Farmers and farm workers were also told to attend refresher courses on farm safety .

The order followed a report by the Agriculture, Fisheries and Conservation Department that the bird flu outbreak last December that led to the culling of 90,000 chickens at a Yuen Long farm and nearby areas was probably caused by the droppings of wild birds and spread by farm workers whose clothing or equipment may have been contaminated.

The report was prepared by the expert team including University of Hong Kong microbiologist Yuen Kwok-yung and head of the university's laboratory animal unit Lo Sing-shun.

Director of Agriculture, Fisheries and Conservation Cheung Siu-hing said the department had inspected the two infected chicken sheds in the farm on December 3 and 5 respectively, just days before the department received a report of the outbreak on December 8.

Assistant director Thomas Sit Hon-chung admitted there was room for improvement for both the department and the farms, such as beefing up inspections on the biosafety measures of farms.

The team found current vaccines to be efficient as the infection and death rates were 0.2 percent and 7.2 percent for chickens which have received vaccinations, compared to 43 percent and 83.3 percent for those without vaccinations.

Sit would not say if the mainland could be the source of infection with unreported outbreaks, but said avian flu was not unusual in southern China, Vietnam and Cambodia and places with a high population of water fowl such as ducks. Sit said infected water fowl might not display symptoms but the virus could contaminate the water causing such infections.

"As with many epidemiological studies of this nature, it is difficult to determine the exact cause of the outbreak. The investigation group considered that the H5N1 virus was most likely to have been introduced to the farm by wild birds," Sit said.

The department has increased the frequency of inspections of farms from once a week to once every five days and doubled the number of specimens for testing from 14 to 30.

Farm owners and workers must attend refresher courses organized by the department to update them on biosafety measures and procedures.