The United Nations warns that apparently healthy ducks may pose a new risk factor in Asia for the spread of the deadly bird-flu virus.

U.N. officials say a new laboratory study has found that the quantity of virus excreted by healthy looking ducks is nearly as much as the amount of virus released by visibly diseased chickens.

Three U.N. agencies issued a joint statement Thursday, calling for an urgent effort in areas affected by bird flu to advise residents on methods of limiting their risk of infection. 

The U.N. statement says protective measures include scalding poultry before plucking, and treating any water intended for human consumption if it has been in contact with ducks.

Bird flu outbreaks have killed nearly 25 people in Asia this year.  The disease has been reported in Cambodia, China, Indonesia, Japan, South Korea, Laos, Thailand and Vietnam.  More than 100 million birds have been slaughtered in an attempt to stop the spread of bird flu.