Communicable disease factsheets

West Nile virus - Fact Sheet

West Nile virus (WNV) is transmitted to humans by infected mosquitoes (probably mainly by Culex sp.) and the reservoir for the virus is a wide range of birds.

Background

West Nile virus (WNV) is transmitted to humans by infected mosquitoes (probably mainly by Culex sp.) and the reservoir for the virus is a wide range of birds.

The majority of humans infected with the virus display only mild flu-like symptoms or no symptoms at all. However, some susceptible individuals may suffer severe illness due to encephalitis that may result in death.

WNV can infect horses, many types of birds, dogs, cats, bats, chipmunks, skunks, squirrels, rabbits and alligators. Infected people and horses do not usually pass on the virus (NB there has been transmission via infected human blood). The virus spreads geographically through transfer from bird to mosquito to bird, and by wind-dispersed mosquitoes.

WNV has been reported for many years in eastern Mediterranean and southern European countries. WNV has been present in the United States since 1999 when it was first identified in the New York City area. Expert opinion is that the WNV probably entered the United States through infected mosquitoes being carried on international aircraft.

The presence in Australia of a very close relative of WNV, Kunjin virus, may mitigate against the entry and spread of WNV in Northern Australia, although it may spread epidemically in southern Australia.

Human illness from WNV is rare, even in areas where the virus has been reported. It has been estimated that over 200,000 people have been infected with WNV in the USA. Most people who become infected with WNV will have either no symptoms or only mild ones. However, on rare occasions, infection can result in severe and sometimes fatal illness.

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Symptoms

Mild Infection

Most WNV infections are mild and often clinically inapparent. Approximately 20% of those infected develop a mild illness (West Nile fever). The incubation period is thought to range from 3 to 14 days. Symptoms generally last 3 to 6 days.

Reports from earlier outbreaks describe the mild form of WNV infection as a febrile illness of sudden onset often accompanied by malaise, headache, anorexia, myalgia, nausea, rash, vomiting, lymphadenopathy, eye pain.

The full clinical spectrum of West Nile fever has not been determined in the United States.

Severe Infection

Approximately 1 in 150 infections will result in severe neurological disease. The most significant risk factor for developing severe neurological disease is advanced age.
Encephalitis is more commonly reported than meningitis. In recent outbreaks, symptoms occurring among patients hospitalized with severe disease include fever, gastrointestinal symptoms, weakness, and a change in mental status.

A minority of patients with severe disease developed a maculopapular or morbilliform rash involving the neck, trunk, arms, or legs. Several patients experienced severe muscle weakness and flaccid paralysis.

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