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Bird flu could be spreading from birds to people more frequently than official figures suggest, despite being hard to catch, says a study published in Archives of Internal Medicine yesterday (9 January).

The authors say that although transmission may require direct contact with infected birds, there may be many more infected people than previously thought in areas where the deadly H5N1 virus is prevalent.

These unrecognised infected people would have milder symptoms than those the bird flu 'radar' of doctors and surveillance systems detects.

The researchers conducted their study in FilaBavi, an area of northern Vietnam that has experienced H5N1 outbreaks in poultry. There, they added two questions to an existing quarterly survey.

The first asked whether people had been sick with a cough and either fever or breathing difficulties in the past six months. The second asked if they had been in contact with healthy, sick or dead poultry during the same period. The team found a direct link between the answers.

People in households where there had been sick or dead poultry were more likely to have been ill, whereas households with healthy birds were not. And people who had direct contact with sick or dead poultry were much more likely to have had a flu-like illness.

Albert Osterhaus, professor of virology at the Erasmus Medical Center in the Netherlands, says however that without further study it is difficult to rely on the link between survey answers. "It's a pity that no virology and serology was carried out" to see whether those surveyed were infected with H5N1, he says. 

The study's authors say that the flu-like disease they report is much milder than the confirmed human cases of bird flu.

They believe that the official numbers, which have led to reports that transmission to humans is rare, represent only a small proportion of severe cases — those who have reached the advanced health care services only available in large cities.

The reality, they say, might be that the transmission of the virus is "frequent", requires direct contact with dead or sick poultry, and often leads to mild flu-like symptoms.

Link to full abstract of paper in Archives of Internal Medicine

Reference: Archives of Internal Medicine 166, 119 (2006)

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