Nine human West Nile Virus (WNV) cases have been identified in Philadelphia during the 2021 season. Seven adult residents developed neuroinvasive disease and two developed febrile illness, and eight were subsequently hospitalized. Positive mosquito pools continue to be identified across the City. While the number of human cases of WNV and location of outbreaks has varied by season across the country, identification of mosquitoes infected with WNV remains the best predictor of increased risk for human infection in a given location. Increased risk for human infection will continue in Philadelphia until the first hard frost occurs.

From May through October, the Philadelphia Department of Public Health (PDPH), in collaboration with the PA Department of Environmental Protection, conducts mosquito management activities throughout Philadelphia, which entails surveillance and identification of mosquito pools, testing pools for WNV, and control activities (eliminating breeding sites, larviciding catch basins, ground spraying, etc.). Since May 2021, 52 locations in Philadelphia have been treated to control adult mosquito populations.

Click here for more information on West Nile Virus and other arboviruses including disease guidance and reporting information.

Click here to download the latest weekly report of West Nile Virus in a printable format.

Click here to view travel-related arboviral infections surveillance data.

Click here to download the 2019 season summary report of West Nile Virus in a printable format.

2021 Philadelphia West Nile Virus Mosquito and Human Case Surveillance

Cumulative 2021 Season Update:

Week 39 (9/12-10/2) 2021 Season (5/1-10/2)
WNV Positive Pools 3 224
Humans Tested** 0 15
Human WNV Cases 0 9
**Testing among case reports received by PDPH only

2021 Geographic Distribution of Positive Mosquito Pools and Human Cases:

Geographic Distribution of Positive Mosquito Pools and Human Cases

Advise your patients to:

  • Use repellent with DEET (≥20% to also prevent tick bites), Picaridin, or oil of lemon eucalyptus when outdoors
  • Regularly check and remove standing water outside their home (e.g., unused pools, tires)
  • Keep well-fitted screens on windows and doors
  • Report mosquito problems and dead bird sightings to the PDPH Vector Control Program’s Mosquito Complaint Line at 215-685-9000

WNV Testing and Reporting Requirements for Human Surveillance in Philadelphia:

  • Collect CSF (from symptom onset—day 8) and serum (day 8—14) from patients with unexplained encephalitis or meningitis for WNV IgM testing.
  • Report suspected and confirmed WNV infections (neuroinvasive or non-neuroinvasive illness) and encephalitis cases to PDPH by telephone at 215-685-6742 (or 215-686-4514 after hours). Reports also may be faxed to 215-238-6947.

State and National West Nile Virus Activity Reports

Updated: 10/15/2021