A disease known to have struck at least 20 people and killed at least three persons in the New York City area in the summer of 1999 may not be St. Louis encephalitis, as was first suspected, but West Nile fever, a disease that has never before been encountered in the Western Hemisphere.
This fits with the fact that the West Nile virus was discovered in a number of birds that died in and near the Bronx Zoo in New York during the same time period.
West Nile virus was first found in Uganda in 1937. It occurs in parts of Africa and Asia and infrequently in Europe and the Middle East. It has never been seen in birds or people in the Western Hemisphere, according to the U.S. Centers for Disease Control and Prevention.
In 1996, West Nile virus caused 90,000 cases of fever and 17 deaths in Romania. But the closest West Nile fever has hitherto come to the U.S. is in France where it was found in the 1960's. St. Louis encephalitis is also not native to New York. In the U.S. it rarely ventures outside the Southeast.
The confusion between the West Nile virus and St. Louis encephalitis is understandable. The two diseases are closely related. The viruses responsible for them belong to the same family (flavivirus). Both diseases are mosquito-borne. They are carried by the same type of mosquito (Culex). And the route is the same: from birds to mosquitoes to people.
The rare fatal cases generally involve infants and small children (under age 5), the aged and people with an impaired immune system.
Dangers of Global Travel
Global travel today can transport exotic viruses to new areas. If a virus enters a new area, the entire mosquito population may be susceptible, putting everyone living in that area at risk.
Tests are underway to determine definitively whether the New York infections were caused by St. Louis encephalitis or the West Nile virus. Whichever disease it turns out to be, insecticide control of local mosquito populations will be key to prevention of disease outbreaks.
Since this story was written (on Sept. 24, 1999), the virus in New York City has been confirmed as the West Nile virus. The autumnal migration of birds has now raised concern that birds from New York carrying the West Nile virus may go southward and carry the threat of West Nile fever with them.