Advisory Council on Food and Environmental Hygiene

Results of Monitoring for H5 Avian Influenza Viruses in Markets

Purpose

This paper provides information on the results of the Administration's on-going monitoring programme for H5 avian influenza viruses in wholesale and retail markets.

Background

2. In 1997 an H5N1 avian influenza virus (hereinafter called "Chicken-97") caused disease in poultry and in man in Hong Kong. 18 people developed the disease as a result of infection with this virus and six of them died. The incident led to the depopulation and disinfection of all poultry markets and all chicken farms, and the virus was eliminated from Hong Kong.

3. When trade in poultry resumed in 1998, stringent measures were introduced to prevent H5 avian influenza viruses from re-establishing in Hong Kong markets. This programme was based on tests and examinations to establish that poultry entering our markets were free from infection with H5 viruses. To achieve this goal, the testing programme covered poultry at the point of production and, for imported birds, the point of entry into Hong Kong.

The testing programme

4. The system in place for mainland poultry destined for Hong Kong includes the following elements:

  1. ) only farms meeting stringent hygiene standards and registered with mainland quarantine authorities are permitted to send poultry to Hong Kong

  2. ) all consignments of poultry must be bled 5 days before being sent to Hong Kong and examined clinically for signs of any disease

  3. ) once tested, these birds must be kept separate from other birds until they reach Hong Kong

  4. ) the birds must be re-examined on the day of departure for Hong Kong for signs of disease

  5. ) on arrival in Hong Kong, 13 birds from each consignment are tested and the birds are again checked for signs of any disease

5. A similar system is in place for local poultry farms. These farms are inspected on average twice a month to ensure hygiene standards are maintained and all consignments of birds are bled and tested in the 7 days prior to marketing.

6. All these programmes rely on detection of antibody to H5 viruses. Tests are done on the day the samples are collected and the results are available before birds are released to the market. In 2000-01 we conducted 296,000 blood tests for H5 avian influenza antibodies.

7. We have also implemented a segregation policy for waterfowl since the 1997 outbreak, because these birds are known carriers of H5 influenza viruses. Ducks and geese destined for the market must be kept separate from other poultry at all stages of production, transport and marketing. They can no longer be sold live in retail markets. Imported live birds are sent directly to Western Wholesale Poultry Market where they are slaughtered. As with other poultry, every consignment of birds is bled and tested for antibodies to H5 virus. If any bird is found to be positive on the blood test we discard all viscera from that consignment to prevent H5 influenza virus from entering the retail markets.

The monitoring programme

8. To assess whether all these control measures are effective, we have in place a monitoring programme covering both wholesale and retail markets. This programme has two components. The first involves the examination of dead birds from markets whenever there is an indication of an increase in mortality. This is based on the premise that the Chicken-97 virus was highly pathogenic and, if present, would cause sickness and death in birds. The second component involves culturing environment swabs collected in markets and poultry faeces for influenza viruses.

9. Currently, we collect 300 faecal samples per week from FEHD retail market complexes and an additional 50 faecal samples per week from selected fresh provision shops. We also collect 100 samples every 10 days from the Cheung Sha Wan wholesale poultry market. In the past 12 months we have tested approximately 28,000 samples for viruses from retail and wholesale markets.

10. Our monitoring programme for waterfowl involves culture of faeces from 13 birds from each consignment, checks on faecal samples collected from under goose and duck cages in the wholesale market every month, and culture of viscera from seropositive consignments.

Results of H5 virus monitoring programmes

11. Since the introduction of the monitoring programme in 1998, the chicken-97 influenza virus that caused disease in man has not been found. This virus was unique among avian influenza viruses in that it caused serious disease in both poultry and man. It was eliminated from Hong Kong as a result of the depopulation exercise in late 1997.

12. Government staff working in retail and wholesale markets have not detected increased mortality in poultry in any markets. Consequently, no samples from dead birds have been submitted for culture. In addition, no H5 influenza viruses have been isolated from poultry at Cheung Sha Wan Wholesale Poultry market.

13. Since testing in the poultry markets commenced in 1998, we have isolated 38 H5N1 viruses from waterfowl in Western Wholesale Poultry Market (Table 1). These viruses have been examined for their relationships to other H5 avian influenza viruses, in particular the Chicken-97 virus. We have found that they are all related to a strain of virus first detected in Guangdong province in 1996 - the so-called Goose Guangdong 96 virus (hereinafter referred to as "Goose-96"), although not all of them are identical.

14. In April this year we confirmed the presence of an H5N1 avian influenza virus in one stall in a retail market for the first time. There was no evidence of illness in any birds in this market. This particular virus, also a Goose-96-like virus, was isolated from faecal specimens collected from under a chicken cage. Repeat testing of birds and faecal samples in this market on two occasions after the initial samples were collected failed to reveal any more viruses, indicating that the virus had been a transient resident in the market.

Significance of results

15. The results of this market surveillance programme suggest that Goose-96-like viruses are becoming widespread in waterfowl. Fifteen of these have been isolated from samples collected in the past 2 months and we are now detecting infection in ducks, which up until December 2000 had not been positive for these viruses (attached Table). This confirms that the segregation of waterfowls from chickens is essential.

16. The Goose-96-like viruses are different from the Chicken-97 virus in that seven out of the eight genes are different. Goose-96 is not considered to pose an immediate public health risk. Unlike the Chicken-97 virus, experimental infection of mice with strains of this virus does not cause serious disease. However, there is always a possibility that Goose-96 may mix with other influenza virus strains. We have to remain vigilant and continue our control, surveillance and monitoring systems on imported and local birds. Despite the presence of H5 viruses in the environment, the absence of such viruses from retail markets for over 3 years demonstrates that the control measures have been effective. However, no programme based on sampling can provide full guarantee that all birds entering our markets are free from infection with H5 influenza viruses. Therefore, all parties involved in the live poultry trade, including poultry farmers, wholesalers and retailers, are reminded regularly to continue to observe good hygiene practices.

17. We are reviewing our existing control, monitoring and surveillance systems with a view to identifying areas that need strengthening. We have also requested mainland authorities to increase the number of samples collected on farms. We are also assessing whether the market surveillance programme needs to be expanded.

Agriculture, Fisheries and Conservation Department
May 2001




Table

H5 influenza viruses isolated from Western Wholesale Poultry Market

Year
Number of viruses isolated
Comments
1998
0
 
1999
4
Viruses all isolated from one set of samples collected from under cages.
2000
16
Viruses isolated from four sets of samples.

Virus first isolated from duck faeces in December 2000.

Ten of these isolates were from viscera from one consignment of geese
2001
(up to end of April)
18
Viruses isolated from 11 separate sets of samples, including four sets collected from ducks.

 

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