Health Advisory

Summary:

• The Fairfax County Health Department (FCHD) has identified the first West Nile virus (WNV) positive mosquito pools in the Fairfax Health District (FHD) in 2026; the mosquitoes were collected on June 30 and July 2 in eastern and northwest areas of the FHD.
• WNV is found in mosquitoes in the FHD every year. These positive pools were collected later than the average first positive pool but are within the historical range of when the first positive pools have been collected. The average date for first positive pool from 2004-2025 was mid-June. The range for first positive pool from 2004-2025 has been from early May to mid/late July.
• Patients with a compatible clinical illness (meningitis, encephalitis, acute flaccid paralysis) should be tested for WNV, especially those at highest risk of serious WNV disease including older adults and those who are immunocompromised.

Suggested Actions:

• Providers should consider WNV in their differential diagnosis for compatible clinical illnesses until the end of mosquito season, generally at the end of October. Providers should also consider testing for other arboviruses, such as La Crosse virus and St. Louis Encephalitis virus, in patients with encephalitis.
• Patients should be informed of the risk of mosquitoes in transmitting diseases, the steps they can take to protect themselves from mosquitoes, and ways reduce the number of mosquitoes around their homes. Recommendations include:
  • Using a mosquito repellent that contains DEET, picaridin, IR3535, Oil of Lemon Eucalyptus (OLE), para-menthane-diol (PMD) or 2-undecanone.
  • Wearing long pants and long sleeves to cover exposed skin.
  • Avoiding areas where mosquitoes are especially common during peak biting times, particularly at dawn and dusk.
  • Taking steps to control mosquitoes around their yards by “tipping and tossing” items that hold water such as tires, buckets, flowerpots, corrugated downspout extensions, and other containers; and using larvicides, such as Mosquito Dunks, in water that cannot be eliminated.
  • Keeping doors, windows, and screens in good repair.
• If you have any questions regarding laboratory testing for WNV and other arboviruses, please contact the Fairfax County Health Department Environmental Epidemiology Program at 703-246-2433 or hdcd@fairfaxcounty.gov.
• Per the Virginia Regulations for Disease Reporting and Control, healthcare providers are required to report all confirmed cases of WNV to their local health department.

Additional Information:

• Over the last ten years (2016-2025), 31 cases of WNV have been reported to FCHD, an average of 3 cases reported every year. Of these cases, 65% had neuroinvasive disease and 2 died. The cases were geographically distributed throughout the entire Health District.
• Most persons with WNV infection are asymptomatic (70-80%) or have only mild illness with fever, headache and muscle aches that can last days to weeks. Such cases are rarely diagnosed or reported. Fewer than 1% develop neuroinvasive disease (meningitis, encephalitis, acute flaccid paralysis).
• WNV is primarily spread by infected Culex mosquitoes which obtain the virus from certain bird species. Summers with higher rain levels correlate to greater numbers of mosquitoes.
• FCHD’s Division of Environmental Health routinely traps and tests mosquitoes for WNV throughout the FHD every mosquito season.
• FHD residents may request a mosquito inspection of their yard from FCHD’s Division of Environmental Health at 703-246-2201, TTY 711. This inspection will result in recommendations to decrease mosquito populations on residents’ property.