The swine-flu outbreak stressed the New York City Health Department's influenza lab, prompting urgent response to a never-before-seen viral threat.

In the days after the H1N1 strain first appeared, the virus led to an exponential increase in the lab's workload, with technicians being able to test more flu samples daily than ever before. At the height of the outbreak, more than 200 department employees worked 12- to 18-hour days, officials said. Typically, the workers had eight-hour shifts.

Now health officials are looking worriedly toward fall, the traditional beginning of flu season. They're concerned a severe mutant strain of swine flu may emerge and stretch their resources to alarming lengths. "If we had an epidemic of the most severe form of H1N1, that's certainly going to test our resources," Health Department commissioner Dr. Thomas Farley said. He added, "We think we can respond to any future strain."

The persistent virus first appeared in April in Queens and since then New York City consistently led the nation in swine flu deaths, national figures show.

The city's first swine flu death occurred May 16. Nearly 40 more people have died since of H1N1-related illness, and hundreds of thousands have been sickened. The city has spent $10 million in its swine flu response. The Health Department alone spent $4 million, with most used to bolster the lab, officials said.

The city's flu lab was one of the first in the country that could confirm swine flu cases, rather than having to send samples to the national Centers for Disease Control and Prevention.

"As the number of samples of confirmed H1N1 increased [nationwide], the CDC reached a point when they knew it was time to move on to the next point of local laboratory testing," said Dr. Sara Beatrice, the city's flu lab director.

In days, the city went from flying samples by courier to the CDC in Atlanta for confirmation, which could take days, to confirming H1N1 in the lab within six hours. The local testing allowed the city to better pinpoint swine flu clusters, research the movement of the virus and help control its spread.

New York City is one of 102 municipalities nationwide with the capability. Health departments in Nassau and Suffolk cannot confirm swine flu and depend on the state Health Department. Since performing its own H1N1 confirmation in early May, the city lab now has the capacity to test 150 samples daily. Typically, the lab tests about 100 medical samples for seasonal flu for the year, officials said.

In the weeks after the outbreak, more manpower was needed to research the virus. The lab's staff went from three technicians a day to 15 or 20. The staff worked around the clock with newly purchased, complex testing equipment. Technicians used the machines to analyze nose and throat swabs. To confirm swine flu, they isolate protein-producing RNA, or genetic material, indicative of the virus.

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Microbiologist Cherry-Ann Da Costa Carter said she and her weary colleagues didn't mind the increased workload in those first weeks. "We were just pumped up with adrenaline," she said. "We looked forward to using our skills - but not the casualties."

By early this month, the lab had tested more than 2,300 specimens for swine flu and confirmed more than 1,260 H1N1 virus cases. The lab does not test all suspected swine flu cases. Testing is usually reserved for large outbreaks.

The CDC, using mathematical models based on populous cities where the virus was most prevalent, estimated at least 1 million Americans have contracted H1N1. It said 500,000 New Yorkers most likely contracted the virus. Luckily for the city, most swine flu cases have been mild.

Circumstances would be different if the virus was more potent. At a June hearing at City Hall, Farley acknowledged the city would need federal emergency aid to battle a more severe swine flu strain. But he said recently his agency, after surviving the virus' first major attack, is ready for what could be a challenging flu season. "I don't want anyone to think we are not adequately prepared to respond to the next wave of this epidemic," he said.