Scientists in the United States are reporting a new strain of bird flu that has infected people in China is able to resist key anti-viral drugs and keep spreading in mammals.

A mutation in a new strain of bird flu infecting people in China can render it resistant to a key first-line treatment drug without limiting its ability to spread in mammals.

The discovery, according to a report published by Reuters, means that the newly-discovered H7N9 strain --- unlike other seasonal flu strains, which are often left less less transmissible after growing resistant to medications drugs like Roche's Tamiflu --- is not expected to lose any of its contagiousness, even after a barrage of medication.

Reserachers say the latest revelations about H7N9 doesn't mean the virus will evolve into a human pandemic, but that doctors should be more mindful when prescribing drugs to battle the illness in animals.

"It's important to emphasise that these H7N9 viruses seem to transmit fairly inefficiently overall," Nicole Bouvier, who led the H7N9 study, said in the Reuters report. "But what was surprising about our study was that the drug-resistant virus was no less efficient than the drug-sensitive one. Usually what we see with influenza, is that resistance... also confers a fitness disadvantage on the virus."

The study was published Dec. 9 in the journal Nature Communications.

The H7N9 bird flu was discovered earlier this year in China and has infected at least 139 people overall, resulting in 45 dead in China, Taiwan and Hong Kong.

Although some experts have said there is yet no evidence of any easy or sustained transmission of H7N9 between humans, a scientific analysis of probable transmission published last August strongly indicated it can at times jump between people, making a human pandemic a possible risk.

A separate group of researchers in the U.S. said earlier this week that H7N9 would have to undergo a series of mutations before it could jump from person to person.

Bouvier and her research team at the Mount Sinai School of Medicine, New York, analyzed a sample of a mutated H7N9 virus taken from an infected patient and found it spread between laboratory animals just as efficiently as its non-mutated counterpart, but showed limited ability to infect human cells in a laboratory dish, and was highly resistant to Tamiflu.

"When seasonal influenza viruses gain resistance to drugs, it usually happens at a cost to the virus - the cost being a reduced ability to transmit between hosts and to grow within them," the Bouvier team wrote. "This is unusual."