Four thousand Japanese chickens have died from an illness confirmed as the H5N1 strain of bird flu. The chickens were all being reared at a poultry farm in the Miyazaki prefecture on the southern island of Kyushu. The farmer's remaining 8,000 birds have now been culled.

As well as ordering a nationwide inspection of poultry farms, the Japanese Ministry of Agriculture has banned the shipment of chickens and eggs from farms within a 10 kilometre radius of the farm, which lies in Japan's largest poultry producing region.

This is the first confirmed outbreak of the virus in Japan since 2004. But despite the danger posed to humans by the H5N1 strain, no people appear to have been infected in this latest outbreak. Indeed, Japan has had only a single case of human infection since 2003, and has not lost any of its human population to the disease.

Environmental officials are beginning an investigation to determine the cause of the outbreak. The working hypothesis is that it was brought by migratory birds from elsewhere in Asia.

Scientists are carrying out further analysis of the virus that killed the 4,000 chickens, including DNA analysis, which may allow them to pinpoint its origin, and should enable them to determine just how virulent this particular strain is.

The outbreak has prompted South Korea to toughen its stance on imported Japanese poultry. The country took no meat from Japan last year, and has temporarily extended the restrictions to include all poultry products. ®

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