West Nile virus surveillance maps and statistics by year:
2010 | 2011 | 2012 | 2013 | 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 l 2021
2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 |
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. See .
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.
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 |
![]() |
![]() |
![]() |
West Nile Virus-Positive Cases in Washington, 2021 |
|||||
County |
Human |
Horse / |
Bird | Mosquito Sample |
|
By County of |
By County of |
||||
Benton | 1 | 1 | 2 | 0 | 13 |
Franklin | 0 | 1 | 0 | 0 | 4 |
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 | 51 |
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 |
![]() |
![]() |
![]() |
West Nile Virus-Positive Cases in Washington, 2020 |
|||||
County |
Human |
Horse/Other |
Bird | Mosquito Sample |
|
By County of |
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 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 |
Bird | Mosquito Sample |
|
By County of |
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 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 |
Bird | Mosquito Sample |
|
By County of |
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 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 |
Bird | Mosquito Sample |
|
By County of |
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 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 |
Bird | Mosquito Sample |
|
By County of |
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 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 |
Bird | Mosquito Sample |
|
By County of |
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 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 |
Bird | Mosquito Sample |
|
By County of |
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 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 |
Bird | Mosquito Sample |
|
By County of |
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 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 |
Bird | Mosquito Sample |
||
By County of |
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 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 |
Bird | Mosquito Sample |
|
By County of |
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 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 |
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 West Nile Virus Activity

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

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

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