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West Nile Virus

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In 2021, ten human West Nile Virus (WNV) infections occurred in Philadelphia. Eight adult residents developed neuroinvasive disease and two developed febrile illness. Of these, eight required hospitalization and all were discharged in improved condition. The cases consisted of six males and four females, with an age range of 42-82. The first case of the season was detected on 8/13/2021 and the last case on 10/16/2021. Cases were identified in six ZIP codes across the city, half of them being in the Northeast.

From May through October, the Philadelphia Department of Public Health (PDPH), in collaboration with the PA Department of Environmental Protection, conducted mosquito management activities throughout Philadelphia, which entailed surveillance and identification of mosquito pools, testing pools for WNV, and control activities (eliminating breeding sites, larviciding catch basins, ground spraying, etc.). Throughout the season, 55 locations in Philadelphia were 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 surveillance data for travel-related arboviral infections.

Click here to download the 2021 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

2021 Season (5/1 - 10/31)

WNV Positive Pools


Humans Tested**


Human WNV Cases


** Testing among case reports received by PDPH only

2021 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