Historical West Nile Virus Activity

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 / Other Mammal	Bird	Mosquito Sample
County	By County of Likely Exposure	By County of Residence	Horse / Other Mammal	Bird	Mosquito Sample
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 Mammal	Bird	Mosquito Sample
County	By County of Likely Exposure	By County of Residence	Horse/Other Mammal	Bird	Mosquito Sample
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 Mammal	Bird	Mosquito Sample
County	By County of Likely Exposure	By County of Residence	Horse/Other Mammal	Bird	Mosquito Sample
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 Mammal	Bird	Mosquito Sample
County	By County of Likely Exposure	By County of Residence	Horse/Other Mammal	Bird	Mosquito Sample
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 Mammal	Bird	Mosquito Sample
County	By County of Likely Exposure	By County of Residence	Horse/Other Mammal	Bird	Mosquito Sample
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 Mammal	Bird	Mosquito Sample
County	By County of Likely Exposure	By County of Residence	Horse/Other Mammal	Bird	Mosquito Sample
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 Mammal	Bird	Mosquito Sample
County	By County of Likely Exposure	By County of Residence	Horse/Other Mammal	Bird	Mosquito Sample
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 Mammal	Bird	Mosquito Sample
County	By County of Likely Exposure	By County of Residence	Horse/Other Mammal	Bird	Mosquito Sample
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 Mammal	Bird	Mosquito Sample
County	By County of Likely Exposure	By County of Residence	Horse/Other Mammal	Bird	Mosquito Sample
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 Mammal	Bird	Mosquito Sample
County	By County of Likely Exposure	By County of Residence	Horse/Other Mammal	Bird	Mosquito Sample
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 Mammal	Bird	Mosquito Sample
County	By County of Likely Exposure	By County of Residence	Horse/Other Mammal	Bird	Mosquito Sample
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 Mammal	Bird	Mosquito Sample
County	By County of Likely Exposure	By County of Residence	Horse/Other Mammal	Bird	Mosquito Sample
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 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.