CANBERRA, June 26 -- When a sick giant petrel was brought to a wildlife rescue group on South Australia's Fleurieu Peninsula on June 14, the bird was just another distressed seabird in need of care. Testing results received five days later told a different story. The bird carried H5N1, the highly pathogenic avian influenza strain that has devastated wild bird populations across six continents over the past five years, and which, until this month, had never been confirmed in an animal on Australian soil.

It was the third H5N1 case recorded in Australia in less than a fortnight. Two migratory seabirds, a brown skua and a second bird undergoing confirmatory testing, had already died of the virus near Esperance on the southern coast of Weste...