Progress of the West Nile virus in the United States

The West Nile virus quickly spread across North America after its introduction in 1999. Current theory is that it was introduced accidentally by an air traveller who was infected before he arrived at New York. The disease spread quickly through infected birds. Mosquitoes spread the disease to mammals. It was mainly noted in horses but also appeared in a number of other species. The first human cases usually followed within three months of the first appearance of infected birds in the area except where cold weather interrupted the mosquito vectors. The rest of the article will deal only with human cases.

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Differences in surveillance and reporting between health departments and generally increased surveillance as the disease spread cause some problems in direct comparison of the number of cases and the mortality rate. The true mortality rate is thought to be much lower because most cases are so mild they go undiagnosed. Some estimates put severe cases at only 1% of all cases. Most but not all mild cases go undiagnosed. Some mild cases are discovered during blood donation screening. 1,039 West Nile-tainted blood donations were discovered between 2003 and mid-2005. 30 cases of West Nile from blood transfusion were known, the majority from 2002 before blood screening was instituted.

The numbers for 1999-2007 are:

The following maps show the progress of human cases of West Nile across the United States. States are colored according to the percentage of all U.S. West Nile cases they represented in that year. Zero percent (no cases) is colored white; less than 1% is blue; between 1% and 5% is green; between 5% and 10% is yellow, and greater than 10% of cases in a year is red.