Health experts says they're investigating the apparent spread of a Tamiflu-resistant swine flu virus.

"USA Today" reports the virus has been identified in four patients at Duke University Medical Center in Durham, North Carolina and five cases in Wales.

Researchers say the clusters appear to be the first in which a virus resistant to the antiviral Tamiflu has spread from person to person.

Duke's Daniel Sexton, who is leading the hospital's investigation, says if the Tamiflu-resistant virus spreads widely, H1N1 will become tougher to treat and may cost more lives. Sexton says three of the four Duke patients have died and the fourth is extremely ill but appears to be recovering.

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

(Copyright 2009 by VERTEXNews/Newsroom Solutions)