For Kathy Zappulla of Smithtown, an unexpected letter from her daughter's school seven years ago with word of a head lice problem served as a jolting wake-up call.

The news helped transform Zappulla from a mom with no formal training in entomology to one of Long Island's leading experts on the insects.

Zappulla, who at the time picked up a case of the bloodsucking head crawlers herself, as did her son, decided to fight lice by opening a salon devoted to delousing infested tresses. She and her team work heads while peering through magnifying lenses, catching every louse and the eggs glued to strands of hair.

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"Our youngest client was a 2-month-old baby," she said, also noting her oldest, a 98-year-old great-grandmother. Yet Zappulla, who now owns three salons, is just one among numerous proprietors running head-delousing enterprises throughout Long Island and the greater metro area.

When it comes to head lice, business is booming.

Now, research by an Illinois entomologist who has scanned the genetic makeup of lice from throughout the United States helps explain why the creatures seem suddenly so prevalent, so difficult to kill.

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Kyong Yoon of Southern Illinois University has found that the tiny bloodsuckers circulating in more than two dozen states, including New York, are resistant to virtually every treatment on store shelves.

The result, according to Yoon and scientists elsewhere, has been the emergence of superstrains that won't die no matter how many times certain pesticide-containing treatments are applied.

"These products are available over the counter and chances are you will have a considerable amount of resistance in kdr lice," he said, using the acronym that stands for "knockdown-resistance," a widely dispersed gene mutation among lice across the country.

Experts, such as Deborah Altschuler, president of the National Pediculosis Association, a nonprofit educational resource on head lice in Massachusetts, caution consumers -- especially parents -- to stop using the in-home products when they fail the first time. "Children shouldn't be repeatedly exposed to pesticides," she said.

Just as bacterial superbugs have developed mutations allowing them to outmaneuver multiple types of antibiotics, so it is with pesticide-defying lice. Overexposure to the chemical compounds used against them has armed the insects for battle. Pesticide resistance is the way lice fight back and they've become very efficient at it, Yoon said.

Resistance, he added, didn't happen overnight, but has been building for years.

Yoon has found the insects are adept at repelling the class of insecticides called pyrethroids, which are contained in drugstore delousing products. Other studies have found that lice developed mutations against at least two prescription-grade treatments years earlier.

'Lice are lice'

Zappulla is unconvinced that resistant lice are more problematic than those susceptible to the chemicals. Before insecticide treatments emerged in the 20th century, she said, people were also bedeviled by the pests, and it wasn't because they were resistant. It's simply tedious to get lice and their nits -- eggs -- out of someone's hair.

"Lice are lice," said Zappulla, who prefers nontoxic methods of ridding hair of infestations. She uses a mint-based rinse that she says "stuns the bugs."

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But Zappulla also has developed expertise in removing nits with a special comb and has created a kit for parents to do it themselves.

Zappulla speaks frequently at PTA meetings in both Nassau and Suffolk counties, helping parents recognize a case of head lice. School-age children are the demographic most affected by the insects, although people of any age can get them.

"Lice are spread through head-to-head contact," said Zappulla, who notes that lice don't leap or fly, they simply crawl. She blames the cellphone-snapped "selfie" as a possible new mode of transmission.

Children's advocates say many parents thought head lice had vanished with a bygone century. "The story of head lice is really a story about children," said Altschuler, a proponent of combing as the preferred method of nitpicking.

Schools in some parts of the country have been closed temporarily, she said, because of lice outbreaks. "Children need to be checked regularly and anything that has found its way to their hair needs to be removed. Checking for lice should be as routine as brushing one's teeth."

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In an email, Janice McPhee, president of the New York State Association of School Nurses, cited a position paper from the school nurses' national organization that emphasized screening programs have not made a significant impact on head lice incidence. School districts, however, are free to set their own policies: "It is a local decision but should be based upon the current [scientific] evidence," McPhee said.

Schools in Nassau and Suffolk counties have long maintained "no nit" policies.

Port Washington district has its policy posted on individual schools' websites, stating parents "must notify the school nurse" when children have lice. Affected students are not allowed to return to class until there are no signs of nits. Similar policies exist throughout Nassau school districts.

In Suffolk, Betty Ann Grunseich, a school nurse at East Moriches Elementary, said the insects are not much of a problem at her school. She doesn't conduct lice checks.

"Moms are good about checking their children," Grunseich said. "They usually call me up and say, 'My child has head lice.' " Grunseich said she examines the child after treatment. Her examination, she said, extends to children's contacts, such as siblings and friends. "We get proactive fast so it doesn't get totally out of hand," Grunseich said.

Head lice -- Pediculus capitis -- like bedbugs, affect only humans and are a nuisance, but not public health threats. They don't transmit disease, but bacterial infections can arise as a result of constant scratching, doctors say.

Reliable data on annual head lice cases in the United States are sketchy, according to the Centers for Disease Control and Prevention, which broadly estimates about 6 million to 12 million infestations, mostly among children 3 to 11. Infestations are not reportable in New York or other states.

Unlike mosquitoes, both female and male lice bite -- and they're tiny -- about the size of the period at the end of this sentence. Lice have small black eyes and a pair of roving, segmented antennae. A human head is their planet Earth; everything above and beyond that head is, from a louse's perspective, deep space.

Instead of a long "drinking straw" proboscis common among other bloodsucking insects, lice are armed with a snoutlike projection outfitted with teeth, -- infinitesimal needlelike stylets, -- that grasp the scalp. Lice emit an anti-clotting agent in their saliva, ensuring that blood clots don't interrupt meals.

Cemented to hair

Females additionally are endowed with powerful adhesion molecules that cement one nit per strand of hair, usually at the base of the shaft. Eggs hatch in about five to 10 days, but empty hatched eggs remain securely cemented to the hair.

Dr. Nika Finelt, a dermatologist with the North Shore-LIJ Health System in Manhasset, is urging calm. She also is not keen on the term superlice. "I am not downplaying the resistance issue, I am just saying that people should not panic over the genetic mutations," Finelt said.

Other experts are not easily dismissing Yoon's scientific contributions. He found resistance mutations, specifically targeting pyrethroid insecticides, were "saturated" among lice in 25 out of the 30 states he studied. Pyrethroids are nerve-destroying chemicals for numerous insects, including mosquitoes, but resistant lice survive these neurotoxins because they have genetically altered the pathway required for the insecticide to work.

"Look, this isn't an urban myth. This is a sad state of affairs just like any other resistance problem," said Dr. Michele Green, a dermatologist at Lenox Hill Hospital in Manhattan. She said superlice genuinely exist based on numerous scientific studies, not just Yoon's.

Green, a physician who has treated difficult skin disorders, said she comes unglued at the sight of lice working their way through a patient's hair.

Altschuler, meanwhile, contends the nation as a whole passed the epidemic stage of head lice infestations years ago. The problem now, she argues, is that lice are endemic, meaning they are entrenched -- here to stay. She said resistance has been notably rising since the 1990s, when evidence emerged that lice were repelling malathion and lindane, two potent pesticides prescribed by doctors in ultralow doses to treat louse infestations.

Lindane was banned in 2009 for agricultural uses by the Stockholm Convention, an international health and environmental treaty. Lindane, however, still can be prescribed as a pharmaceutical to treat head and body lice.

Full-strength malathion is used against a vast range of agricultural pests and also in assaults on West Nile-carrying mosquitoes. Lice have flourished, Altschuler said, despite the combat.

"We can't allow head lice to become the new normal," she said.