Surveillance and control measures required to protect our community against avian influenza (AI) H7N9

Dear Member,

The Advisory Committee on Communicable Diseases (the Committee) of the Hong Kong Medical Association met Mr. Christopher WONG, Deputy Secretary (Food) of Food and Health Bureau (FHB), Dr. LEUNG Ting Hung, Controller, Centre for Health Protection, Dr. Allen CHAN, Senior Veterinary Officer of Centre for Food Safety, Food and Environmental Hygiene Department, and Dr. Elaine LEE and Dr. Benedict TSANG, Veterinary Officers of Agriculture, Fisheries and Conservation Department in an extended meeting held on 17 December 2013 to discuss on the surveillance and control measures required to protect our community against avian influenza (AI) H7N9.

Epidemiological Assessment

1.    The AI H7N9 outbreak appears to be at its zoonosis stage. Even for the more recent cases in Guangdong, evidence does not suggest a substantial risk of human-to-human transmission. For the two reported cases in Hong Kong, the gene sequences do not show obvious changes, in relation to re-assortment or drug resistance. There is no change in risk assessment for community transmission.

2.    The continuing occurrence of multiple poultry to human transmission across Eastern and Southern China implies that the H7N9 virus is persisting in some form in a near human host. Substantial human exposure must have occurred, even though there has not been reliable data on the relative proportions of exposed individuals who would develop severe disease, milder disease, just seroconversion or no trace of infection at all. While the H7N9 virus does not appear to have an ability to maintain sustained human-to-human transmission in the general community at the current stage, caution must still be exercised to guard against nosocomial transmission in high-risk hospital environments.

3.    History has shown us that more harmful mutant or re-assortment AI strains could emerge where the low pathogenic AI strains were allowed to persist and replicate in birds or poultry. Continuing vigilance is therefore warranted with the ongoing zoonotic outbreak.

4.    Taking the difficulty in differentiating human H7N9 infections from other respiratory infections, less than desired performance of the current sampling and testing procedures with its predominant localization in the lower respiratory tract and the detection of other, often less severe, cases in the national influenza surveillance system, the reported figures likely represent only a portion of all human cases.

Risk Assessment

5.    The Committee highlighted the new challenges posed by the emergent H7N9 virus. The virus, though capable of causing lethal human diseases, is largely silent among poultry. The term “lowly pathogenic” refers to its pathogenicity in birds only. Many of the existing surveillance measures, like the sentinel chickens, have been designed for “highly pathogenic” AI viruses, like H5N1, which cause manifest disease among poultry. For H7N9, the viral yield was low even with molecular polymerase chain reaction (PCR) tools at putative poultry markets linked to the outbreak in Shanghai. In absence of guidance by dead chicken / disease manifestations, the current surveillance mechanisms may not be effective in picking up low level of viral circulation.

6.    Some experts from Guangdong believed that the source of infected chickens for wholesale markets in Guangdong might have come from small backyard farms rather than the larger and better controlled farms. Cross-infection might have occurred in the wholesale markets when chickens mixed together. By the time the chickens got to the retails markets, they might cross-infect further, as unsold chicken were allowed to stay overnight.

7.    However, the existing data does not allow us to pinpoint the backyard poultry as the source or only source of the H7N9. From theoretical consideration of the transmission dynamics, huge aggregation of poultry may be more effective in sustaining low level of virus isolation than isolated chickens in backyard farms.

8.    Poultry come from many different sources into Shanghai and the numbers are expected to increase markedly with the strong demand for chickens during the Lunar New Year, making it virtually impossible contain the cross-transmission risk. In this regard, the Shanghai municipality has announced its plan to close the live poultry market for a month during the coming Chinese New Year.   

9.    While live poultry from affected areas carries the highest risk, frozen poultry is not risk-free, as the AI virus may be preserved under frozen condition and can resume activity when the environment become warmer. Although gastrointestinal symptoms were not prominent among the reported human cases, this does Not exclude the possibility of enteric route of transmission altogether.

Current Surveillance and Control Measures

10.  According to FHB, around 7,000 live chickens (plus other minor poultry) are imported daily to Hong Kong from registered farms in the Mainland, mainly in Guangdong. Live poultry bound for Hong Kong are quarantined and observed for 5 days to confirm that they have no clinical signs of infection with AI. They are also subject to rapid PCR testing against AI, including H5 and H7 (30 samples per consignment of around 1,000 birds) before they are released from the farms. The consignments are then transported by designated vehicles registered with the relevant entry-exit quarantine and inspection authorities. When they arrive at the Man Kam To Boundary Control Point in Hong Kong, another 30 chickens per consignment are sampled for rapid PCR testing. The consignments are held at the wholesale market until satisfactory testing results have been obtained. Should there be any positive result, the whole batch of chickens will be safely disposed of.

11.  About 10,000 live chickens are released from the 29 active licensed local farms every day.  This number may fluctuate according to market demand. All local chicken farms are operating under stringent biosafety regulations. No backyard poultry are allowed in Hong Kong since 2006. Rapid PCR and virus isolation tests are conducted by staff of Agriculture, Fisheries and Conservation Department regularly for AI surveillance.  Local farms are also required to place sentinel chickens in each batch of the H5-vaccinated chickens for timely detection of AI infection. The poultry will only be transported to the wholesale market after satisfactory test results have been obtained.

12.  To avoid cross-infection between imported and local chickens, different holding areas were built to separate the two categories of poultry, as well as other minor poultry, in the wholesale market. The chickens are normally sold within one to two days. The wholesale market, as well as the cages and vehicles used in the transportation of the poultry, are subject to thorough cleansing and disinfection every day. 

13.  Currently, there are 132 licensed live poultry retail outlets. Any unsold live poultry are slaughtered at the end of each day. Stringent hygienic measures with frequent cleansing and disinfection are undertaken at all levels of the live poultry supply chain.

14.  For chilled and frozen poultry imported from the Mainland. The supplying farms, namely associated poultry farms, are subject to similar regulation and surveillance as those farms supplying live poultry to Hong Kong. To control smuggling of raw meat or poultry across the boundary, specially trained detection dogs are deployed at the boundary to monitor any such illegal activity.

15.  There is an ongoing surveillance program on wild birds covering both H5 and H7 AI. From available scientific data, the yield of H7N9 from sampling of wild birds is expected to be very low anyway.

Recommendations made by the Committee

16.  The Committee appreciated the efforts of the relevant bureau and departments in putting into place those important safety measures as mentioned above. However, the Committee also warned of an impending crisis in consumer confidence with the continuing reports of cases in Guangdong and Hong Kong. Chicken sale has already declined, causing substantial backlog in local farms. With the failure of existing surveillance measures to track the H7N9, we have effectively fallen back to using human cases as sentinel for H7N9 in poultry. Workload pressure in health care facilities has been mounting, and such situation would not be sustainable. In the interest of all parties and to avoid chaos as experienced in the past, something definitive has to be done to restore consumer confidence before the coming Chinese New Year.

17.  Serological testing was raised by the FHB in the discussion with the Mainland authorities earlier. There had since been discussions among the experts about some technical issues. As these issues have largely been resolved now, the Committee urged the government to request our Mainland counterparts to introduce the serological testing without any further delay, AND in case of any difficulty in introducing such testing in the Mainland farms, testing should be initiated at our boundary control point and at local farms.

18.  H7N9, being of lower pathogenicity to birds, might not induce a very strong anti-haemoagglutinin titre in birds as the more pathogenic H5N1. False positives can easily occur with the usual haemoagglutination inhibition tests as a result of cross-reaction to other more benign H7 viruses. However, H7 seroprevalence should NOT be high in our Guangdong supplying farms if effective biosafety measures have been implemented. Neutralization test would be much more time-consuming and tedious, and could involve biohazards. Local data would need to be collected with the rolling out of the serological testing so as to inform the adjustment of the relevant action criteria.

19.  In response to a question from FHB on whether there was a more robust alternative than enlarging the PCR viral sampling from 30 chickens per batch to, say, 120 in case of a positive result on serological testing from PCR-negative chickens, the Committee considered it very impractical and prohibitively costly to conduct a huge number of PCR tests to exclude a low percentage of potentially lethal H7N9 virus carriage in a serologically positive batch of chicken. While a suitable seroprevalence threshold would have to be set to allow for cross-reactions with other more benign viruses, it would be more cost-effective to slaughter the whole batch of possibly affected poultry if the seroprevalence suggested either a substantial probability of H7N9 or substantial lapses in the biosafety measures of the supplying farms.

20.  With the continuing spread of the H7N9 virus among birds and poultry over a wide area, there is an obvious need to guard against possible introduction of the virus into the local farms. Besides separating imported and local chickens in the wholesale market, complete segregation of all personnel and vehicles handling imported chickens from those handling local chickens should be considered to avoid cross-contamination and back-tracking to local farms.

21.  Should an H7N9 outbreak of substantial size occur in Guangdong, it would suggest lapses in the existing control measures. Under such situation, more decisive actions, like complete banning of import of live children, has to be considered to avoid spread of H7N9 into Hong Kong.

22.  Although the government has a clear contingency plan for influenza pandemic threats, there is a need for ongoing assessment of the evolving situation. Suitable adjustments would also need to be made to meet the new challenges posed by the emergent H7N9 virus. No public health response system can be expected to have an unlimited surge capacity to cope with a crisis of any scale. With the limited capacities of the rapid testing system, isolation facilities and advanced life support machines, like the extra-corporal membrane oxygenation (ECMO), in the public sector, clear priorities should be set on the target groups for screening, isolation and advanced life support, so that suitable scaling down of coverage or rechannelling of less likely or milder cases to alternative facilities could be made to contain the workload in case of overwhelming demand.

23.  With high tourist flow during Christmas and Chinese New Year holidays, visitors to affected areas would need to pay serious attention to various hygienic and precautionary measures as included under the current advice from the Centre for Health Protection:

*Do not visit live poultry markets. Avoid contact with poultry, birds and their droppings. If contact has been made, thoroughly wash hands with soap;
* Poultry and eggs should be thoroughly cooked before eating;
* Wash hands frequently with soap, especially before touching the mouth, nose or eyes, handling food or eating; after going to the toilet or touching public installations or equipment (including escalator handrails, elevator control panels and door knobs); or when hands are dirtied by respiratory secretions after coughing or sneezing;
* Cover the nose and mouth while sneezing or coughing, hold the spit with a tissue and put it into a covered dustbin;
* Avoid crowded places and contact with fever patients; and
* Wear masks when respiratory symptoms develop or when taking care of fever patients.

 

Regards,

The Hong Kong Medical Association