Credit: Andrei Sauko from iStock Photo
Philadelphia health officials are urging residents to take precautions after detecting West Nile virus in mosquitoes near Pennypack Park in Northeast Philadelphia during routine surveillance testing. The discovery marks the first infected mosquito pool of the 2025 season, collected on June 18th in the city's Northeast section.
The Philadelphia Department of Public Health has issued a health advisory to area medical providers, instructing them to look out for virus symptoms in patients and test them for West Nile when appropriate from now through October's end or until the first frost. No human cases have been reported in the city as of Thursday, though positive mosquito pools are anticipated to be identified across Philadelphia in the coming weeks.
Understanding West Nile Virus
West Nile virus, originally discovered in Uganda in 1937, first appeared in the United States in 1999 in New York. The virus is primarily transmitted by Culex tarsalis mosquitoes, which are the primary vectors of West Nile virus. These mosquitoes become infected when they feed on birds carrying the virus, and then transmit the virus to humans and other animals through their bite. While other mosquito species can also be infected with West Nile virus, Culex mosquitoes are the most important carriers in the transmission cycle between birds and humans.
Nationally, the highest incidence rates are observed in states of the central great plains, with South Dakota, Wyoming and North Dakota leading in incidence. The CDC reports that in 2022, 1,132 cases were reported across the United States.
The majority of people bitten by a West Nile-infected mosquito don't get the virus, and most don't experience any symptoms. About 20% of people will develop a flulike illness with fever, rash, diarrhea, vomiting, headache, fatigue, joint pain, and body aches. However, fewer than 1% will develop severe disease, and about one out of 10 people die from a severe case, according to Emily Kehoe, the city's vector-borne disease surveillance coordinator.
In rare cases, West Nile virus can cause encephalitis or meningitis, which result in swelling of the brain and of the membranes around the brain and spinal cord. Older adults and those with weakened immune systems are at higher risk for severe illness.
From 2020 to 2024 in Philadelphia, four to nine people were diagnosed with West Nile virus that caused neurological complications each year, with at least one case being fatal during each of those years. West Nile virus season in the United States generally runs from summer through fall, with peak activity typically occurring from now through early September when mosquito populations are at their highest and most active, especially during dusk and dawn.
Each year, mosquitoes infected with West Nile virus are detected in parks citywide, with Pennypack Park encompassing about 1,600 acres of woodlands, meadows, and wetlands in the city's Holme Circle neighborhood. The Philadelphia Department of Public Health conducts mosquito management activities from May through October in collaboration with the PA Department of Environmental Protection, including surveillance, testing, and control activities such as eliminating breeding sites and larviciding catch basins.
Health officials recommend several precautions to protect against mosquito bites. Residents should use bug repellent with DEET, picaridin, or oil of lemon eucalyptus while outdoors, wear long-sleeved shirts and long pants, and use well-fitted screens on windows and doors to keep out mosquitoes. Additionally, residents should regularly empty standing water outside the home, drill holes in the bottom of trash cans, and turn over plastic kiddie pools at day's end.
People should avoid the outdoors at dusk and dawn when mosquitoes are most active, and if outdoors during this time, wear long sleeves and pants and use insect repellent. Standing water should be eliminated from containers such as flowerpots, buckets, and barrels, and water in pet dishes should be changed weekly.
There is no vaccine and no specific treatment for West Nile virus. Rest, fluids, and pain medication can help relieve symptoms, and most people fully recover. However, individuals should seek medical care immediately if they develop symptoms of severe West Nile virus, including fever, headache, neck stiffness, or confusion.
All suspected and confirmed cases of West Nile virus should be reported to the Philadelphia Department of Public Health at 215-685-6741. Residents can also report mosquito problems to the city health department's Mosquito Complaint hotline at 215-685-9000.
