Scientists from some 100 countries began a two-day conference in Rome to try to define the exact role of migratory birds in the spread of avian flu, one of several mysteries puzzling researchers nearly three years after the first outbreaks.

"There are still a lot of unknowns about this type of bird, about how the virus spreads. There´s a feeling of powerlessness and ignorance which pushed us to call this international conference," said Gideon Bruckner, a leading bird flu expert with the World Organisation for Animal Health (OIE).

Some 300 experts on bird flu are meeting Tuesday and Wednesday at the headquarters of the UN Food and Agriculture Organisation (FAO) in Rome.

"The more time passes the more we realize that the epidemiology of the virus is complex. Many aspects are little known or even unknown, and we are currently at a crossroads," said Joseph Domenech, FAO´s head of veterinary operations.

"The recent spread of the disease in Europe has shown that long distance transport of HPAI virus is a reality but many epidemioogical and virological facts remain to be better scientificially documented," said Domenech.

"The question today is to know what exact role they can play as reservoirs and carriers of highly pathogenic avian influenza," he added.

Scientists are puzzled over why some species of wild acquatic birds are more efficient carriers of the disease than others.

The most efficient prevention of a human pandemic is still based on the control of the disease at source in domestic birds.

Bird flu -- of which the highly pathogenic H5N1 virus is but one of its forms -- "is continuing to evolve at high speed and is also blowing away all the established results," said US expert Robert G. Webster.

The only certain element "is that the acquatic migratory birds are natural resevoirs of all the flu viruses," he said.

But "there are also enormous differences in the prevalance according to the species," said David Stallknecht, a specialist of the American university of Georgia.

Some acquatic birds which contracted the virus developed antibodies, others appeared to be genetically protected against the disease, while still others die, according to Dutch virologist Ron Fouchier.

"We have to change the sampling methods," to better understand how each species reacts to the virus, he said. "The virus varies considerably depending on the bird harbouring it."

But even more basic methods will first have to be strengthened if research is to be effective, said Juan Lubroth, an animal health expert with FAO.

"It´s essential to strengthen veterinary services in some developing countries," if scientists are to accumulate better data.

"For example, some official reports talk about dead ducks and geese but without any detail about the species or the sub-species to which they belong," he said.