West Nile virus in the United States
The West Nile virus quickly spread across the United States after the first reported cases in Queens, New York in 1999. The virus is believed to have entered in an infected bird or mosquito, although there is no clear evidence. 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.
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 reported number of infected in 2009 was 720, but the estimated total number of infected the same year was 54,000. 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. In addition, some more severe but non-neuroinvasive cases are not reported to the CDC. 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. In the first ten years since the virus arrived in the U.S., over 1,100 deaths occurred with human cases reported from every U.S. state except Maine, Alaska and Hawaii. (Animal cases have been occasionally found in Maine and in Puerto Rico.)
In 2012, there was a widespread outbreak with the second-highest total case numbers. Maine and Puerto Rico reported one case each.
Maps of progression
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.
Locations of reported cases by year 2001-2011
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- USGS Disease Maps
- "West Nile Virus Activity --- United States, 2009" (MMWR 59(25), July 2, 2010)
- West Nile activity maps, Centers for Disease Control and Prevention (CDC), 2009
- "West Nile Virus". CDC. Retrieved 21 May 2013.