General information

Human infection is most often the result of bites from infected mosquitoes. The largest outbreaks occurred in Greece, Israel, Romania, Russia and USA. Outbreak sites are on major birds migratory routes. In its original range, WNV was prevalent throughout Africa, parts of Europe, Middle East, West Asia, and Australia. Since its introduction in 1999 into USA, the virus has spread and is now widely established from Canada to Venezuela.

Infection with WNV is either asymptomatic (no symptoms) in around 80% of infected people, or can lead to West Nile fever or severe West Nile disease.About 20% of people who become infected with WNV will develop West Nile fever. Symptoms include fever, headache, tiredness, and body aches, nausea, vomiting, occasionally with a skin rash (on the trunk of the body) and swollen lymph glands.

WNV cases have been reported from all around the world. WNV infection is usually mild and self-limiting. It is calculated that 80% of those infected with WNV are asymptomatic. When symptomatic, and after an incubation period of 2–15 days, there can be fever, fatigue, headache, malaise, muscle pain, nausea, vomiting, lymphadenopathy and a transient rash. Neuroinvasive disease (meningitis, encephalitis, or acute flaccid paralysis) develops in <1% of infected persons. Direct detection of the virus from serum or cerebrospinal fluid can be carried out by molecular techniques or cell culture. Viremia is only present for a brief period of time (up to 7 days). Serology constitutes the mainstay of WNV diagnosis. The mean time from the detection of viral RNA to IgM has been reported as 4 days, and to IgG detection as 8 days. If serum is collected within 8 days of illness onset, the absence of detectable virus specific IgM does not rule out the diagnosis of WNV infection. In neuroinvasive WNV disease specific IgM is almost always detectable by the time when the symptoms appear. Detectable IgM can persist up to one year.

The symptoms of severe disease (also called neuroinvasive disease, such as West Nile encephalitis or meningitis or West Nile poliomyelitis) include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. It is estimated that approximately 1 in 150 persons infected with the West Nile virus will develop a more severe form of disease. Serious illness can occur in people of any age, however people over the age of 50 and some immunocompromised persons (for example, transplant patients) are at the highest risk for getting severely ill when infected with WNV.

The incubation period is usually 3 to 14 days.

West Nile virus can be diagnosed by a number of different tests:

• IgG antibody sero-conversion (or significant increase in antibody titers) in two serial specimen collected at a one week interval by enzyme-linked immunosorbent assay (ELISA);
• Ig M antibody , Ig G 
• neutralisation assays;
• viral detection by reverse transcription polymerase chain reaction (RT-PCR) assay, and
• virus isolation by cell culture.

IgM can be detected in nearly all cerebrospinal fluid (CSF) and serum specimens received from WNV infected patients at the time of their clinical presentation. Serum IgM antibody may persist for more than a year.

Effective prevention of human WNV infections depends on the development of comprehensive, integrated mosquito surveillance and control programmes in areas where the virus occurs !