Produced by the Royal College of Physicians of Edinburgh and Royal College of Physicians and Surgeons of Glasgow

West Nile Virus

  • Professor D Williams, Hennepin County Medical Centre, Minneapolis, USA


In recent years, there have been serious outbreaks of West Nile Virus in the US, where the outbreaks caused a number of fatalities, and in some parts of Europe. Prof David Williams provides an overview of the virus, its symptoms, treatment and prevention.

Key Points

  • West Nile Virus is named after the West Nile district of Uganda where it was discovered.
  • Large birds are probably the main hosts, and bites from infected Culex mosquitos transmit the virus to humans.
  • Person-to-person spread does not occur other than by blood transfusions, organ donation and transplacentally.
  • Infected humans are usually asymptomatic. About 1 in 5 suffer a feverish illness with headache and joint pains, and 1 in 150 suffer nervous system disease usually encephalitis or aseptic meningitis.
  • The virus is susceptible to ribavirin and interferon alpha 2B in the laboratory, but the effectiveness of these drugs in patients is not known.
  • Prevention is by public control of mosquitos and personal measures against being bitten.

Declaration of interests: No conflict of interests declared


The West Nile Virus is an RNA virus belonging to the Flavivirus group. It is classified in the Japanese encephalitis virus serocomplex. The virus was first isolated and identified in 1937 in the West Nile district of Uganda. It was first linked with encephalitis in 1957 during an outbreak in Israel. Since the mid 1990s the frequency and apparent clinical severity of West Nile Virus outbreaks have increased. Outbreaks have occurred recently in Romania, Russia and the US. In the USA in 2002 and 2003 there were 2900 and 2700 cases respectively of meningoencephalitis with more than 200 deaths in both years. The first cases in the US occurred in the Northeast in 1999, but how or why the virus appeared in the US remains unknown. It has been postulated that travel and commerce may have played a role.


The virus can infect humans, birds, and various mammals. However, large birds appear to be the principal hosts, as they are able to survive long-term viremia. Dead crows have become harbingers of human cases, giving new meaning to the phrase, ‘as the crow flies’ (or falls). The mosquito most commonly implicated in transmission belongs to the genus Culex. The mosquito and the virus are able to survive through the winter months.

Humans are infected following a bite by an infected mosquito typically in the late summer months. The virus is located in the mosquito’s salivary glands. Person-to-per