Dr Leon Bennun, Director of Science, Policy and Information for BirdLife International, raises an interesting possibility in the BBC’s Green Room series – he points out that the outbreaks lie mainly on major trade routes, rather than on the routes taken by migrating birds. And countries which have implemented strict controls on importing and movement of poultry, such as Japan and South Korea, have had no outbreaks after the controls were imposed:
In fact, countries which have not yet developed a large-scale intensive poultry industry have also been largely spared. The UN Food and Agriculture Organization (FAO) reports that in Laos, 42 out of 45 outbreaks affected intensive poultry units.
So – it could be that avian flu is largely caused by the overcrowded, insanitary conditions in many large intensive poultry-farms. As Bennun says of the dying swans being reported across Europe:
They may have caught the disease from other wild birds; but this is unlikely given the tens of thousands of waterfowl that have tested negative for H5N1 over the last decade. Much more likely is that before starting out, they picked up the virus from farms, either from infected poultry or their faeces. Mute swans often graze agricultural fields, and are likely to have come into contact with poultry manure spread as a fertiliser.
If wild birds had been spreading the disease across continents there would have been trails of outbreaks following migration routes; but this hasn’t happened…