Health workers clad in masks began culling thousands of chickens at a markeplace in Hong Kong on Wednesday, a day after authorities raised the bird flu alert level to "serious" following an outbreak at a farm.
The outbreak near the border with China was the city's first in five years despite mass vaccination of the birds, prompting concern that the virus might have mutated.
"A vaccine's effectiveness on Wednesday will not be the same like what it was five years ago. So this time, could it be that the vaccine may not be as effective in fighting the virus? This possibility remains," said Ho Pak-leung, a microbiologist at the University of Hong Kong.
Workers clad in masks, white medical suits and black rubber gloves began the culling in a wholesale market on Wednesday. Culling also continued for the second day in areas within a 3 km radius of the infected farm.
A government spokeswoman said some 3,000 chickens had been destroyed on Tuesday but the process may take sometime to complete.
"We aim to complete the task as soon as possible," the spokeswoman said without giving a time frame.
Hong Kong health authorities raised the city's bird flu alert level to "serious" on Tuesday after the H5 virus killed dozens of chickens at a farm.
Laboratories in the city were now trying to determine the precise identity of the virus. A leading expert said it was likely to turn out to be the highly pathogenic H5N1 strain, which turns up regularly in flocks in Asia, parts of Europe and Africa.
Although H5N1 is mainly a disease among birds, it may mutate into a form that spreads easily among people.
If that happens, it could trigger a pandemic and kill millions. Even in its current hard to catch form, H5N1 has infected 387 people since 2003, killing 245 of them.