ATLANTA (AP) -- Health officials are reporting what may be the first instance of a Tamiflu-resistant swine flu virus spreading from one person to another.

It happened in July at a camp in western North Carolina, where two teenage girls -- cabin mates -- were diagnosed with the same drug-resistant strain of swine flu.

Tamiflu is one of two flu medicines that help against swine flu, and health officials have been closely watching for signs that the virus is mutating, making the drugs ineffective.

The Centers for Disease Control and Prevention is aware of nine U.S. cases of Tamiflu-resistance since swine flu first appeared in April, but all the others were single cases. In this instance, there seemed to be a spread.

"That was the concerning thing about these cases," said Dr. Zack Moore, a respiratory disease epidemiologist for the North Carolina Department of Health and Human Services.

The virus may have spread from one girl to the other, or it's possible that the girls got it from another camper. It's also possible that they each developed a resistant strain independently, but that's unlikely, Moore added.

Both girls had been given Tamiflu before they got sick -- as a preventive measure -- after an outbreak of swine flu at the camp. They were among more than 600 campers and camp staff treated.

That may have been part of the problem: Overuse of medicines can contribute to viruses becoming drug resistant.

The CDC this week issued revised guidance advising against giving flu drugs to prevent illness in most healthy people, even if they may have been exposed to an infected person.

The CDC recommends fast treatment with Tamiflu or Relenza for anyone hospitalized with a flu-like illness. They also advise prompt treatment at the first sign of symptoms for those at high risk for serious complications, including pregnant women, children younger than 5, and people with certain chronic conditions like asthma and heart disease.

"Tamiflu is still considered effective. This is just a reminder we need to be cautious with these drugs," Moore said.

The North Carolina cases are reported in Thursday's issue of a CDC publication, Morbidity and Mortality Weekly Report.

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