thumb|300px|[[United States Geological Survey|USGS map of the United States showing reported cases of West Nile virus infection by county in 2012. Cumulative total of the entire country: 1,590 cases. (Current as of Aug 28, 2012).]]
thumb|300px|U.S. West Nile virus (WNV) neuroinvasive disease incidence as of Sep 4, 2012
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. Since the virus has become widely established in the U.S., an average of 130 deaths a year have occurred.
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. It is believed that the elderly or people with weak immune systems are most vulnerable to serious illness or death if bitten by a mosquito infected by West Nile. Most 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 highest death toll and second-highest total case numbers. Maine and Puerto Rico reported one case each, the first time the disease was reported in those places.
Discovery in the United States
In August 1999, the New York State Department of Environmental Conservation (NYSDEC) began receiving reports of large die-offs of crows in parks across New York City. On August 9, dead wild crows were discovered outside both the Queens Zoo and the Bronx Zoo, prompting head pathologist Tracey McNamara to begin investigating what was going on. At the same time, Dr. Deborah Asnis, an infectious disease specialist at Flushing Hospital Medical Center in Queens, noticed two male patients who were suffering from similar, unusual symptoms. The two patients, aged 60 and 75 respectively, were exhibiting sudden paralysis in their arms and legs, as well as disorientation and high fevers. Lab testing also showed elevated numbers of white blood cells within the spinal fluid of both males.
During the 2012 West Nile virus outbreak in Texas, 1,868 cases were reported. Male patients, persons >65 years of age, and minorities were at highest risk for neuroinvasive disease. At total of 1,868 cases including 844 (45%) WNND (West Nile neuroinvasive disease) cases and 89 deaths (case-fatality rate 5%). Dates of onset ranged from May 1, 2012, through December 6, 2012. The outbreak peaked during week 33 (mid-August) with 225 reported cases, which is historically the same peak for all reported WNV cases in Texas during 2002–2011. The median time from date of symptom onset to date of official report to TxDSHS was 27 days (range 6–274 days). Of the 254 counties in Texas, 135 (53%) reported a WNV case. The overall incidence rate for the state was 7.8 cases per 100,000 population. Almost half of the cases were reported from the northeastern quadrant of the state, including the Dallas/Fort Worth metroplex (902 [48%] cases): Dallas (396 [21%]), Tarrant (259 [14%]), Collin (64 [3%]), and Denton (183 [10%]) counties. These 4 counties had a combined incidence rate of 16 cases per 100,000 population.
Cases
{| class="wikitable sortable"
!
!Cases
!Deaths
!Mortality Rate<br>of<br>reported cases
|-
|1999
|align=right| 62
|align=right| 7
|align=right| 11%
|-
|2000
|align=right| 21
|align=right| 2
|align=right| 10%
|-
|2001
|align=right| 66
|align=right| 10
|align=right| 15%
|-
|2002
|align=right| 4,156
|align=right| 284
|align=right| 7%
|-
|2003
|align=right| 9,862
|align=right| 264
|align=right| 3%
|-
|2004
|align=right| 2,539
|align=right| 100
|align=right| 4%
|-
|2005
|align=right| 3,000
|align=right| 119
|align=right| 4%
|-
|2006
|align=right| 4,269
|align=right| 177
|align=right| 4%
|-
|2007
|align=right| 3,623
|align=right| 124
|align=right| 3%
|-
|2008
|align=right| 1,356
|align=right| 44
|align=right| 3%
|-
|2009
|align=right| 720
|align=right| 32
|align=right| 4%
|-
|2010
|align=right| 1,021
|align=right| 57
|align=right| 6%
|-
|2011
|align=right| 712
|align=right| 43
|align=right| 6%
|-
|2012
|align=right| 5,674
|align=right| 286
|align=right| 5%
|-
|2013
|align=right| 2,469
|align=right| 119
|align=right| 5%
|-
|2014
|align=right| 2,205
|align=right| 97
|align=right| 4%
|-
|2015
|align=right| 2,175
|align=right| 146
|align=right| 7%
|-
|2016
|align=right| 2,149
|align=right| 106
|align=right| 5%
|-
|2017
|align=right| 2,097
|align=right| 146
|align=right| 7%
|-
|2018
|align=right| 2,647
|align=right| 167
|align=right| 6%
|-
|2019*
|align=right| 971
|align=right| 60
|align=right| 6%
|-
|2020**
|align=right| 731
|align=right| 66
|align=right| 9%
|-
|2021
|align=right| 2,911
|align=right| 227
|align=right| 8%
|-
|2022
|align=right| 1,126
|align=right| 90
|align=right| 8%
|-
|}
Sources: Final Cumulative Maps & Data for 1999–2018, Preliminary Maps & Data for 2019, and West Nile Virus Disease Cases by State 2020, Centers for Disease Control and Prevention
<nowiki>*2019 also had 104 presumptive cases observed in blood donors</nowiki>
<nowiki>** as of December 15, 2020, also 127 presumptive cases observed in blood donors</nowiki>
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.
<gallery>
Image:1999_west_nile_map.png|1999
Image:2000_west_nile_map.png|2000
Image:2001_west_nile_map.png|2001
Image:2002_west_nile_map.png|2002
Image:2003_west_nile_map.png|2003
Image:2004 november west nile map.png|2004
Image:2005_west_nile_map.png|2005
</gallery>
Counties of reported cases by year
<gallery>
File:West nile virus cases US 2002.jpg|2001
File:West Nile virus cases in United States map- May 2003.gif|2002
File:West Nile virus cases in United States map- 2004.gif|2003
File:West nile virus case in United States 2006.png|2006
File:West nile virus case in United States 2007.png|2007
File:West nile virus case in United States 2008.png|2008
File:West nile virus cases in United States 2009.png|2009
File:West nile virus cases in United States 2010.png|2010
File:West nile virus cases in United States 2011.png|2011
File:West Nile virus map United States 2012 outbreak.png|2012
File:West Nile virus cases in United States 2013.png|2013
File:West Nile virus cases in United States 2014.png|2014
File:West Nile virus cases in United States 2015.png|2015
File:West Nile virus cases in United States 2016.png|2016
</gallery>
See also
- West Nile virus
- West Nile fever
- List of West Nile virus outbreaks
References
External links
- CDC West Nile Virus Statistics, Surveillance, and Control
- West Nile Cases Drop as Immunities Emerge, Experts Say
- CDC Information Page For Current Cases and Deaths For West Nile Virus