A type of bird flu that has killed millions of chickens is becoming more infectious to mammals and scientists fear it could cause the next worldwide pandemic in
humans.
The avian flu has forced authorities to slaughter millions of chickens and other fowl in Asia to stem outbreaks in recent years. Thousands more have been killed in the US and elsewhere.
Already the flu has passed from birds to humans in Hong Kong, killing six of 18 people infected in 1997, and human cases have been reported since then in Vietnam and Thailand.
Now China-based researchers studying the H5N1 strain of the flu report that it has been changing over the years 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 Chen Hualan of the Animal Influenza Laboratory of the Chinese Ministry of Agriculture.
Human infections from bird flu remain rare, but the World Health Organization (WHO) considers it a potential major threat.
Two possibilities exist 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 human cases have derived from birds, and no evidence has arisen of the bird flu being passed from person to person.
Even more worrisome, the experts say, would be sudden change that could be caused should the flu combine with a human flu in someone's body. The two viruses could swap genes and create a potent hybrid as deadly as the bird strain and as contagious as a regular human strain.
Klaus Stohr, head of the WHO's global influenza program, noted the new study confirms that the virus is evolving fairly rapidly, and viruses that are pathogenic for chickens are more apt to be transmitted to humans because humans are in contact with chickens.
The tests in mice act as a magnifying glass to help understand how dangerous the virus might be for humans, Stohr said. While seeing no cause for panic, Stohr observed that the virus' ability to infect humans raises the possibility of a new pandemic of dangerous flu.
The new Chinese analysis looked at samples of the H5N1 virus collected from ducks between 1991 to 2002. The researchers tested the effect of the viruses in mice and found that the samples collected between 1999 to 2000 were less dangerous than those from 2001 to 2002. How the viruses evolved with increasing lethality is not clear, but it may have occurred in farms where pigs and ducks live in close proximity
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