Philadelphia has mosquitoes testing positive for West Nile virus for the first time this summer, the city’s Department of Public Health said Thursday.

West Nile virus is the nation’s leading mosquito-borne disease, according to the Centers for Disease Control and Prevention. Most people infected do not experience symptoms, but about one in five develop a fever, headaches, and overall fatigue. In rare cases — about one out of 150 infections — the disease can be severe and even fatal, causing inflammation of the brain and spinal cord. People over the age of 50 at are higher risk of serious illness.

Over the past two decades, the number of severe West Nile cases in Philadelphia has ranged from zero to 24, according to the Health Department. Last year, there were 10, with West Nile-positive mosquitoes found citywide.

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Elsewhere in Pennsylvania, positive mosquitoes have been identified so far this year in Bucks, Cumberland, and Montgomery Counties, according to the Pennsylvania Department of Environmental Protection.

“Keeping mosquitoes at bay is the best way to ensure that you don’t get bitten, and you don’t get West Nile,” said Deputy Health Commissioner Palak Raval-Nelson in a news release.

The Department of Public Health offered tips on mosquito-proofing Philly homes and neighborhoods:

  • Get rid of anything that can hold water outside, from soda bottle caps to discarded tires.

  • Once or twice a week, empty water from flowerpots, pet food and water dishes, birdbaths, swimming pool covers, buckets, barrels, cans, and other items outside your home.

  • Empty and store wading pools for kids on their side.

  • Clean clogged rain gutters.

  • Keep well-fitted screens on both windows and doors.

Philadelphia residents may also want to consider staying indoors when mosquitoes are most active at dawn, dusk, and the early evening, wearing long-sleeved shirts and long pants, and applying insect repellent on exposed skin when outdoors. To be effective, repellent should include DEET, Picardin, Oil of Lemon Eucalyptus, or PMD as part of their ingredients.

To report mosquito problems in your neighborhood, call the Health Department’s Mosquito Complaint hotline at 215-685-9000.