The majority of cases in the country have been in six states - Texas, Louisiana, Mississippi, Michigan, Oklahoma, and South Dakota. Among the national total, 66 have died.
An unseasonably warm winter allowed mosquitoes to breed all year and is partly to blame for the problem, officials said.
States and counties are regularly checking for infected mosquitoes as a way of identifying areas that need spraying. On Friday, Pennsylvania reported finding 67 affected mosquitoes.
"We're on track to exceed the highest recorded year ever for mosquito infection," said Amanda Whitman, a spokeswoman for the Pennsylvania Department of Environmental Protection. "On average, each day, we're identifying 75 to 125 mosquitoes that test positive."
As in Pennsylvania, state officials in New Jersey are working closely with counties, providing lab testing and other assistance.
"West Nile virus cases tend to increase in the late summer and fall, and residents should take steps to prevent mosquito bites," said New Jersey Department of Health Commissioner Mary E. O'Dowd.
"Residents should protect themselves by using repellent, wearing long sleeves and pants, and avoiding outdoors during dawn and dusk, when mosquitoes are most active," she said.
Over the last few weeks, communities in the Philadelphia area have been sprayed even as health officials urged residents to work to reduce mosquito populations by eliminating standing water on their properties.
On Friday, some of Manhattan's most expensive neighborhoods - the Upper West Side and parts of Central Park - also were sprayed.
Older residents and those with compromised immune systems are the most vulnerable to the virus, which can lead to West Nile fever, encephalitis, and meningitis.
In Pennsylvania, one human case has been reported in Philadelphia, along with one each in Bucks, Chester, York, Centre, Franklin, Luzerne, and Lebanon Counties. Three additional cases were reported in Delaware County, three in Lancaster County, and two in Lehigh County. A total of seven were logged last year.
In New Jersey, eight cases have been reported: two in Essex County and one each in Bergen, Hudson, Mercer, Middlesex, Monmouth, and Ocean Counties. No deaths have been recorded. Seven cases were reported last year, with no fatalities.
"The surveillance and management of chronic mosquito-producing sites continues on a year-round basis, and includes wetlands management and the introduction of biological control agencies, such as mosquito-eating fish and tiny crustaceans, or copepods," said Bob Kent, administrator of the New Jersey DEP's Office of Mosquito Control Coordination.
The testing of the mosquito population "shows us where we have to focus our attention," Kent said Friday. "This has been a bad year, but we have an ambitious mosquito-control program."
But, he added, "we can't compare what's happening in the Northeast to Texas," which has 45 percent of the cases in the nation.
Testing birds for the virus will begin about mid-April and last until late October. Crows and blue jays are usually the focus, but the virus has also been found in hawks and robins.
In Gloucester County, one of four birds submitted to the state Department of Health on Wednesday tested positive for the virus, officials said Thursday.
"The mosquitoes started forming early this year," DEP spokesman Larry Ragonese said. "If you can't access [the breeding areas] by ground, you do flyovers. We provide funds and labs" for testing.
The majority of West Nile victims are infected between June and September, and 80 percent of them show no symptoms, CDC officials said. Those with mild or moderate symptoms have fever, headaches, rash, fatigue, nausea, and swollen lymph nodes. The symptoms appear three to 14 days after being bitten by an infected mosquito, the CDC said.
This week, New Jersey and Pennsylvania officials urged residents to clean out gutters and drains where water can accumulate and act as a breeding ground for insects. They also called on residents to dispose of old tires, turn over plastic wading pools and wheelbarrows, change birdbath water, and make sure all window and door screens are in good condition.
The West Nile season "goes on to Oct. 31," the Pennsylvania DEP's Whitman said. "We've still got two months to go."
Contact Edward Colimore
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