The mosquito found May 3 in Exeter Township, Berks County, was the earliest detected since state testing began in 2000, according to the state departments of Environmental Protection and Health.
DEP Secretary Mike Krancer said the virus-carrying insects usually do not show up until June, but this year's unseasonably warm weather accelerated the cycle. He said the DEP staff is monitoring mosquito populations and spraying to reduce the threat to public health.
In humans, the virus can result in inflammation of the brain. Officials says most infected people do not become ill, but older adults and people with compromised immune systems have the highest risk of getting sick and developing complications.
Official urged Pennsylvanians to take precautions at home to remove stagnant water, which is where most mosquitoes breed.
Among other things, they recommended disposing of tires and containers where water can collect, drilling holes in the bottoms of recycling bins and aerating ornamental pools or stocking them with fish. They also suggested making sure screens fit tightly in door and window openings, and avoiding the outdoors as much as possible at dawn and dusk, when mosquitoes are the most active.