The H5N1 strain of bird flu is most likely to enter North America through infected poultry trade, researchers say.

British and American researchers studied how H5N1 moved out of China, across Asia and Europe and into the Middle East and Africa. The poultry trade was often the source, with migrating fowl then spreading it, they concluded.

"We conclude that the most effective strategy to prevent H5N1 from being introduced into the Western Hemisphere would be strict controls or a ban on the importation of poultry and wild birds into the Americas and stronger enforcement to curb illegal trade," they wrote in this week's online issue of the Proceedings of the National Academy of Sciences.

The virus has infected 258 people and killed 153, mostly in Vietnam, Indonesia, Thailand and China. More than 200 million birds have also been killed or slaughtered because of H5N1 since 2003.

In Europe, much of the spread of H5N1 followed a cold period when wild birds moved west looking for warmer climes. Some of the birds carried H5N1, said David Gibbons, head of conservation science with the U.K.'s Royal Society for the Protection of Birds.

In almost all cases, H5N1 was detected in wild birds that were dead, said Peter Marra, an avian ecologist with the Smithsonian's Migratory Bird Center.

The findings point to the importance of surveillance programs for early detection of bird diseases like H5N1, Marra said, noting that Canada, Mexico and other countries import day-old chicks from other regions while the U.S. does not.

About three-quarters of new diseases originate in animals, the researchers noted.

Genetic testing allowed most of the H5N1 outbreaks to be traced to the poultry trade, migratory birds or wild birds, but some outbreaks remained, such as those in South Korea, Russia, India, Pakistan and Cameroon, the researchers said.

Illegal trade in chicken feces for fertilizer and fish food could be a culprit.

On Monday, international experts said that compensating poultry farmers quickly and fairly is key to fighting bird flu.

The experts released the report ahead of a bird flu meeting by the World Bank, the UN Food and Agriculture Organization and the International Food Policy Research Institute.