EMed

West Nile Fever

Background

West Nile Virus (WNV), spread by infected mosquitos, can produce a spectrum of disease including severe meningoencephalitis. The USA had it's first recognized outbreak in 1999; it has since spread across North America and Canada.

WNV has recently been confirmed in two Irish holidaymakers suffering from febrile illness on return from holidays in the Algarve (Portugal). The National Disease Surveillance Centre has updated its advice for clinicians, advising that testing for WNV be considered in patients with suggestive symptoms - including those without a history of recent travel in North America.

  • Few infected people will be able to recall any mosquito bite
  • The incubation period is 3-14 days
  • Most WNV infections are clinically unapparent. 
  • About 20% develop mild illness.
    • Febrile illness of sudden onset with malaise, anorexia, vomiting, eye pain, headache, myalgia, rash and lymphadenopathy
  • Severe Infection
    • About 1 in 150 infections result in severe neurological disease with a case fatality of 4-14%

Beware

Please beware of suggestive illness in individuals over the age of 50 (regardless of their travel history) but to be have a high index of suspicion of those with a history of recent travel to North America and to request diagnostic testing accordingly.

A case of encephalitis or meningitis, especially in patients aged >50:

Encephalitis

Any person with suspected viral encephalitis with all of the following :

  1. Fever over 38°C and;
  2. Altered mental state and/or other evidence of cortical involvement (e.g., focal neurological findings, seizures) and;
  3. CSF pleocytosis with predominant lymphocytes and/or elevated protein with a negative Gram stain and culture and;
  4. No alternative microbiological cause identified, e.g., herpes simplex virus

Any person with suspected viral (aseptic) meningitis with all of the following criteria:

  1. Fever over 38° C and
  2. Headache, stiff neck and/or other meningeal signs and
  3. CSF pleocytosis with predominant lymphocytes and/or elevated protein and a negative Gram stain and culture and
  4. No alternative microbiological cause identified, e.g., enterovirus
  • In order to identify WNV and other commoner causes of viral CNS disease, the NVRL advises that a sample of CSF and of blood be taken for virological analysis
  • More complete clinical information on WNV can be found on the  at National Disease Surveillance Centre's website
  • Information to clinicians is also available on the NDSC site

Content drafted by Dr Íomhar O' Sullivan.