Role of migrating ducks revealed in new study

WASHINGTON, Nov 2 (Reuters) - Migrating ducks, geese, and swans spread the H5N1 bird flu virus from Russia to Romania, Turkey and Ukraine, researchers said on Thursday.

A careful analysis of the spread of the virus from central Asia into eastern Europe in the autumn of 2005 shows that wild birds, especially mallard ducks, were the chief spreaders of the virus.

"We conclude that the spread of (highly pathogenic avian influenza) H5N1 virus from Russia and Kazakhstan to the Black Sea basin is consistent in space and time with the hypothesis that birds in the Anatidae family have seeded the virus along their autumn migration routes," the researchers wrote in the journal Emerging Infectious Diseases.

Anatidae include geese, ducks and swans, some of which are killed by H5N1, and other species of which often show no ill effects from the virus but which can spread it. Mallard ducks are the main suspect.

Bird flu remains mainly an animal disease, but has infected 256 people since late 2003, killing 152 of them, according to the World Health Organization.

Experts fear the virus could mutate and spark a human influenza pandemic, which could kill millions.

Domestic ducks and chickens can spread the virus to people, and the poultry trade is held responsible for some spread, but experts have also long believed that wild birds also can carry the virus over long distances.

Marius Gilbert of the Free University of Brussels and colleagues used satellite data to figure out the start of the autumn migration, and plotted known seasonal migration pathways against the actual outbreaks of H5N1.

They noted that adult birds can transmit the virus easily to juveniles during the molting season, when they cannot fly, and also noted that the virus can survive in cold standing water -- where many species of birds congregate while molting.

"The initial outbreaks of (highly pathogenic) H5N1 virus in Romania, Turkey, and Ukraine occurred close to wetlands frequented by overwintering migratory waterfowl," they wrote.

"These locations were clearly far from any known location where ... H5N1 virus had been recorded, while the timing and location match the autumn wildfowl migration ahead of the approaching wave of frost."

Bird experts around the world are testing many different species to see which ones carry and transmit the virus.

The researchers said that their study "by no means excludes the role of the poultry trade as an important, complementary transmission pathway."

U.S. and Canadian officials say they expect highly pathogenic strains of H5N1 to eventually arrive in the Americas, saying it is only a matter of time.