updated 6/28/2004 5:59:32 PM ET 2004-06-28T21:59:32

A strain of bird flu that scientists fear could lead to a worldwide pandemic in humans is becoming more infectious to mammals.

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Millions of chickens and other fowl have had to be slaughtered in Asia — and thousands more have been killed in the United States and elsewhere — to stem outbreaks of avian flu in recent years. Scientists say it is only a matter of time before the virus adapts to spread among humans.

The flu already passed from birds to humans in Hong Kong in 1997, killing six of 18 infected people. Since then human cases also have been reported in Vietnam and Thailand.

Strain becoming more lethal
Now China-based researchers studying the H5N1 flu strain report that over the years it is changing to become more dangerous to mammals. Their research, based on tests in mice, is reported in Monday’s issue of Proceedings of the National Academy of Sciences.

“Our results demonstrate that while circulating in domestic ducks, H5N1 viruses gradually acquired the characteristics that make them lethal in mice,” reported the team led by Hualan Chen of the Animal Influenza Laboratory of the Chinese Ministry of Agriculture.

While human infections from bird flu remain rare, the World Health Organization considers it a potential major threat to people.

There are two possibilities for the bird virus becoming a serious danger to people.

Viruses constantly mutate and this one could accumulate enough genetic changes to become good at passing between humans. So far the human cases have derived from birds and there is no evidence of the bird flu being passed from person to person.

Virus is rapidly evolving, expert says
More scary, the experts say, would be a sudden change caused by combining with a human flu in someone’s body. The two viruses could swap genes and create a potent hybrid with the deadliness of the bird strain and the contagiousness of a regular human strain.

Klaus Stohr, head of the WHO global influenza program, noted that the new study confirms that the virus is evolving fairly rapidly, and viruses which are pathogenic for chickens are more likely to be transmitted to humans because humans are in contact with chickens.

The tests in mice are a type of magnifying glass to help understand how dangerous the virus might be for humans, Stohr said in a telephone interview.

While saying there is “no cause for panic,” Stohr observed that the virus’ ability to infect humans raises the possibility of a new worldwide epidemic, or pandemic, of dangerous flu.

Dr. Linda Lambert, an influenza program officer at the U.S. National Institute of Allergy and Infectious Disease, said the flu virus has an error-prone replication system.

It plays Russian roulette with its genome, she said, and could get a change that allows it to spread from person to person.

Details of Chinese study
The new Chinese analysis looked at samples of the H5N1 virus collected from ducks in China in 1991-2002. Ducks can carry this flu without appearing ill.

The researchers tested the effect of the viruses in mice and found that the samples collected in 1999-2000 were less dangerous than those from 2001-2002.

They rated the viruses as low, middle or high pathogens for mice.

Those rated low infected the lungs in modest amounts. In the middle group the viruses infected the lungs and in some cases the spleen and brain. The group rated high invaded the lungs, spleen, kidney and brains of the mice.

“While circulating in ducks, the natural host, H5N1 viruses gradually acquired the ability to infect and kill mammals,” said researcher Hualan Chen.

How the viruses evolved increasing lethality is not clear, but the researchers say it may have occurred in farms where pigs and ducks live in close proximity, allowing the virus to move back and forth between mammals and fowl.

They say there have been no reports of the H5N1 virus being isolated from pigs, but pigs have been infected with the virus in experiments.

An alternative possibility is that the virus was passed from ducks to humans and then back again. But, they add, “the transmission of the virus from humans back to ducks is difficult to envision.”

The research was funded by grants from the Chinese National Key Basic Research Program, Chinese National S&T Plan, U.S. National Institutes of Health and the American Lebanese Syrian Associated Charities.

© 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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