Since the first bird flu outbreak hit Hong Kong in 1997, so many measures have been imposed on the chicken trade to prevent new ones that it is disconcerting to still find chickens dying in mysterious circumstances. The measures included stepping up the screening of live chickens imported from the mainland, regular closure and cleaning of chicken stalls, centralised slaughtering of ducks and geese, and banning chicken stalls from also selling quails to stop the cross-mutation of viruses from both species. Yet, they have failed to prevent the sudden deaths of 120 chickens last week at a farm in Yuen Long. Once again, the precautionary measure of mass killing was taken. All 110,000 birds at the farm were culled, even though it has yet to be confirmed whether the chickens died of the H5 strain of influenza virus that could infect humans. Yesterday, the discovery of 29 dead chickens at a stall in Tsuen Wan market led to the killing of 300 others kept at that stall. This was the third time in four years that mass culling has had to be conducted to eliminate a possible health risk. In 1997, all 1.5 million chickens in the territory were killed, after the H5N1 virus killed six people. In May last year, two million were slaughtered, although no one was killed in that outbreak. Surely, the mass killing of poultry as a health measure cannot and should not become routine. Apart from the costs involved, it does not seem to address the problem at its root. The trouble is so much about the origin of the virus and how it spreads remains unknown. Centralised slaughtering of chickens may help, but the problem appears to lie in rearing practices in the farm and the stalls.