New research has shown that avian influenza viruses make mallard ducks thinner than other ducks.

Researchers tested thousands of ducks migrating through Sweden. The results showed that the viruses do affect the birds, contrary to conventional wisdom that the pathogens have no effect on them.

"Mallard ducks are a main reservoir [hosts a virus without becoming ill and thus serves to spread it] for low-pathogenic avian influenza virus in nature, yet surprisingly little is known about how infection affects these birds," Jonas Waldenstrom of Sweden's Kalmar University, Albert Osterhaus of Erasmus Medical Centre in Rotterdam and colleagues reported in the Proceedings of the Royal Society B: Biological Sciences.

"We analysed 10,000 samples from migratory mallards in Sweden for presence of influenza virus and were able to demonstrate that infected birds were leaner than uninfected birds, and that weight loss was related to the amount of virus shed in their faeces," said Waldenstrom's team.

Inter-continental spread of bird flu

"Although many mallard populations are migratory, the short virus shedding times (often less than a week) imply that individual birds are not long-distance dispersers of the virus on a continental scale," they added.

Waldenstrom's team found that infection did not affect how fast or how far the birds migrated. On average, the ducks were infected eight days and spread the virus for just three of them in their droppings.

"The short virus shedding time suggests that individual mallards are less likely to spread the virus at continental or intercontinental scales," they wrote.