Do migrating birds spread Bird Flu?

It’s the bird migratory season again. With the start of the season, another threat comes. The Bird Flu.

In 2005 an outbreak of the H5N1 'bird flu' virus in South East Asia led to widespread fear with predictions that the intercontinental migration of wild birds could lead to global pandemic. Such fears were never realised, and now research published in the Journal of Applied Ecology reveals why the global spread of bird flu by direct migration of wildfowl is unlikely.

The highly pathogenic H5N1 virus is primarily a disease of poultry, often resulting in mass mortality. However, the virus can also infect other species, including wild birds and humans. Some wild ducks, geese and swans can carry the virus asymptomatically, (without showing any symptoms) meaning that they have the potential to spread the virus as they migrate.

"The actual risk of H5N1 spread through migratory birds depended on whether infected individuals were capable of migratory movements while shedding virus, and the distance over which such individuals could travel. Our research has answered these questions using analysis of infection and migratory routes and timings for many bird species" says Dr Nicolas Gaidet.

His team analysed 228 birds from 19 species using satellite telemetry from 2006 to 2009 over the bird flu affected areas of Asia, Europe and Africa. The results indicated that migrating wildfowl are capable of dispersing the virus over extensive distances as much as 2900km before symptoms become apparent.

However, while this is theoretically possible the team found that direct virus dispersal by migrating birds would require asymptomatic infection to coincide precisely with the migration season. The results revealed a very small 'window' of between 5 to 15 days when dispersal of the virus over 500 km could occur.

It is crucial to the spread of disease over such a distance that an infected bird must not be showing the symptoms of infection. If the symptoms are evident then it is highly likely that the individual may not migrate, or at least they will be unable to cover the distance as well as a healthy bird.

Read the research paper here