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 British Ecological Society's Journal of Applied Ecology reveals why the global spread of bird flu by direct migration of wildfowl is unlikely but also provides a new framework for quantifying the risk of avian-borne diseases.
The highly pathogenic H5N1 bird flu virus is primarily a disease of poultry, often resulting in mass mortality for infected flocks. However, the virus can also infect other species, including wild birds and humans. Experimental infection has also revealed that some wild ducks, geese and swans can carry the virus asymptomatically, that is before the symptoms of the virus become apparent, meaning that they have the potential to spread the virus as they migrate.
"The potential risks to humans led to extensive media coverage often focusing on migratory birds, which fuelled public concern and led to calls for the mass culling of wild birds," said lead author Dr Nicolas Gaidet. "However, 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."
Dr Gaidet's 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 do have the potential to disperse H5N1 over extensive distances as mass migration can result in infected birds covering 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.