The odds of migrating birds rapidly spreading bird flu over immense distances are rather low, new research suggests. The findings, published in the Journal of Applied Ecology, have implications for our understanding of the spread of other bird-borne diseases, such as West Nile Virus. The H5N1 strain of influenza has been circulating in both wild and domestic flocks of birds for several years. Public health experts are keeping a close eye on the spread of the disease which has also infected over 500 people since 2003, 300 of whom died. Migrating birds have often been held responsible for the spread of the disease around the globe. Certain species of duck, goose and swan are often fingered as the chief culprits, as they can be infected and pass on the virus while remaining asymptomatic themselves. '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,' recalls lead author of the paper Dr Nicolas Gaidet of CIRAD (Centre de coopération internationale en recherche agronomique pour le développement) in France. '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 and his team scoured the scientific literature for information on the duration of asymptomatic infection in key species of wildfowl. They also attached tiny satellite transmitters to over 200 birds from 19 species of wildfowl, including mallards (Anas platyrhynchos), bar-headed geese (Anser indicus) and whooper swans (Cygnus Cygnus), all of which have been suspected of transmitting the virus over long distances during migration. This gave the team an unprecedented insight into the migratory journeys of the birds. By applying their knowledge of the duration of asymptomatic infection to the satellite-tagged birds, the team built up a picture of the likelihood of these birds actually spreading the flu virus during their migration. The researchers found that in theory, migratory birds could indeed disperse H5N1 over truly immense distances; some species can travel almost 3,000 kilometres (km) within timeframes that are compatible with the duration of asymptomatic infection. However, in fact the chances of this happening are extremely rare. The satellite tags revealed that birds tend not to complete their migrations in a single go. Instead, they break up the journey into a few long flights lasting between one and four days. Between each leg of their trip, they will typically rest at stopover sites. Crucially, these stopover periods usually last longer than the period of infection. As the team points out, this would prevent the birds from dispersing the virus over several consecutive but interrupted long-distance movements. 'Intercontinental virus dispersion would therefore probably require relay transmission between a series of successively infected migratory birds,' the researchers write. They conclude that: 'The likelihood of ... virus dispersal over long distances by individual wildfowl is low: we estimate that for an individual migratory bird there are, on average, only 5-15 days per year when infection could result in the dispersal of HPAI [highly pathogenic avian influenza] H5Na virus over 500 km.' They add that to spread the disease, the bird must remain asymptomatic; a bird with flu symptoms is unlikely to migrate, and if it does embark on the journey, it will probably not cover the same kinds of distances as a healthy bird. The findings have implications for our understanding of the spread of other diseases, including West Nile virus, bacterial diseases such as salmonellosis, and tick-borne Lyme disease.
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