An international team, which included experts from China, Indonesia, Thailand, the US and Vietnam, say that domestic ducks possibly pose a major threat of spreading avian flu to animals and humans, as not only do they carry the virus with few visible signs, the virus is able to mutate in them, which means they could trigger a human pandemic.
The team studied mallard ducks and tested the ducks in the laboratory in order to establish if ducks could possibly play a role in spreading further outbreaks.
As a direct result of bird flu outbreaks in several East Asian nations since 2003, millions of chicken and other fowl have died or been destroyed.
The deadliest strain of the disease, H5N1, has killed a total of 54 people in Vietnam,Thailand and in Cambodia since late 2003.
Humans catch the disease through close contact with live infected birds.
Infected birds excrete the virus in their faeces, which then dries, becomes pulverized, and is then inhaled, and humans catch the disease through close contact with live infected birds.
Until now the main focus has been on infected chickens, but more recently cases have been reported in domestic ducks.
Dr Robert Webster, from St Jude Children's Research Hospital in Memphis, and colleagues, now believe these animals have become the "Trojan horse" of the virus - 'a silent reservoir of infection'.
In their study the researchers infected two four-week-old mallard ducks with various strains of the virus isolated in 1997-2004 from humans or poultry throughout South Asia, including H5N1.
Four hours later they placed these ducks in the same cage as two other non-infected ducks, and over the next 21 days the researchers observed what happened to the birds.
They found that both of the infected ducks shed the virus, mainly through the upper respiratory tract rather than in their droppings and the other two ducks caught the virus, and one was completely cleared of it by day seven.
The team found that the viral characteristics had changed since 2002, and the ducks infected with H5N1 from 2003 or 2004 shed the virus for 11-17 days, a longer transmission time than pre-2002 strains.
Dr Webster and colleagues says this shows that the H5N1 viruses are reverting to nonpathogenicity in ducks, and since this newer strain is still potentially harmful to humans, ducks may play a role in spreading further outbreaks.
Although the virus no longer caused disease in healthy ducks exposed to it, it was still able to cause disease in chickens.
Of course the very real danger is that a mutation will arise that will mean the virus can spread directly between humans.
A spokeswoman from the UK's Health Protection Agency says the research offers some interesting new information relating to disease in ducks, but the implications for human health or human transmission remain speculative.
The World Health Organization says that the spread of avian influenza viruses in animals needs to be monitored, as the viruses have proven their ability to jump the species barrier.
Dr Juan Lubroth, an expert in avian flu from the Food and Agriculture Organization, confirms that the lab work supports what is being seen in Asia, and says it is important to control the infection, by vaccinating healthy animals, culling infected ones and encouraging better farming practices.
He says if farmers are financially compensated for any animals lost to culling it would encourage them to alert authorities to cases of infection.
The findings appear in Proceedings of the National Academy of Sciences.