Scientists in the United States are puzzled that the infection seems, so far, to be moving north rather than the warmer south, as previously expected. Others point out, however that the infection is following the well-known Atlantic flyway for migrating birds. The West Nile virus is one of a group that causes encephalitis or inflammation of the brain. Closely related viruses are the St. Louis encephalitis and Japanese encephalitis. There is currently no vaccine against West Nile Virus encephalitis.
Many people show no symptoms.
A study in Egypt reported that 50 per cent of the people exposed to the virus developed antibodies but showed no symptoms. However, other people, especially the elderly and those with weakened immune systems, can become very ill. If illness occurs, it usually happens within five to15 days of being bitten by an infected mosquito. There were epidemics in Israel in the 1950s and South Africa in 1974. Fatality rates can range from three per cent to 15 per cent of cases.