Progress of the West Nile virus in the United States
Encyclopedia
The West Nile virus
quickly spread across North America
after its introduction 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. Mosquito
es 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.
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 . 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.)
The numbers for 1999-2009 are:
* As of september 20, 2011
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.
West Nile virus
West Nile virus is a virus of the family Flaviviridae. Part of the Japanese encephalitis antigenic complex of viruses, it is found in both tropical and temperate regions. It mainly infects birds, but is known to infect humans, horses, dogs, cats, bats, chipmunks, skunks, squirrels, domestic...
quickly spread across North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
after its introduction 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. Mosquito
Mosquito
Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...
es 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.
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 . 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.)
The numbers for 1999-2009 are:
| Cases | Deaths | Mortality Rate of reported cases |
|
|---|---|---|---|
| 1999 | 62 | 7 | 11% |
| 2000 | 21 | 2 | 10% |
| 2001 | 66 | 10 | 15% |
| 2002 | 4156 | 284 | 7% |
| 2003 | 9862 | 264 | 3% |
| 2004 | 2539 | 100 | 4% |
| 2005 | 3000 | 119 | 4% |
| 2006 | 4269 | 177 | 4% |
| 2007 | 3623 | 124 | 3% |
| 2008 | 1356 | 44 | 3% |
| 2009 | 720 | 32 | 4% |
| 2010 | 1021 | 57 | 6% |
| 2011* | 268 | 13 | 5% |
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.