More than 140 million chickens have been slaughtered in the region in a bid to halt the disease.
A new study reports that the H5N1 virus could kill some ducks after causing only mild symptoms which could lead to undetected trasmission of the virus in flocks.
Public health experts say the avian flu virus is mutating, and fear it could develop the ability to spread easily from person to person, and kill millions in a flu pandemic.
Dr. Robert Webster of St. Jude Children's Research Hospital in Memphis, Tennessee, and colleagues found more evidence of the virus mutating.
"Wild waterfowl, including ducks, are natural hosts of influenza A viruses," they wrote.
"These viruses rarely caused disease in ducks until 2002, when some H5N1 strains became highly pathogenic," they said, adding that their study showed the viruses were again becoming harmless to ducks.
