The reason for the deaths of 3,500 chickens at a farm in Yuen Long this week is a mystery. Bird flu was ruled out in the first round of tests, but no one is exactly sure what killed the birds.
Officers from the Agriculture, Fisheries and Conservation Department, as well as vets, are checking hygiene, ventilation and vaccinations at the farm and all local chicken farms.
There are many theories as to how the birds died. Professor David Hui Shu-cheong, from Chinese University, doesn't believe it was bird flu. He thinks that Newcastle disease is more likely to blame. It is a highly infectious virus that has caused mass chicken deaths in the past.
But director of Hong Kong Poultry (Kamei Chicken) Development, Regal Cheng Chin-keung, says the deaths were not caused by disease. He says the air became "too stuffy" when fans stopped working in the chicken pens after a storm, and thinks this is what caused the deaths.
Cheng says that his company is now cleaning the farm three times a day instead of just once a day like before.