Advisory Council on Food and Environmental Hygiene
INVESTIGATION OF A CHICKEN FARM
EXPOSED TO AN H5 INFLUENZA VIRUS
To advise members of the result of investigation into a chicken farm in Hong Kong exposed to an H5 influenza virus.
2. On 19 October, AFCD, through its regular farm surveillance programme, detected low level of antibodies to H5 avian influenza virus in chickens in a small farm at Ngau Tam Mei, Yuen Long.
THE INITIAL INVESTIGATION
3. Initial blood tests on chickens in this farm showed that 60% of the older chickens (totaling 7,500 in number) had developed low-level antibodies to H5 influenza virus. One other batch of younger chickens (2,500 in number) kept in a separate house, however, had no antibodies to H5 virus.
4. Chickens on the farm showed no signs of illness or increase in mortality. As a precautionary measure in preventing the virus from escaping to other farms and to the market, we isolated this farm on October 19 and prohibited movement of chickens onto or off the premises.
5. On 20 October, we collected 240 environment samples from all over the farm premises and 480 tracheal and faecal swabs from the chickens for isolation of influenza viruses. We also arranged with Professor Shortridge's laboratory at Hong Kong University to conduct confirmatory testing on our blood test results.
6. In addition, we visited and collected representative blood samples from all 133 chicken farms in Hong Kong between October 20 and 22, and all other poultry farms (88 pigeon and 7 quail farms) from October 23 to 26, to determine whether H5 influenza virus had broken out from this farm.
THE INITIAL RESULTS
7. Professor Shortridge's laboratory confirmed our test results indicating that chickens in this farm had indeed been exposed to an H5 avian influenza virus. The Government Virus Unit has also conducted separate testing on some of the samples collected, using methods developed by the Centers for Diseases Control and Prevention, USA, and obtained similar correlating results.
8. Blood samples collected from all other poultry farms showed no evidence of infection suggesting that the virus had not spread to other poultry farms.
9. We completed culture of all 720 samples collected from this farm by 29 October and isolated no influenza viruses. However, the batch of younger chickens had started to develop antibodies by 30 October with 11% of them became positive. As they were negative before, the change suggested recent infection. We decided to carry out further investigation to clarify these seemingly contradictory results.
10. On 2 November, we placed an additional 200 individually tagged, seronegative birds into the farm. These birds had never been exposed to an H5 virus before and were highly susceptible indicators as to whether the virus was still circulating in the farm. We followed this group, as well as the groups of older and younger chickens for another 2 weeks to monitor for the development and changes in antibodies, and to isolate the virus.
11. Throughout the investigation period, chickens in this farm remained healthy. None of the individually tagged, seronegative birds had shown any reaction to H5 avian influenza virus. The percentage of younger birds reacting in the blood test remained at a similar level (14%) as before while that of the older group fell from 60% to 29%. We have also isolated no virus from the additional 1000 swabs collected. All these results suggested that the H5 virus no longer remained in the farm by November.
12. We believe that chickens on this farm have been exposed to an H5 avian influenza virus some time ago. However, the behaviour of this virus is entirely unlike the very virulent H5N1 avian influenza virus we had in 1997. It caused no disease in chickens in this farm, elicited only a low level antibody response, and failed to spread through the younger birds, in contrast to the higher level of infection in older birds (14% infection rate against 60%).
13. Despite intensive effort, we have not isolated the virus. All these results suggested that the virus had not established in the farm and chickens were no longer affected by it by November.
14. We discussed these results with the Special Investigation Group on H5 avian influenza comprising local medical experts, microbiologist and virologist. The group agreed that chickens in this farm posed no public health risk and could be sold in the market.
FOLLOW UP ACTION
15. We lifted all restrictions on this farm on 20 November and it resumed normal operation. We will continue to monitor it closely for evidence of infection.
16. This case shows that H5 avian influenza viruses do occasionally cross into domestic poultry population. To prevent any possible spread and contamination of local poultry markets, we have stepped up inspection to local farms and the wholesale market at Cheung Sha Wan to ensure that they are regularly cleaned and disinfected. We have also increased testing at local farms to ensure that all local chickens are tested for H5 avian influenza virus before they are sent to markets, and stepped up surveillance and monitoring for virus activities at wholesale and retail markets.
17. Similarly, FEHD have increased their inspection and cleaning at retail markets. They have also strengthened clinical checks and sampling on imported birds for H5 testing at import control points.
Agriculture, Fisheries and Conservation Department