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West Nile fever

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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  • West Nile virus causes sporadic cases and outbreaks of human and equine disease in Europe (western Mediterranean and southern Russia in 1962-64, Belarus and Ukraine in the 1970s and 1980s, Romania in 1996-97, Czechland in 1997, and Italy in 1998)
  • vector for transmission - mosquitoes
  • environmental factors, including human activities, that enhance population densities of vector mosquitoes (heavy rains followed by floods, irrigation, higher than usual temperature, or formation of ecologic niches that enable mass breeding of mosquitoes) could increase the incidence of West Nile fever
  • Clinical features:
    • incubation period of 3-15 days
    • in general asymptomatic but may cause a mild flu-like illness
      • a severe illness will develop in less than 1% of those infected - generally aseptic meningitis or acute encephalitis associated with fever; both presentations may be associated with a flaccid paralysis which is characteristic. Other distinguishing features may include lymphadenopathy, rash, abdominal pain, nausea, diarrhoea, conjunctivitis and pharyngitis
      • severe and symptomatic disease are both more common in older patients with fatalities most often occurring in those over 50 years of age
  • West Nile Fever in the USA
    • there have been 153 confirmed human cases and four deaths (reported up to August 2003) (2) in 2003
  • there has been advice (2) that, despite the risk of transmission-associated West Nile virus infection occurring in the UK being low, potential blood donors who have travelled to any part of the United States and Canada should not give blood for 28 days after leaving the region (or for 28 days after the onset of any symptoms consistent with West Nile Virus infection)


  1. Emerg Infect Dis. 1999 Sep-Oct;5(5):643-50.
  2. CMO's Update (August 2003). Department of Health, UK.

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