Historical West Nile Virus Activity

West Nile virus surveillance maps and statistics by year

For reports on West Nile virus surveillance and the distribution of mosquitoes in the state, see Zoonotic Disease Data and Reports.

In addition, the Washington Tracking Network presents 2006-2019 vector mosquito species, Culex pipiens and Cx. tarsalis test data by county. The network also includes bird and mammal test data. View data in a table, chart, and map format and export data for your own use.

Surveillance Maps: Human Case Map 1 shows counties where the infected person was most likely exposed and Map 2 shows counties where the infected person lived. Environmental Detection Map 3 shows counties where infections in horses, birds, and mosquitoes occurred.

2022

2022 West Nile Virus Activity

Map of Washington where people may have gotten west nile virus with no counties colored in.
Map of Washington with counties that have cases of red nile virus in red.
Map of Washington where there are infected houses/other mammals, birds, and vector mosquitoes.
West Nile Virus-Positive Cases in Washington, 2022
County   Human  Horse/Other Mammal    Bird    Mosquito Sample  
By County of Likely Exposure  By County of Residence 
Benton 0 0 0 0 5
Clallam 0 1 0 0 0
King 0 1 0 0 0
Kittitas 0 0 0 0 1
Pierce 0 1 0 0 0
Spokane 0 1 0 0 0
Multi-county Exposure* 0        
In-state Acquired 0        
Out-of-state Acquired 3        
Unknown Exposure Location ** 1        
Total 4 4 0 0 6***

* "Multi-county Exposure" represents human cases that had potential exposure in more than one county.

** "Unknown Exposure Location" represents human cases that had potential exposure in other states or countries in addition to Washington State. These cases are not shown on Map 1. Human Cases by County of Likely Exposure.

*** Of these 6 positive pools, one is a pool of Aedes vexans collected in Kittitas County tested positive for West Nile virus (WNV). Aedes vexans is not considered to be directly involved in the maintenance of WNV in the bird-mosquito cycle.

Maps and statistics are updated daily by 3 PM. This table shows cases of West Nile virus reported in the state.

2021

 2021 West Nile Virus Activity

Map 1. Human cases by county of likely exposure Map 2. Human cases by county of residence Map 3. Infected horses/other mammals, birds, and mosquitoes
WA map with red section showing county with WNV cases
WA map with red section showing county with WNV cases
WA map with red section showing county with WNV cases

West Nile Virus-Positive Cases in Washington, 2021

County

Human

Horse /
Other
Mammal

 Bird Mosquito
Sample

By County of
Likely Exposure

By County of
Residence
Benton 1 1 2 0 13
Franklin 0 1 0 0 5
Grant 0 0 0 0 7
Kittitas 0 0 2 0 0
Klickitat 0 0 1 0 0
Walla Walla 0 1 0 0 11
Whatcom 0 0 1 0 0
Yakima 1 1 5 0 16
Multi-county Exposure* 1        
In-state Acquired 3        
Out-of-state Acquired 1        
Totals 4   11 0 52
2020

 2020 West Nile Virus Activity

Map 1. Human cases by county of likely exposure Map 2. Human cases by county of residence Map 3. Infected horses/other mammals, birds, and mosquitoes
WA map with red section showing county with WNV cases
WA map with red section showing county with WNV cases
WA map with red section showing county with WNV cases

West Nile Virus-Positive Cases in Washington, 2020

County

Human

Horse/Other
Mammal

 Bird Mosquito
Sample

By County of
Likely Exposure

 By County of Residence
Benton 1 1 1 0 8
Franklin 0 0 0 0 1
Yakima 1 1 1 0 5
Multi-county Exposure* 0        
In-state Acquired 2        
Out-of-state Acquired 0        
Totals 2     0 14
2019

 2019 West Nile Virus Activity

Map 1. Human cases by county of likely exposure Map 2. Human cases by county of residence Map 3. Infected horses/other mammals, birds, and mosquitoes

2019 West Nile Virus-Positive Cases in Washington

County

Human

Horse/Other
Mammal

 Bird Mosquito
Sample

By County of
Likely Exposure

 By County of Residence
Benton 4 5 0 0 9
Franklin 0 0 1 0 0
Grant 0 0 0 0 16
Klickitat 0 0 1 0 0
Yakima 0 0 0 0 2
Multi-county Exposure* 0        
In-state Acquired 4        
Out-of-state Acquired 0        
Unknown Exposure Location** 1        
Totals 5   2   27

* "Multi-county Exposure" represents human cases that had potential exposure in more than one county. ** "Unknown Exposure Location" represents human cases that had potential exposure in other states or countries in addition to Washington State. These cases are not shown on Map 1. Human Cases by County of Likely Exposure. Not listed in the table: two presumptive viremic blood donors were identified. One resides in Yakima County and one resides in Benton County. Both were likely exposed in the county of residence.

2018

 2018 West Nile Virus Activity

Map 1. Human cases by county of likely exposure Map 2. Human cases by county of residence Map 3. Infected horses/other mammals, birds, and mosquitoes

2018 West Nile Virus-Positive Cases in Washington

County

Human

Horse/Other
Mammal

 Bird Mosquito
Sample

By County of
Likely Exposure

 By County of Residence
Benton 0 0 0 0 9
Franklin 0 0 0 0 1
Grant 0 0 1 0 29
King 1 1 0 0 0
Pierce 0 0 0 0 4
Spokane 0 0 0 0 3
Stevens 0 0 1 0 0
Thurston 0 1 0 0 0
Whatcom 0 1 0 0 0
Yakima 0 0 0 0 3
Multi-county Exposure* 0        
In-state Acquired 1        
Out-of-state Acquired 2        
Totals 3   2 0 49

* "Multi-county Exposure" represents human cases that had potential exposure in more than one county. These cases are not shown on Map 1. Human Cases by County of Likely Exposure.

2017

2017 West Nile Virus Activity

Map 1. Human cases by county of likely exposure

Map 2. Human cases by county of residence

Map 3. Infected horses/other mammals, birds, and mosquitoes

 

 

2017 West Nile Virus-Positive Cases in Washington

County

Human

Horse/Other
Mammal

 Bird Mosquito
Sample

By County of
Likely Exposure

 By County of
Residence
Benton 0 0 1 0 9
Clark 0 1 0 0 0
Grant 0 0 0 0 14
King 0 2 0 0 0
Kittitas 0 0 1 0 0
Lincoln 0 0 1 2 0
Okanogan 1 1 0 0 0
Pierce 0 1 0 0 0
Spokane 7 8 6 1 1
Walla Walla 0 0 0 1 0
Yakima 0 0 0 0 10
Multi-county Exposure* 0        
In-state Acquired 8        
Out-of-state Acquired 5        
Totals 13   9 4 34

* "Multi-county Exposure" represents human cases that had potential exposure in more than one county. These cases are not shown on Map 1. Human Cases by County of Likely Exposure.

2016

2016 West Nile Virus Activity

Map 1. Human cases by county of likely exposure

Map 2. Human cases by county of residence

Map 3. Infected horses/other mammals, birds, and mosquitoes

 

 

2016 West Nile Virus-Positive Cases in Washington

County

Human

Horse/Other
Mammal

 Bird Mosquito
Sample

By County of
Likely Exposure

 By County of
Residence
Adams 0 0 0 0 1
Benton 3 3 1 0 25
Franklin 0 0 1 0 6
Grant 0 0 1 0 51
Kittitas 0 0 1 0 0
Lincoln 0 0 2 0 0
Okanogan 0 0 4 0 0
Pend Oreille 0 0 2 0 0
Spokane 3 3 8 2 0
Stevens 0 0 6 0 1
Walla Walla 1 1 0 0 0
Whitman 1 1 0 0 0
Yakima 1 1 1 0 11
Multi-county Exposure* 0        
In-state Acquired 9        
Out-of-state Acquired 0        
Totals 9   27 2 95

* "Multi-county Exposure" represents human cases that had potential exposure in more than one county. These cases are not shown on Map 1. Human Cases by County of Likely Exposure. Not listed in table: Three presumptive viremic blood donors were identified. Two reside in Grant County, and one resides in Asotin County.

2015

2015 West Nile Virus Activity

Map 1. Human cases by county of likely exposure

Map 2. Human cases by county of residence

Map 3. Infected horses/other mammals, birds, and mosquitoes

 

 

2015 West Nile Virus-Positive Cases in Washington

County

Human

Horse/Other
Mammal

 Bird Mosquito
Sample

By County of
Likely Exposure

 By County of
Residence
Adams 1 0 1 0 0
Benton 12 13 4 5 27
Columbia 0 0 1 0 0
Douglas 0 0 1 0 0
Franklin 2 2 2 0 17
Grant 4 3 7 1 107
King 0 1 0 0 0
Kittitas 0 0 8 1 1
Lincoln 0 0 1 0 0
Snohomish 0 1 0 0 0
Walla Walla 0 0 3 0 1
Whatcom 0 2 0 0 0
Yakima 2 2 8 0 5
Multi-county Exposure* 1        
In-state Acquired 22        
Out-of-state Acquired 2        
Totals 24   36 7 157

* "Multi-county Exposure" represents human cases that had potential exposure in more than one county. These cases are not shown on Map 1. Human Cases by County of Likely Exposure. Not listed in table: Seven presumptive viremic blood donors were identified. Two of the donors reside in Benton County, and one donor each in Franklin, Grant, Kittitas, Walla Walla, and Yakima counties. All donors were most likely exposed in their county of residence.

2014

2014 West Nile Virus Activity

Map 1. Human cases by county of likely exposure

Map 2. Human cases by county of residence

Map 3. Infected horses/other mammals, birds, and mosquitoes

 

 

2014 West Nile Virus-Positive Cases in Washington

County

Human

Horse/Other
Mammal

 Bird Mosquito
Sample

By County of
Likely Exposure

 By County of
Residence
Benton 7 7 1 0 14
Franklin 0 0 1 0 26
Grant 1 0 1 0 34
Grays Harbor 0 1 0 0 0
King 0 1 0 0 0
Okanogan 0 0 1 0 0
Pierce 0 1 0 0 0
Walla Walla 1 1 0 0 5
Yakima 1 1 1 0 1
Multi-county Exposure* 0        
In-state Acquired 10        
Out-of-state Acquired 2        
Totals 12   5 0 80

* "Multi-county Exposure" represents human cases that had potential exposure in more than one county. These cases are not shown on Map 1. Human Cases by County of Likely Exposure.

2013

2013 West Nile Virus Activity

Map 1. Human cases by county of likely exposure

Map 2. Human cases by county of residence

Map 3. Infected horses/other mammals, birds, and mosquitoes

 

 

2013 West Nile Virus-Positive Cases in Washington

County

Human

Horse/Other
Mammal

 Bird Mosquito
Sample

By County of
Likely Exposure

 By County of
Residence
Benton 0 0 0 0 1
Franklin 0 0 1 0 5
Grant 0 0 0 0 2
Skagit 0 1 0 0 0
Spokane 0 0 0 0 1
Yakima 0 0 1 0 9
Multi-county Exposure* 0        
Total In-state Acquired 0        
Out-of-state Acquired 1        
Totals 1   2 0 18

* "Multi-county Exposure" represents human cases that had potential exposure in more than one county. These cases are not shown on Map 1. Human Cases by County of Likely Exposure.

2012

2012 West Nile Virus Activity

Map 1. Human cases by county of likely exposure

Map 2. Human cases by county of residence

Map 3. Infected horses/other mammals, birds, and mosquitoes

 

 

2012 West Nile Virus-Positive Cases in Washington

  

County

  

Human

Horse/Other
Mammal

 Bird Mosquito
Sample

By County of
Likely Exposure

 By County of
Residence
Benton   1 1 1 0 0
Clark   0 1 0 0 0
Pierce   0 1 0 0 0
Yakima   1 1 0 0 0
Multi-county Exposure*   0        
Total In-state Acquired   2        
Out-of-state Acquired   2        
Totals   4   1 0 5

* "Multi-county Exposure" represents human cases that had potential exposure in more than one county. These cases are not shown on Map 1. Human Cases by County of Likely Exposure.

2011

2011 West Nile Virus Activity

Map 1. Human cases by county of likely exposure

Map 2. Human cases by county of residence

Map 3. Infected horses/other mammals, birds, and mosquitoes

 

 
2011 West Nile Virus-Positive Cases in Washington

County

Human

Horse/Other
Mammal

 Bird Mosquito
Sample

By County of
Likely Exposure

 By County of
Residence
Franklin 0 0 0 0 1
Grant 0 0 0 0 3
Yakima 0 0 0 0 1
Multi-county Exposure* 0        
Total In-state Acquired 0        
Out-of-state Acquired 0        
Totals 0   0 0 5

* "Multi-county Exposure" represents human cases that had potential exposure in more than one county. These cases are not shown on Map 1. Human Cases by County of Likely Exposure.

2010

2010 West Nile Virus Activity

Map 1. Human cases by county of likely exposure

Map 2. Human cases by county of residence

Map 3. Infected horses/other mammals, birds, and mosquitoes

 

 
2010 West Nile Virus-Positive Cases in Washington

County

Human

Horse/Other
Mammal

 Bird Mosquito
Sample
By County of
Likely Exposure		 By County of
Residence
Benton 0 0 0 0 6
Chelan 0 1 0 0 0
Grant 1 1 0 2 115
Yakima 0 0 0 0 5
Multi-county Exposure* 0        
Total In-state Acquired 1        
Out-of-state Acquired 1        
Totals 2   0 2 126

* "Multi-county Exposure" represents human cases that had potential exposure in more than one county. These cases are not shown on Map 1. Human Cases by County of Likely Exposure.

2009

2009 West Nile Virus Activity

2009 West Nile Virus-Positive Cases in Washington
County Human Horse/ Other
Mammal		 Bird

Mosquito
Sample
Adam 0 2 0 3
Benton 9 23 7 128
Franklin 0 3 2 45
Grant 1 11 0 10
Grays Harbor 0 0 1 0
King* 1 0 2 0
Kittitas 0 16 2 2
Klickitat 2 0 0 3
Lewis 0 0 1 0
Mason 0 0 1 0
Pierce 0 0 1 0
Spokane** 2 0 1 0
Walla Walla 0 0 4 1
Whatcom* 1 0 0 0
Whitman 1 0 0 0
Yakima 21 18 0 154
Totals 38 73 22 346

Table represents in-state acquired West Nile virus human cases, and cases are listed by county of residence. *Exposures in eastern Washington. ** Exposures potentially in county of residence or in other eastern Washington counties.

2008

 2008 West Nile Virus Activity

2008 West Nile Virus-Positive Cases in Washington
County Human Horse/Other
Mammal		 Bird Mosquito
Sample
Benton 0 4 10 14
Grant 0 10 1 2
King 1 0 3 0
Kittitas 0 1 0 0
Lewis 0 0 1 0
Pierce 0 0 3 0
Thurston 0 0 1 0
Yakima 2 26 5 41
Totals 3 41 24 57

Table represents in-state acquired West Nile virus human cases, and cases are listed by county of residence.

2007

2007 West Nile Virus Activity

2007 West Nile Virus-Positive Cases in Washington
County Human Horse/Other
Mammal		 Bird Mosquito
Sample
Yakima 0 9 1 0
Totals 0 9 1 0

Table represents in-state acquired West Nile virus human cases, and cases are listed by county of residence.

2006

2006 West Nile Virus Activity

2006 West Nile Virus-Positive Cases in Washington
County Human Horse/Other
Mammal		 Bird Mosquito
Sample
Clark 1 0 0 0
Cowlitz 0 0 1 0
Island 0 0 1 0
King 0 1 6 0
Pierce 2 0 0 0
Snohomish 0 0 2 0
Yakima 0 5 3 0
Totals 3 6 13 0

Table represents in-state acquired West Nile virus human cases, and cases are listed by county of residence.

2005

2005 West Nile Virus Activity

2005 West Nile Virus-Positive Cases in Washington
County Human Horse/Other
Mammal		 Bird Mosquito
Sample
Yakima 0 1 1 2
Totals 0 1 1 2

Table represents in-state acquired West Nile virus human cases, and cases are listed by county of residence.

2004

2004 West Nile Virus Activity

2004 West Nile Virus-Positive Cases in Washington
County Human Horse/Other
Mammal		 Bird Mosquito
Sample
  0 0 0 0
Totals 0 0 0 0

Table represents in-state acquired West Nile virus human cases, and cases are listed by county of residence.

2003

2003 West Nile Virus Activity

2003 West Nile Virus-Positive Cases in Washington
County Human Horse/Other
Mammal		 Bird

Mosquito
Sample
  0 0 0 0
Totals 0 0 0 0

Table represents in-state acquired West Nile virus human cases, and cases are listed by county of residence.

2002

2002 West Nile Virus Activity

2002 West Nile Virus-Positive Cases in Washington
County Human Horse/Other
Mammal		 Bird

Mosquito
Sample
Island 0 1 0 0
Pend Orielle 0 0 1 0
Pierce 0 0 1 0
Snohomish 0 0 1 0
Thurston 0 0 1 0
Whatcom 0 1 0 0
Totals 0 2 4 0

Table represents in-state acquired West Nile virus human cases, and cases are listed by county of residence.