US block processed chicken shipments from China

Poultry shipments also likely spread bird flu - expert

"We forget that there is an enormous commercial industry with the movement of animals all the time. That, to me, is the most obvious thing to look for," said Kennedy Shortridge, who spent three decades studying influenza viruses.

"Don't rush to blame migratory birds straightaway," he said in Singapore at a bird flu conference organised by the Lancet medical journal.

Shortridge's assertions would probably not sit well with many experts in this field, who have credited the spread of the H5N1 virus in parts of Europe, Africa and the Middle East in the past few months to wild migratory birds from China's Qinghai Lake.

An outbreak of the virus in Qinghai Lake last May killed thousands of birds and that particular strain of the virus has since been found in affected places in Europe, Africa and the Middle East.

That gave rise to the popular theory the virus was probably brought to these places by surviving wild birds from Qinghai, in remote western China.

But Shortridge questioned this, saying: "Birds go north-south, they don't usually go east-west."

"There's a railway line that runs from there to one side of Qinghai Lake and there's a road that goes to the other side. If you look at the movements with H5N1, they don't seem to tie in with migratory bird routes for the simple reason they seem to follow the Trans-Siberian railway," he added.

"Lots of people don't realise that there's movement of poultry from one country to another, even to Nigeria, where we've got bird flu. People are transporting all sorts of poultry meat."

Since re-emerging in Asia in late 2003, the H5N1 virus has killed 113 people out of 205 reported infections, most notably from Asia, Turkey and Egypt.

Although it is predominantly a bird disease and most of the victims contracted the virus directly from birds, experts fear it could mutate into a form that will transmit easily among people and trigger a pandemic of catastrophic proportions.

However, not everyone was quick to debunk the migratory bird demon.

Hiroshi Kida of the department of diseases control at Japan's Hokkaido University said migratory birds and water as agents of viral transmission cannot be underestimated. H5N1 can be preserved in frozen lakes and ponds, and be carried from place to place by wild birds - not necessarily in one, spectacular sojourn, but many short-haul flights.

"Not one bird can do this, but many, many birds are involved in many, many short flights, helped by water," Kida, who gave a lecture at the conference, told Reuters later.

"Through water-borne transmission, the virus is carried to Europe, Africa."