Bird flu: how far and fast can it spread?

by N. Gopal Raj

Although migrating wild birds are the prime suspects in the spread of the H5N1 virus, there are no definitive conclusions.

This year, the deadly strain of bird flu known as H5N1 has dramatically expanded its range out of countries in East and South-East Asia where outbreaks began in 2003. Since then, this bird flu has led to the death or slaughter of some 140 million domestic birds, resulting in losses to the Asian poultry industry estimated at around $10 billion. Worse, the virus has shown that it is capable of infecting humans and claimed the lives of 60 people.

Until recently, the outbreaks were restricted to Indonesia, Vietnam, Thailand, Laos, Cambodia, and China, points out the Food and Agriculture Organisation (FAO). In May to July this year, many thousand wild waterbirds were found to have died at Qinghai Lake in western China, and the H5N1 virus was isolated from the dead birds.

In late July, outbreaks of the virus in poultry and wild birds were reported from Siberia in Russia and in neighbouring Kazakhstan. In early August, authorities in Mongolia reported that many migratory birds at two lakes died after being infected by the lethal strain.

Recently, outbreaks of this bird flu have been confirmed in Turkey and Romania. In both countries, thousands of domestic birds have been destroyed in and around the places where the virus was discovered, and the cull is continuing in a desperate attempt to stop the disease from spreading.

Although migrating wild birds are the prime suspects in this spread of bird flu, the extent of their involvement is uncertain. The evidence implicating wild birds is circumstantial. A "smoking gun" in the form of migratory birds infected with H5N1 shedding the virus in their droppings and secretions and healthy enough to fly long distances has still not been found (see "Are wild bird villains or victims," The Hindu, August 27).

Lack of such evidence, however, does not rule out the possibility that some species of migratory birds are capable of harbouring the virus and remaining healthy. After visiting Russia, an expert team from the World Organisation for Animal Health (OIE) recently concluded that "in certain conditions migratory birds could carry the Asian H5N1 influenza virus to other parts of the world."

In the absence of specific information about which species of migratory birds might be capable of acting as carriers of the virus, countries in Europe, the Middle East, South Asia, and even South-East Asia that are currently free of the bird flu are making frenzied preparations in case wild birds bring the virus to their shores.

If migrating wild birds are indeed carrying the H5N1 virus, there would be a risk of the virus being passed on to poultry flocks. Wild birds infected by bird flu shed vast quantities of the virus in their droppings and secretions. So poultry flocks can become infected with these viruses in a number of ways, including:

Poultry being free to mix with wild birds.

Wild birds may defecate while flying and their droppings can fall in open fields where there are poultry.

Water that poultry drink comes from a source, like a lake, that is contaminated by droppings from infected wild birds.

(The Hindu)