West Nile Virus

Example of a mosquito that transmits WNV
West Nile virus

West Nile virus (WNV) is the most common of several mosquito-borne viruses in the United States that can infect people. The virus exists in nature primarily through a transmission cycle involving many different species of mosquitoes and birds. Mosquitoes become infected with WNV when they feed on WNV infected birds.

To submit dead crows or blue jays for WNV testing, contact your local health department.

Most people that are infected with WNV by the bite of an infected mosquito will have no symptoms; however, approximately 20 percent of the people that are infected may experience a range of flu-like symptoms, and less than 1 percent of people that are infected will develop severe illness. As with any disease, the public should respond appropriately with increased awareness of WNV and practicing personal protective measures when exposed to mosquitoes.

For more information about this disease, visit the CDC's West Nile virus home page.

On the map below, each dot represents one case of WNV that occurred in 2021. Each dot is placed randomly in the patient's county of residence. The gradient refers to WNV incidence rates for each county in Tennesee from 2011-2021, which was calculated using county population data from the 2020 U.S. Census.

In 2021, WNV activity increased as compared to previous years, with a total of 5 human cases. Two of the cases occurred in Shelby County, and one case occurred in the East, West, and Mid-Cumberland regions respectively.

There were also 2 WNV positive equine cases in 2021, one in Davidson County and one in Weakley County. WNV positive mosquito pools were detected in Davidson and Shelby County, 2 of the 3 counties where mosquito trapping was conducted.

WNV case map across Tennessee by county

In Tennessee, illness onset for cases of WNV is mainly reported in the second half of the year. The number of reported WNV cases is lowest in the winter (December-March), slightly increases in the late spring to mid-summer (April-July), peaks in the late summer (August-September), and then gradually declines through autumn (October-November).

Signs and symptoms of WNV typically begin 1-2 weeks after being bitten by an infected mosquito. Late spring through mid-summer is the time of year when Tennesseans are most at risk for contracting WNV.

WNV cases by onset week
Note: The n=155 in this chart refers to reported cases of WNV from 2011-2021 with recorded illness onset dates. Thus, this figure only accounts for a portion of the total number of WNV cases reported during this time period.

Anyone who spends time outdoors in Tennessee is at risk of contracting WNV. However, persons over 50 years of age are at highest risk of developing the most severe form of the disease, and persons over the age of 70 with pre-existing health problems are at greatest risk for death.

WNV cases by age group and reported sex

In regards to trends over the past decade, case numbers for WNV were on the lower end of the spectrum in 2021. The dip in cases seen in 2020 can be identified in vector-borne diseases across the board during that year, likely due to a number of factors surrounding the COVID-19 pandemic.

WNV cases by year and hospitalization status

Most people infected with WNV do not devleop any symptoms. About 1 in 5 people who are infected will develop a mild febrile illness, which includes a fever in addition to one or more of the following symptoms:

  • Headache
  • Body aches
  • Joint pains
  • Vomiting
  • Diarrhea
  • Rash
  • Fatigue and weakness lasting for weeks or months

About 1 in 150 people who are infected develop a severe illness affecting the central nervous system, such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes that surround the brain and spinal cord). Symptoms of severe illness include:

  • High fever
  • Headache
  • Neck stiffness
  • Stupor
  • Disorientation
  • Coma
  • Tremors and/or convulsions
  • Muscle weakness
  • Vision loss
  • Numbness and/or paralysis

For more information, see the CDC's WNV Symptoms, Diagnosis, & Treatment web page.

WNV is diagnosed by a healthcare provider, who can order blood tests to look for evidence of WNV infection should you begin to experience the signs and symptoms listed above. 

For more information, see the CDC's WNV Symptoms, Diagnosis, & Treatment web page.

Year Human Horses Birds Mosquito Pools
  Positive Positive Positive Positive
2000 0 0 0 0
2001 0 1 46 0
2002 56 141 823 307
2003 26 103 275 308
2004 14 15 34 405
2005 18 7 12 574
2006 22 8 1 626
2007 11 4 7 600
2008 19 6 3 658
2009 8 5 1 488
2010 4 3 0 403
2011 18 3 0 995
2012 33 6 6 916
2013 24 4 1 824
2014 16 0 0 728
2015 8 1 0 694
2016 7 0 0 776
2017 32 3 32 1671
2018 13 12 2 1119
2019 4 1 0 488
2020 0 0 0 725
2021 5 2 0 666