BEIJING, Nov. 1 (Xinhuanet) -- The World Health Organization or WHO warns that bird flu might be transmitted from ducks to humans, even though they show no external signs of having the disease.

The UN health agency´s influenza chief Dr. Klaus Stohr says experiments in the United States show that unlike chickens, seemingly healthy ducks can also infect humans, China Radio International reported on Monday.

    He suggests governments urgently invest in research to discover just how widespread bird flu is in domestic ducks, adding that no human cases can be directly traced to ducks.

    The deadly H5N1 strain of bird flu appeared throughout Asia early this year, ravaging poultry farms and sparking a region-wide health scare.

    Authorities in Asia culled tens of millions of birds in an attempt to thwart the spread of the disease, but it resurfaced in July.