Mutation in a new strain of bird flu in China can be resistant to first-line treatment drug, CBS News reports.

H7N9 bird flu emerged earlier this year in China and has infected at least 139 people so far in China, Taiwan and Hong Kong, killing 45 of them.

A separate team of researchers in the United States said this week that while it is not impossible that H7N9 could become easily transmissible from person to person, it would need to undergo multiple mutations to do that. So scientists around the world are keeping a watchful eye, on alert for any sign the virus might develop such potential.

Antiviral drugs such as Roche's Tamiflu, known generically as oseltamivir, are useful for treating flu infections, but human cases of Tamiflu-resistant H7N9 were found fairly swiftly after the current outbreak began.

For their study, the team at the Mount Sinai School of Medicine in New York analysed a mutated H7N9 virus taken from a sample from an infected patient in China, examining its resistance to drugs and its infectivity.

They found it was highly resistant to Tamiflu, but also that it still had the ability to infect human cells in a laboratory dish, and spread between laboratory animals just as efficiently as its non-mutated counterpart.

And since it is known that treating flu with antivirals can lead to drug resistance this study further underscores the need of prudent use of antivirals in H7N9 influenza infections.