalescent)
    • The best evidence for infection is a significant change (fourfold difference) in titer on 2 appropriately timed specimens done at the same laboratory
    • IgG usually negative in 1st 2 weeks
    • Single positive titer may represent past or current infection

Differential Diagnosis

• Herpes encephalitis
• Brain abscess
• Viral meningitis
• Bacterial meningitis
• Brain tumor