Epidemic experts say they are investigating the apparent spread of Tamiflu-resistant swine flu virus among four patients at Duke University Medical Center in Durham, N.C., and five in a hospital in Wales.

These clusters appear to be the first in which a virus resistant to the antiviral Tamiflu, a mainstay of flu treat, has spread from person to person, researchers said Friday.

If Tamiflu-resistant virus spreads widely, swine flu will become tougher to treat and may cost more lives, says Duke's Daniel Sexton, who is leading the hospital's investigation.

Doctors say investigations of the two hospital outbreaks are underway, but the preliminary genetic evidence suggests that the virus spread among patients at the hospitals.

"The four patients involved in this situation had the same resistance pattern," says Sexton, adding that researchers at the Centers for Disease Control and Prevention are now testing virus samples from Duke to see whether they're identical.

"The resistance pattern suggests that might be the case," he says.

All of the Duke patients were located in a ward for people with cancer or severe blood disorders. All were severely ill and were highly susceptible to infections, Sexton says.

The patients became ill with flu in October. When they didn't respond to Tamiflu two weeks ago, the hospital sent specimens of their virus to see whether it might be resistant to antiviral treatment. The results came back this week, prompting Duke to launch an investigation.

CDC spokesman Dave Daigle confirmed that three CDC epidemiologists have arrived at Duke to assist in the investigation, alongside infectious disease experts from the hospital and the state health department. The research team is also trying to determine whether Tamiflu-resistant flu is circulating elsewhere in North Carolina.

Three of the four Duke patients have died, Sexton says. The fourth is extremely ill but is being treated with another antiviral called Relenza and appears to be recovering.

In Wales, doctors have confirmed five Tamiflu-resistant swine flu cases in one ward of an unidentified hospital. Three more patients on the ward are being tested for drug-resistant virus; a ninth patient is infected with virus that is still susceptible to Tamiflu.

"At present we believe the risk to the general healthy population is low," Britian's Health Protection Agency said in a statement, noting that there is no evidence that the resistant virus is any more virulent than any other form of flu. A community-wide investigation is ongoing, the agency said.

So far, like the swine flu virus isolated from the Duke patients, the Wales virus remains sensitive to the antiviral drug Relenza.

As of Friday, the World Health Organization has reported 57 cases of Tamiflu-resistant virus worldwide.