Changes in the avian flu virus have made it less deadly to ducks, potentially turning them into medical Trojan horses where the flu can hide while continuing to infect other birds and humans.

Waterfowl such as ducks have been natural hosts of this type of influenza before but rarely became ill from it until 2002, when an evolving strain killed of a large number of the birds.

Since then, however, the virus has continued to change, reverting to a form less dangerous to ducks but still able to cause illness and death in chickens and humans, according to a study in Tuesday's issue of Proceedings of the National Academy of Sciences.

"These results suggest that the duck has become the Trojan horse of Asian H5N1 influenza viruses," reported a research team led by Robert G. Webster of St. Jude Children's Research Hospital in Memphis, Tennessee.

"The ducks that are unaffected by these viruses continue to circulate these viruses, presenting a pandemic threat," the team said.

The researchers infected domestic ducks with flu isolated at various times.

They found that ducks infected with H5N1 from 2003 or 2004 were contagious for 11-17 days, a longer transmission time than pre-2002 strains.

Source: China Daily