Jun 28, 2005 (CIDRAP News) – About 5,000 migratory birds have died of avian influenza at a wildlife sanctuary in northwestern China, close to five times as many as reported previously, United Nations officials who visited the site said today.

Also today, the World Health Organization (WHO) said Vietnam had confirmed another human case of H5N1 infection. That brings the WHO"e;s tally of human cases in Vietnam since December to 60, with the number of deaths remaining at 18. The case occurred in May in the northern province of Ha Tay.

WHO and United Nations Food and Agriculture Organization (FAO) officials reported the increased death toll among birds at a refuge in Qinghai province, according to an Agence France-Presse (AFP) report today.

Initial reports of the outbreak, in May, said 178 birds had died of H5N1 flu. China later increased the number to 519 in an official report, and a Chinese official subsequently raised the number to more than 1,000. The dead birds were said to include five species.

AFP quoted Julie Hall, the WHO"e;s communicable-disease expert in China, as saying, "This is the first time we"e;ve seen large numbers of migratory birds dying from bird flu. So the virus has obviously changed to be more pathogenic to animals. What it means for humans we don"e;t know."

WHO and FAO officials voiced concern that China had tested only 12 birds, all of them dead, AFP reported. The officials worried that infected but asymptomatic birds could spread the virus to distant places when they migrate in August and September.

Hall called for the immediate testing and electronic tagging of birds to identify infected ones and trace where they go, the story said. No birds have been killed to control the outbreak, because some of the birds are protected species.

Hall said no human cases have been reported, but the presence of the virus in new bird species could mean an increased risk for humans.

WHO officials expressed concern that only two people have been tested for avian flu in connection with the outbreak, AFP reported. They also urged China to supply virus sequencing information as soon as possible so that experts can assess whether the virus has become more pathogenic.

A WHO official said 20 birds a day are still dying at the refuge, but the outbreak appears to be waning, according to an Associated Press report today.

Wild waterfowl are recognized as the natural reservoir for all influenza A viruses and commonly carry them without getting sick, according to the WHO.