A frightening strain of bird flu that can kill people is mutating into an ever more deadly form in ducks and needs to be controlled quickly, U.S. and Chinese researchers reported on Monday.

They found steady changes in the so-called H5N1 virus infecting flocks of apparently healthy ducks that made the virus more likely to kill mammals such as mice - and perhaps people, too.

“Our findings suggest that immediate action is needed to prevent the transmission of highly pathogenic avian influenza viruses from the apparently healthy ducks into chickens or mammalian hosts,” the researchers write in this week’s issue of the Proceedings of the National Academy of Sciences.

The H5N1 virus was first seen in Hong Kong in 1997 and has reappeared in southeast Asia, killing 24 people in Vietnam and Thailand this year. Whenever it appears in poultry officials move quickly to destroy the birds to prevent its spread.

Unlike ordinary influenza, so far H5N1 cannot be spread from person to person, so it does not cause human epidemics. But flu experts say the virus, which mutates quickly, could acquire this ability at any time.

Hualan Chen of the Harbin Veterinary Research Institute and colleagues analyzed 21 different samples of the H5N1 bird flu virus taken from healthy flocks of ducks in southern China between 1999 and 2002.

They inoculated groups of chickens, mice, and ducks with virus samples taken in different years.

The ducks never got sick, but most of the virus samples made chickens ill and killed them.

The key issue is mice, which are mammals like humans and more likely to react as humans do. “We observed an increasing level of pathogenicity to mice with the progression of time,” the researchers wrote.

“Viruses isolated in 1999 and 2000 were less pathogenic (deadly) to mice than those isolated in 2001 and 2002,” they added.

They found some expected changes in genes associated with how deadly a virus is and said their findings suggest the virus is evolving.

To date more than 100 million birds have been culled or have died from bird flu, which experts suspect was spread across Asia by migratory birds or wild fowl.

Different strains are infecting flocks around the world but the H5N1 strain is the one that most worries health experts.

SOURCE: Proceedings of the National Academy of Sciences, June 28, 2004.