Bird flu threatens humans
2004-06-30 09:42
Washington - A type of bird flu that has killed millions of chickens is becoming more infectious to mammals. 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 United States 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 Hualan Chen of the Animal Influenza Laboratory of the Chinese Ministry of Agriculture.
Human infections from bird flu remain rare, but the World Health Organisation considers it a potential major threat.
Mutate constantly
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 swop 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 programme, 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's ability to infect humans raises the possibility of a new worldwide epidemic, or pandemic, of dangerous flu.
Dr Linda Lambert, an influenza programme officer at the US national institute of allergy and infectious diseases, 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 would allow it to spread among people.
The new Chinese analysis looked at samples of the H5N1 virus collected from ducks in China in 1991-2002. Ducks can carry the 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.
Ducks the natural host
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 Chen.
How the viruses evolved with increasing lethality is not clear, but the researchers say it may have occurred in farms where pigs and ducks live in close proximity, which would have allowed the virus to move back and forth between mammals and fowl.
They said they have no reports of the H5N1 virus being isolated from pigs, but pigs have been infected with the virus in experiments.
Another possibility is that the virus was passed from ducks to humans and back again. But, they added, "The transmission of the virus from humans back to ducks is difficult to envision."
- SAPA