Health and animal experts Monday called for the mass vaccination of poultry in Asia to stop the spread of the virulent H5N1 bird flu virus, which has claimed dozens of human lives in the region.

The call came at the opening of a three-day U.N. conference in the Malaysian capital, Kuala Lumpur.

The U.N. World Health Organization says prevention needs to focus on small-scale farming- where there is a close mingling of humans, pigs, chickens, and ducks in backyard farms.

Peter Cordingley, the WHO's spokesman for the Western Pacific, says changes are also needed in traditional wet markets. "There are wild animals and they're slaughtered on the spot and there's guts and stuff on the floor. In the past I think that's been culturally acceptable, but in these days of emerging disease quite clearly it's not appropriate anymore so we're looking to have actions on that front as well," he said.

Health experts say these practices boost the risk the bird flu virus and the human flu virus could mix and mutate into a form more easily transmitted to humans.

They agree the virus is unpredictable. For example, in areas of Southeast Asia there are ducks infected with the virus that show no symptoms, but excrete the virus, possibly infecting chickens.

At the same time, in China's remote Qinghai Province, 6,000 migratory birds - which are supposed to carry the virus but show no symptoms - suddenly dropped dead last month.

Mr. Cordingley said this is confusing. "So what's going on? Do we have different viruses? One in China and one in Southeast Asia? This virus is puzzling us," he said.

The World Health Organization has conducted studies in Vietnam, where the majority of human cases have sprung up. Mr. Cordingley says, luckily, the evidence suggests there is time to stem a human epidemic. "Basically it's still a big and growing problem, but indications that we'd had that it was getting a lot worse very quickly - that wasn't the case. Basically we're exactly at the same place as we were before…the threat has grown by the month this year, we still think it's a very serious threat to global health," he said.

U.N. health experts at the summit are calling on regional governments now to devote more money and resources to combat the spread of the virus.