West Nile Fever

The Cause

This is a flavivirus infection first identified in Uganda in 1937.

Distribution

Until 1999 it was only recognised to occur in Africa (widespread, including Egypt), Asia, Middle East (including Israel) and Europe (e.g. in Portugal, Hungary, Slovakia, Romania, Czech Republic, Italy, Spain, Greece, Austria, Albania, Poland and France. It is likely to occur elsewhere but may be difficult to confirm when laboratory testing is not available. Cases have also been identified in South India.

Since the 1990s, 3 trends have emerged:

  1. increased frequency of outbreaks
  2. increase in recognised severe illness in humans
  3. high death rates in birds which are an intermediate host usually accompanying human outbreaks.

In 1999 the virus appeared in New York City and has spread rapidly throughout North America with the highest numbers of confirmed human cases, in Illinois, Michigan, Ohio, Louisiana and Indiana throughout the summer months.

In Canada virus had been detected in five provinces (Nova Scotia, Quebec, Ontario, Manitoba, Saskatchewan), with confirmed human cases in Ontario and Quebec.

Transmission

It is usually transmitted to man by the bite of an infected Culex mosquito. The virus has occasionally been isolated from ticks. It infects a wide range of birds that can spread the virus through migration and also can act as a reservoir of infection, and sometimes other animals. There is no documented evidence as yet of person to person or animal to person transmission.

The Illness

  • Incubation period is 3-14 days.
  • Most infections are asymptomatic. Those who become ill, can experience fever, photophobic, conjunctivitis, lymphadenopathy and a maculopapular rash. A few case progress to a meningo-encephalitis and rarely, this can be fatal.
  • Virus can be isolated from the blood if facilities are available. Usually the diagnosis is made when high levels of antibodies are present in the blood.
  • There is no vaccine and no effective antiviral therapy. Treatment is symptomatic.

Recommendations for Travellers

Care should be taken to minimize mosquito bites especially during and after rainy seasons and when epidemics are known to be occurring. Medical advice can be sought if symptoms develop and are unresolving following travel to a risk area.

Many viruses can be detected in the blood during the viraemic phase of the illness and this may be a risk to blood recipients after a blood donation. A specific recommendation was issued early in the summer of 2003 stating that travellers should not donate blood for one month after returning from travel to the US because of fears about transmitting West Nile Virus.