If current trends continue into late summer and fall, when most West Nile cases - in humans as well as mosquitoes - typically appear, this year could be the region's worst since 2003, at least for the insects.

That year, 237 people in Pennsylvania and 34 in New Jersey were infected, the highest in either state since the disease first appeared in New York City in 1999 and gradually spread nationwide.

What this means for human health is unknown. Studies have shown that the vast majority of people infected do not get sick but develop immunity for life. And large numbers of people have been exposed over time, perhaps explaining part of the drop-off in human cases in recent years.

But officials are taking no chances.

The Pennsylvania Department of Environmental Protection announced yesterday that it was increasing its insecticide spraying program in the counties of Bucks, Delaware, Montgomery and Philadelphia. Health officials cautioned residents to use insect repellent outdoors at dusk and after dark, when mosquitoes are most active.

The number of mosquito pools from those four counties that tested positive for the virus as of yesterday is double what it was by the same date in 2003. (The number tested varies somewhat every year but does not account for the magnitude of the new finding.) Statewide, those counties accounted for almost 70 percent of the positive test results.

New Jersey health officials were not available for comment yesterday. The department's Web site reports 237 positive mosquito pools as of Friday. The highest number by this date in any year since 2000 was 150 pools, in 2005.

And records in Trenton show that while 5 percent of mosquito pools usually have tested positive for the virus by the end of the year, a spokeswoman said in an e-mail message, they are already at 6.5 percent, indicating "both an increase and reaching a certain level of virus activity faster in the season."

Will the trend continue?

Because scientists have less than a decade's worth of experience with the disease in North America, the long-term pattern is unclear.

Roger Nasci, a medical entomologist with the U.S. Centers for Disease Control and Prevention, said regions can experience a bump in activity.

West Nile is "notorious for being focal and sporadic, so finding one relatively small area that has higher transmission activity than other areas is more normal than abnormal," Nasci said.

The virus is affected by mosquito population densities, the kind of mosquitoes (only three of 69 local species serve as hosts), the availability of susceptible birds to act as carriers, and weather patterns that include humidity, rainfall and temperature.

"You put all these complicated factors together, and in certain circumstances you could wind up with a situation that promotes more virus activity," Nasci said.

State surveillance programs are designed for just that reason, he said, "so they have the knowledge to allow them to respond appropriately."

Nationwide, Nasci said this year's "high-risk" areas include Southern California, portions of the Gulf Coast, and "traditional hot spots" in the Dakotas.

Pennsylvania environmental protection officials speculated yesterday that mosquito populations might somehow be overwintering in the southeastern portion of the state instead of dying off at first frost.

"The virus has established a foothold" this year, environmental program manager Nancy Roncetti, of the DEP's southeastern regional office, said at a news conference yesterday.

They said this year's rate of individual mosquitoes that test positive for the virus, an indicator of how pervasive it is in a particular insect population, is higher than in 2003.

Officials advised residents to empty standing water in wading pools, bird baths, flower pots, and any other place with stagnant water, where mosquitoes breed.

"This is something we can't control alone," said Joseph M. DiMino, Montgomery County medical director. "We need everybody's help."

So far this year, officials said, one Pennsylvanian has been infected: a 27-year-old Montgomery County woman who has since recovered. No human cases have been reported in New Jersey.

Nationwide, 236 human cases have been reported, including two deaths, mostly in the South and West.

Officials said most human cases do not arise locally until September and October, and 80 percent of those infected will experience nothing worse than a cold.

Most of the rest will get a serious fever, headache, nausea, or swollen lymph nodes for a week or two; under 1 percent get the more serious neuro-invasive disease, which can cause coma and paralysis. There is no cure.

"It may be reassuring to know that that many people probably have immunities because they were infected in the past, but never got sick," said Leanne Gasink, associate epidemiologist at the Hospital of the University of Pennsylvania.

To further limit the impact in Southeastern Pennsylvania, the DEP's Roncetti said, spraying crews have been brought from elsewhere in the state. By day, they fan out to sample areas of known mosquito infestations or others that have been reported.

At dusk, they begin spraying insecticides in areas with mosquitoes that have tested positive.

Matt Helwig, the state West Nile coordinator, said the the spray is a neurotoxin for mosquitoes and is not harmful to humans. He said it is the same substance used to treat head lice.

Among today's scheduled sprayings, a larvicide will be applied this morning by helicopter over wetlands and the Philadelphia wastewater treatment plant near the airport.

Later in the morning, wetlands at the city's northeast wastewater treatment plant near the Betsy Ross Bridge will be sprayed.

Areas of Collingdale, Colwyn and Yeadon in Delaware County, and portions of Kennett and Pennsbury Townships in Chester County also will be sprayed today.

Links to West Nile updates, including spraying

schedules in Pennsylvania: http://go.philly.com/health