PORTLAND (AP) — An unusual strain of swine flu has infected five young children in Maine, Pennsylvania and Indiana, health officials said Friday.

A young boy of 7 or 8 from Cumberland County became the latest case when he came down with flu symptoms in early October, not long after being exposed to pigs at an agricultural fair, the Maine Center for Disease Control and Prevention said. He and the other four children have recovered, health officials said.

The H3N2 swine-origin strain was confirmed by the U.S. Centers for Disease Control as the same strain identified earlier in three cases in Pennsylvania and one case in Indiana, according to Maine’s state epidemiologist Stephen Sears.

The three infected children in Pennsylvania had all attended an agricultural fair in August where pigs were exhibited, the CDC said. The Indiana child had not been exposed to pigs, but is thought to have been infected in late August by another person who had recently been exposed to pigs, but who did not have flu symptoms.

Unusual strains of influenza pop up virtually every year, health officials said, and there’s nothing to suggest that this H3N2 strain is being widely transmitted from person to person. The infected boy in Maine was treated by a family doctor and is back up and playing, Sears said.

“We’re taking a prudent public health approach to this,” Sears said. “This is an unusual virus, but it doesn’t appear to be spreading in people, so we don’t think it’s a major issue.”

The first case of H3N2 was reported to the federal CDC in late August after a boy under 5 from Indiana experienced a fever, cough, shortness of breath, diarrhea and a sore throat. The boy was hospitalized for treatment and has since recovered.

Like humans, pigs can become infected with influenza viruses and suffer the same symptoms people do. Pigs can become infected not only with swine flu viruses, but with human and avian influenza viruses as well, the CDC says. On rare occasions, humans can become infected with swine flu viruses; the CDC says fewer than two dozen cases have been documented in the last five years.

The H3N2 virus contains a gene picked up from H1N1 swine flu virus that resulted in global pandemic in 2009, the first combination virus to turn up in people since the pandemic, according to the CDC. It is a hybrid of viruses that have infected pigs over the last decade.

Influenza viruses are constantly changing and picking up genes from other viruses, and the H3N2 strain represents a step in the evolutionary process of a virus taking on new genetic properties, said federal CDC spokesman Tom Skinner.

“Right now we don’t see any widespread person-to-person transmission, and that’s the key when it comes to influenza cases,” Skinner said.

Meanwhile, seasonal influenza activity remains low around the country with the arrival of seasonal flu season, the CDC says. The agency said earlier that this year’s vaccine, the same as last year’s, likely would not protect against the new swine strain.

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