US Government

The bird flu H5N1 virus is spreading fast, and the general view presented in the media is that migratory birds are to blame.

Yet, writes Dennis Normile in Science, bird experts have been almost unanimously sceptical about this theory. They argue that sick or dying birds cannot fly very far, and that even if they were carrying the virus, H5N1 should already have arrived in places where it has not.

Moreover, researchers in the US and China have been monitoring wild birds for several years, looking for healthy birds carrying H5N1. So far, both searches have found nothing.

Rather, experts have been arguing that it is human movement of birds that is spreading the disease. Examples include poultry trade and, more unusually, a traveller caught smuggling birds of prey from Thailand to Belgium. Tests showed that these birds were carrying H5N1.

But outbreaks among wildfowl in remote corners of China and Mongolia — where movements of domestic poultry have been ruled out as a cause — are forcing some to change their minds. A Chinese team, for instance, is speculating that mildly infected birds could be carrying the virus long distances.

As researchers scramble to pinpoint the cause, surveillance remains patchy, and efforts to fight bird flu in Asia is failing to get adequate international funding.