Ducks Help Spread Asian Bird Flu

Ducks may be playing a significant role in spreading Asian bird flu, according to a new study. Researchers have shown that the animals can carry and shed the flu-causing H5N1 virus for up to 17 days, posing a long-term danger to other birds as well as humans.

The H5N1 virus likely evolved in domestic ducks in Asia, where it was endemic and produced only mild illness. But in 2002, H5N1 became lethal even for waterfowl, killing several species in a Hong Kong park. Then, in the winter of 2003-04, the virus began its rampage through domestic poultry flocks in Southeast Asia. More than 140 million birds died or were culled, and some 50 people were killed. Despite attempts to stamp out the disease by culling infected flocks, the virus continues to resurface (ScienceNOW, 6 July).

Scientists typically monitor the spread of H5N1 by watching for signs of illness in domestic poultry. But suspecting this approach might not be enough, virologist Robert Webster, of St. Jude Children's Research Hospital in Memphis, Tennessee, and his team infected mallard ducks with H5N1 strains isolated in 2003 and 2004. The ducks survived and shed a virus that proved lethal to chickens for 11 to 17 days, far longer than the 2 to 5 days reported for earlier versions of the virus. This means infected ducks put more virus into the environment than previously thought, increasing the chance of exposure for other animals and humans. H5N1 appears to have reverted to causing only mild or no illness in ducks, allowing them to survive and spread the disease longer, the team reports online this week in Proceedings of the National Academy of Sciences.

In response to the team's early results in mid-2004, Thailand started testing domestic duck flocks and culling those infected, instead of just watching for signs of illness. And a survey by Vietnam's Ministry of Agriculture and Rural Development backed Webster's findings by discovering that a high percentage of domestic ducks in several areas carried H5N1 without signs of illness. As a result, Vietnam has instituted a temporary ban on hatching domestic ducks and is investigating vaccines that would allow farmers to resume raising them.

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