As public health anxieties about a global influenza epidemic rise after confirmation that a deadly form of avian flu has reached Europe from Asia, a troubling question is being asked: how has the virus spread relatively quickly over such long distances? Is Australia at risk from the long march of contagious disease?
Until July, outbreaks of the H5N1 virus were restricted to East Asia. Since then, the infection has been confirmed among birds - chiefly farm poultry - in parts of western China, Asian Russia east of the Ural Mountains, Kazakhstan in Central Asia and, in recent days, in Turkey, Romania, European Russia, Croatia and Greece.
There is increasing evidence that bird flu is being spread by wild waterbirds, mainly in the Anatidae family of ducks, geese and swans - as they migrate from their spring-summer breeding grounds in Russia and other parts of the northern hemisphere to escape the winter freeze there to Europe and Africa.
Migratory birds travel in huge numbers along various overlapping routes called flyways. These aerial highways stretch from the far north of the Eurasian landmass to Africa, India, Australia and New Zealand in the southern hemisphere. Several of these flyways straddle Europe. The one that is bringing waterbirds into the Balkans and the Danube delta, Europe's largest wetlands near the Black Sea, is known to ornithologists as the Black Sea-Mediterranean flyway. Most of the birds using this route are heading for the Middle East, North Africa and East Africa.
If this epic movement of wild birds from north to south and back again every year has indeed become a conveyor belt for avian flu, it has disturbing implications. The United Nations Food and Agriculture Organisation has warned that the northwards migration of birds in spring next year may spread the H5N1 virus further into Europe because birds coming from southern zones will have mixed with migrants from the north. The avian flu virus is shed in birds’ faeces and saliva. It is passed on among ducks, geese and swans that share common water and resting areas with farm poultry, which often happens in Asia, Europe and Africa.
Worse still, in poor areas of Africa and Eastern Europe, people live in close proximity with their poultry and other farm animals, as they do in Asia. This will provide a dangerous crucible for mixing bird and human viruses, increasing the risk that the avian virus could gain the ability to spread easily from person to person and cause a pandemic.
The same mixing may happen in Asia as migratory ducks, geese and swans fly from north-east Asia to South-East Asia, stopping frequently along the way in wetlands for food and rest. But these birds have not been recorded going further south than Indonesia. The several million migratory birds that fly to and from Australia each year along the East Asian-Australasian flyway are shorebirds. Thirty-six species are involved, mainly snipe, godwits, curlews, redshanks, sandpipers, knots, stints, plovers and pratincoles. Some travel up to 26,000 kilometres on their return journeys between Australia and Siberia.
Scientists have reported finding avian influenza viruses among migratory shorebirds, including some of the species that fly to and from Australia. But they say that the incidence is much lower in shorebirds than in migratory ducks, geese and swans. There has been no reported finding of a highly pathogenic flu virus such as H5N1 from shorebirds anywhere in the world.
The FAO has reported that certain species of migratory ducks can carry influenza viruses without showing signs of disease. These birds have probably carried the viruses, with no apparent harm, for centuries, enabling them to build up their own immunity.
Although the deadly H5N1 virus has devastated poultry flocks and killed more than 60 people in South-East Asia, it was never recorded in wild birds before the recent outbreaks of bird flu in Asia, Russia and countries around the Black Sea. In the past, highly pathogenic viruses have been found only very rarely in migratory birds. They were usually found dead within flight range of an outbreak of virulent bird flu among poultry, leading scientists to believe that wild waterfowl were not agents for the onward transmission of the lethal viruses.
But something has changed and it is an ominous development. The World Health Organisation says some migratory birds now appear to be directly spreading the H5N1 virus in its highly pathogenic form over long distances and that further spread to new areas can be expected. It also says there is considerable circumstantial evidence to suggest that when they mix with domestic poultry, wild waterfowl can introduce to farm flocks low pathogenic viruses that then mutate to the highly pathogenic form, including the H5N1.
Still, the FAO points out that the pattern of H5N1 outbreaks in East Asia does not coincide with the migratory routes used by wild birds for all countries.
The UN agencies monitoring bird flu outbreaks say more research is needed to solve the puzzle of how the virus is spread and how it changes into forms deadly to humans as well as birds. Meanwhile, it recommends that wild and farmed birds should not mix.
The risk that deadly avian flu will be imported into Australia by migratory birds seems low. The biggest risk is the one that faces the world as a whole: if the H5N1 virus changes into a strain that can spread easily from person to person and has a high infection and death rate, it would be difficult, if not impossible, to stop. The consequences of a pandemic could be catastrophic.