Animal and
Plant Health
Inspection
Service
Veterinary
Services
November
2022
2021 Summary of West Nile Virus
Equine Cases in the United States
January 1 – December 31, 2021
This report is a descriptive summary of equine West Nile virus (WNV) cases in the United States as
reported to the United States Department of Agriculture (USDA) during 2021. The USDA Animal and
Plant Health Inspection Service (APHIS) Veterinary Services (VS) branch collaborates with the U.S.
Centers for Disease Control and Prevention (CDC) and State veterinary and public health officials to
facilitate communication about arbovirus disease cases in horses and confirm equine cases in each
State. CDC collects arbovirus case information using its ArboNET reporting system, an electronic
surveillance and reporting system for tracking and reporting arboviral activity in humans and animals.
During the transmission season, VS disseminates the equine arbovirus case information to State animal
health officials for confirmation and epidemiological follow-up. The USDA equine website provides
further WNV information, including specific case definitions:
https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/equine. In this
document, ‘reported cases’ of WNV may include presumptive positive and confirmed positive cases.
In 2021, 220 equine cases of WNV were reported from 34 States. Figure 1 shows the annual number of
equine WNV cases reported in the United States from 2008 through 2021. Figure 2 shows the number of
equine WNV cases in each State in 2021, and table 1 provides the number of cases per county or parish
in those States in 2021.
Equine Cases of WNV
700
627
600
500
400
300
200
100
0
179
276
125
87
377
141
225
380
307
493
90
71
220
2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
WNV Cases
Figure 1. Reported U.S. equine cases of West Nile virus, 2008-2021
2021 Summary of West Nile Virus Equine Cases in the United States
Figure 2. Distribution of reported equine WNV cases in 2021 (220 total cases)
Table 1. Number of reported equine WNV cases by State and county, 2021
State County
West Nile Virus
Reported Cases
Alabama Baldwin County 1
Conecuh County 1
Total cases for Alabama 2
Arizona Cochise County 1
Gila County 1
Graham County 3
Maricopa County 8
Pima County 8
Pinal County 2
Total cases for Arizona 23
California Amador County 1
Fresno County 2
USDA-APHIS-VS 2
2021 Summary of West Nile Virus Equine Cases in the United States
State County
West Nile Virus
Reported Cases
Kings County 1
Merced County 1
Sacramento County 2
San Joaquin County 3
Stanislaus County 1
Yuba County 2
Total cases for California 13
Colorado Adams County 3
Boulder County 1
Douglas County 1
Elbert County 1
El Paso County 1
Fremont County 1
Larimer County 1
Logan County 1
Mesa County 1
Morgan County 2
Otero County 1
Weld County 9
Total cases for Colorado 23
Delaware Kent County 1
Total cases for Delaware 1
Florida Glades County 1
Hendry County 1
Lee County 1
Suwannee County 1
Volusia County 1
Total cases for Florida 5
Idaho Ada County 4
Blaine County 1
Canyon County 3
Elmore County 1
Gooding County 1
Lemhi County 2
Payette County 1
Twin Falls County 1
Washington County 1
USDA-APHIS-VS 3
2021 Summary of West Nile Virus Equine Cases in the United States
State County
West Nile Virus
Reported Cases
Total cases for Idaho 15
Illinois Douglas County 3
Moultrie County 1
Richland County 1
Total cases for Illinois 5
Indiana Adams County 6
Elkhart County 1
Monroe County 1
Ripley County 1
Total cases for Indiana 9
Iowa Howard County 2
Total cases for Iowa 2
Kansas Ford County 1
Shawnee County 2
Total cases for Kansas 3
Kentucky Ballard County 1
Christian County 1
Graves County 1
Todd County 1
Trigg County 1
Woodford County 1
Total cases for Kentucky 6
Louisiana Beauregard Parish 1
Total cases for Louisiana 1
Michigan Genesee County 1
Kent County 1
Lenawee County 1
Midland County 1
Montcalm County 1
Sanilac County 1
Van Buren County 1
Total cases for Michigan 7
Minnesota Benton County 1
Crow Wing County 1
Freeborn County 1
Wadena County 1
Total cases for Minnesota 4
USDA-APHIS-VS 4
2021 Summary of West Nile Virus Equine Cases in the United States
State County
West Nile Virus
Reported Cases
Missouri Macon County 1
Moniteau County 2
Total cases for Missouri 3
Montana Phillips County 1
Richland County 1
Total cases for Montana 2
Nebraska Arthur County 1
Cherry County 1
Grant County 1
Holt County 1
Lincoln County 1
Scotts Bluff County 1
Sheridan County 1
Total cases for Nebraska 7
New Mexico Dona Ana County 1
Luna County 1
Otero County 2
Quay County 1
Santa Fe County 1
Total for New Mexico 6
New York Cattaraugus County 1
Chautauqua County 1
Lewis County 1
Onondaga County 1
Yates County 1
Total for New York 5
North Carolina Guilford County 1
Union County 1
Total for North Carolina 2
North Dakota Barnes County 1
McIntosh County 1
Nelson County 1
Ramsey County 1
Total cases for North Dakota 4
Ohio Cuyahoga County 1
Total for Ohio 1
Oklahoma Adair County 1
USDA-APHIS-VS 5
2021 Summary of West Nile Virus Equine Cases in the United States
State County
West Nile Virus
Reported Cases
Ellis County 1
Garfield County 1
Jackson County 1
McClain County 1
Oklahoma County 2
Osage County 1
Texas County 1
Total cases for Oklahoma 9
Oregon Baker County 1
Jackson County 1
Klamath County 1
Malheur County 3
Morrow County 1
Umatilla County 1
Total cases for Oregon 8
Pennsylvania Cumberland County 1
Lancaster County 2
Northumberland County 1
Snyder County 1
Total cases for Pennsylvania 5
South Carolina Bamberg County 1
Berkeley County 2
Colleton County 1
Lexington County 1
Orangeburg County 1
Sumter County 1
Total cases for South Carolina 7
South Dakota Bon Homme County 2
Butte County 1
Jackson County 1
Stanley County 1
Total cases for South Dakota 5
Tennessee Davidson County 1
Weakley County 1
Total cases for Tennessee 2
Texas Freestone County 1
Grayson County 1
USDA-APHIS-VS 6
2021 Summary of West Nile Virus Equine Cases in the United States
State County
West Nile Virus
Reported Cases
Hood County 1
Llano County 1
Lubbock County 2
Parker County 1
Reeves County 1
Scurry County 1
Total cases for Texas 9
Utah Box Elder County 2
Salt Lake County 1
Utah County 1
Weber County 8
Total cases for Utah 12
Washington Benton County 1
Kittitas County 1
Klickitat County 1
Lewis County 1
Yakima County 4
Total cases for Washington 8
Wisconsin Crawford County 1
Total cases for Wisconsin 1
Wyoming Campbell County 1
Converse County 1
Platte County 1
Uinta County 1
Washakie County 1
Total cases for Wyoming 5
Total Reported West Nile Virus Equine Cases for the United States 220
WNV can be a deadly disease in equids, with mortality occurring in roughly 33 percent of all infected
animals. Furthermore, up to 40 percent of affected equids may still exhibit neurological signs as long as
6 months after diagnosis.
1
This susceptibility to the virus makes equids a recognized WNV sentinel
species for the human population. The main public health purpose of confirming and reporting equine
WNV cases is to inform human surveillance efforts. Because equine commercial vaccines are available
and convey good protection against WNV infection, reported cases of WNV in equids tend to be in
unvaccinated or under-vaccinated animals.
1. “Core Vaccination Guidelines: West Nile Virus.” American Association of Equine Practitioners,
https://aaep.org/guidelines/vaccination-guidelines/core-vaccination-guidelines/west-nile-virus. Accessed 9 April 2021.
USDA-APHIS-VS 7
