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West Nile Virus

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During the 2023 season, the Philadelphia Department of Public Health (PDPH) has identified 4 neuroinvasive West Nile virus (WNV) infections. All 4 cases occurred in adults ≥50 years of age who were hospitalized. One case had a significant underlying medical history and was fatal. One adult with WNV fever and 2 presumptive viremic blood donors also were reported in 2023.

The WNV mosquito pool positivity in 2023 is higher than the year-to-date historic median rate (29% vs 8%) but lower than rates observed in 2018, 2021, and 2022 (34–43%), peak WNV activity seasons in Philadelphia. Increased risk for human infection will continue in Philadelphia until the first hard frost occurs.

From May through October, 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 2023, 30 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 2023 West Nile Virus report in a printable format.

Click here to download the 2022 season summary report of West Nile Virus in a printable format.

West Nile Virus Mosquito and Human Case Surveillance, Philadelphia PA, Morbidity Week 42 and Cumulative 2023 Season Update

Week 42 (10/15 - 10/21)

2023 Season (5/1 - 10/21)

Mosquito Pools Tested



WNV Positive Pools



Human WNV Cases



Neuroinvasive WNV



WNV Fever



Human WNV Deaths



Presumptive Viremic Donors



*823 mosquito pools at 95 different mosquito trapping sites

**Testing among case reports received by PDPH only

mapwithlegend_2023 (12)

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 serum (3-8 days after onset) and CSF for patients with unexplained encephalitis or meningitis. Absence of detectable antibodies in serum within 8 days does not exclude possible WNV and samples can be recollected after 8 days if concern continues.
  • Report suspected and confirmed WNV infections (neuroinvasive or non-neuroinvasive illness) and encephalitis cases to PDPH by telephone at 215-685-6741 (or 215-686-4514 after hours).
    • Reports also may be faxed to 215-238-6947.

State and National West Nile Virus Activity Reports