Avian Influenza and the Wet Markets in Hong Kong
Kendra Wu writes about avian influenza in Hong Kong.
Aquatic birds are the natural reservoirs of avian influenza (or “bird flu”). While some bird species carry these viruses with no apparent signs of harm, other bird species, including domestic poultry, develop disease when infected. [1] Apart from being highly contagious among birds, a highly pathogenic subtype of avian influenza A/H5N1 was discovered to have leaped through species barriers to infect humans in 1997 causing 18 people to be infected resulting in six deaths. [2] In fact, since 1997, there have been 385 human cases and 243 deaths reported around the world (as of 19 June 2008). [3] Wet markets - where people trade live animals, fish, fruit and vegetables - were identified as one of the risk factors for H5N1 influenza. [4]
I am currently in Hong Kong, where the first fatal human case occurred.
Local demands for live animals for fresh produce make wet markets popular and widespread in Hong Kong. Presently, there are 64 wet markets [5] in the 18 community districts in Hong Kong averaging 3-4 wet markets per district. However, wet markets also pose public health concerns from the perspective of zoonotic transmission.
Interspecies transmission of influenza viruses involving species other than birds, such as pigs, [6] sea mammals, [7] and humans [8] have occurred sporadically. This type of transmission often involves gene segments mutation, reassortments, or recombining of gene segments. In 1957 and 1968, for instance, both of the influenza pandemics were caused by reassortments of gene segments from the avian and human influenza viruses. [9] However, for the fatal human cases in Hong Kong, this was not the case. For these patients, all genes of the H5N1 virus were of avian origin, indicating that the virus had not undergone genetic reassortment. [6] Since it has been found that visiting a poultry market is a risk factor of infection, [6] direct contact of birds and poultry should be minimized to avoid cross-species transmission. However, wet markets provide a favorable environment for close repeated contacts between humans and birds.
How so? First of all, the animals at wet markets are kept alive and stay in very confined cages from days to weeks. [10] If one of the birds becomes infected by avian influenza, the rest of them will become vulnerable to infection because of the packed and confined environment. In addition, since the animals stay in their cages for a long period of time, and the highly pathogenic viruses can survive for long periods in the environment, [1] there is plenty of time to be infected. Although stalls that sell live aquatic birds do not usually sell live poultry or red-meat animals, the stalls can be next to each other, which provides an environment for cross species transmissions. Daily introduction of new animals also increases the chance of contact between infected and non-infected animals, which provides a favorable condition for disease epidemic. Adding daily close human contacts make wet markets an important potential source of viral infection and transmission.
Once the virus infects humans, H5N1 has the potential to cause severe illness with a high risk of death (the overall case fatality ratio (CFR) is 63% as of 19 June 2008). [2] If this subtype becomes capable of efficient human-to-human transmission, a pandemic could break out in a highly dense urban community, such as Hong Kong, due to the general lack of immunity to this strain of virus among humans.
Hence, the city has put resources into educating the public and developing expertise and medical facilities to detect and control avian influenza, among other infectious diseases. Aside from preventing viral transmission within Hong Kong, as a trading and financial hub, the city recognizes that a high volume of goods and travelers pass by Hong Kong daily. Therefore, the city also puts in efforts to prevent various emerging and re-emerging infectious diseases from entering the city and transmitting to the rest of the world. Based on the stories of how Hong Kong provided support for the discovery of the bacteria that caused the plague in 1894, identifying H5N1 as the cause of the human avian influenza cases in 1997 and isolating the coronavirus (CoV) that caused severe acute respiratory syndrome (SARS) in 2003 almost 110 years later, [11] this investment is necessary.
Preventive Measures
The media in Hong Kong has been reporting new human or poultry cases of avian flu in China, Hong Kong, and sometimes in neighboring regions. The local government also puts out television commercials regularly during prime time to remind the public of how they can protect themselves from bird flu. For instance, they are discouraged from directly contacting live poultry or from blowing poultry aerosols during purchase (for health inspection). During consumption, they are asked to thoroughly cook the meat. In addition, they are also advised to pay attention to personal hygiene and to seek immediate medical attention when symptoms of influenza-like illness occur. Hence, the local community should be rather well informed regarding the risk of this disease and how they may protect themselves from bird flu.
Regulations were also been implemented after the fatal human cases in 1997 so that all aquatic birds, including ducks and geese, are not allowed to be sold live from retail markets. Instead, they have to be sold chilled in the markets to minimize direct contact between live birds and humans. With regard to poultry, chickens have not been banned from retail markets due to the opposition of poultry workers and the continuous demands for live poultry from the local community. As a compromise, the local government asks a sample of each truckload of poultry entering the wet markets in Hong Kong (either locally or from China) to be screened for the level of avian flu immunity. Despite these efforts, H5N1 of multiple new genotypes reappeared in retail markets in 2001. [12] Subsequently, a monthly ‘clean day’ was introduced during which all markets are emptied and cleaned in order to minimize the chance of viral survival at wet markets. Quail, which was newly recognized as a host susceptible to all subtypes of influenza and a potential intermediate host, was not allowed to be sold in the retail markets either. Yet, H5N1 viruses reappeared in late 2002 and in early 2003. In fact, in February 2003, the virus was transmitted to two humans resulting in one death. [2] A second clean day every month was then introduced. The Agriculture, Fisheries and Conservation Department (AFCD) also began to perform routine surveillance among wild birds, pet birds and poultry across the territory. [13] In addition, only designated farms may supply live poultry to the wet markets and these farms are asked to vaccinate their poultry. Farms and private citizens have also since been banned from keeping domestic poultry without licenses. Nevertheless, in June 2008, H5N1 virus was again detected in local retail live-poultry markets again. [14,15]
As a result, all poultry from the poultry stalls in affected markets were asked to be culled blindly in case there was any H5N1-infected poultry that was undetected. Despite the steep economical cost, this practice is an attempt to eradicate the disease from the territory and has been used since 1997. It has since become the gold standard and adopted by countries from around the world.
Discussion
The repeated appearance of the H5N1 virus in wet markets illustrates the difficulty of keeping the virus away from the wet markets. Why is this? While all aquatic birds, including ducks, geese, and quails, are banned to be sold live, chickens are exempted from the ban regulation and are allowed to be sold live at retail markets. However, as long as chickens are alive, they pose a threat to shedding the virus, and presently these animals are allowed to be transported to retail markets which spread across 18 community districts in Hong Kong.
The local government is relying on routine surveillance to identify infected chickens and stop them from entering Hong Kong. Yet, we often struggle with identifying infected poultry unless it becomes very sick from virus infection or if it happens to be sampled during routine surveillance. In a non-endemic area, finding a H5N1-infected chicken through routine surveillance is practically like finding a needle in a haystack. Not finding a positive sample does not mean that the virus is not in the area.
Asking all suppliers to vaccinate imported poultry would minimize the proportion of chickens susceptible to avian influenza from being imported to Hong Kong; however, poultry vaccines of good quality are not widely used in China. [16] Therefore, vaccinated chickens may or may not be protected as much as we would like.
In addition, according to the media, there might be birds and poultry entering Hong Kong via illegal channels, despite government efforts to stop them. Therefore, we cannot be certain that all of the live chickens that are being sold at retail markets are free of avian flu viruses.
Aside from keeping infected birds and poultry from entering Hong Kong, the local government has also asked stall owners to empty and clean their stalls on two designated dates each month. This regulation helps improve the sanitation of wet markets and minimize the chance of viral survival. Based on the markets that the author has visited, stall owners have good compliance. But, is two days per month enough?
New regulation
After the poultry outbreaks in June, the local government decided to require poultry stall owners to cull their live poultry and clean their stalls daily [17] in case any poultry was infected but undetected, so the virus would not be further transmitted. The proposal was faced with opposition from the poultry industry because industry representatives argued that the source of infection was unknown, which meant that the viruses could have been from illegal imports. They further argued that the government was killing the industry by “exaggerating the threat of avian influenza”. Representatives warned that the new regulation would negatively affect Hong Kong’s fine reputation as a food haven. [18]
Despite this, all three political parties opposed the veto from the poultry industry based on public health concerns, [18] which appeared to be the general consensus of the community as well. Hence, the new regulation has been implemented since July 4.
The future
High quality H5N1 poultry vaccines should, in principle, provide good protection to poultry for sale. Unfortunately, some suppliers in China and the illegal imports are not using these vaccines, making their livestock susceptible to avian flu. Considering the uncertainty in identifying flu-infected poultry, keeping live animals away from the general population becomes the most desirable environment that Hong Kong, a highly dense megacity, should aim for. H5N1 is not be the only avian flu virus that we are concerned about. Other highly pathogenic subtypes of avian flu may become capable of leaping through species barriers. In fact, other zoontic transmissions pose a threat too, like the severe acute respiratory syndrome (SARS) that was caused by the previously unrecognized coronavirus (SAR-CoV), which leaped from civet cats to humans. [19] As of today, animals that are found susceptible to H5N1 virus infection include aquatic birds, some species of wild birds, domestic poultry, [1] pigs, [6] palm civets, [20] tigers, [21] and cats. [22]
Live-animal retail markets should be closed. However, it takes significant political will to achieve this because such a policy would alter the public’s food consumption behavior and severely affect the livelihood of poultry workers, who are mostly people of advanced age and low education level. In Hong Kong, for instance, live chickens are in demand due to food culture and religious rituals. Despite successfully banning the sale of other types of live poultry and aquatic birds, live chickens are still being sold at retail wet markets. Nevertheless, the local government would like to ultimately ban the sale of live chickens in retail areas some day.
However, when the general population can no longer purchase live chickens at wet markets, would the number of illegal imports increase? If so, could these underground activities still be controlled? Therefore, apart from closing live-animal retail, illegal imports would need to be controlled.
Since our last pandemic in 1968, the global human population has almost doubled, [23] whereas the chicken population in China alone has multiplied 100 times to keep up with the demand. [24] When demand stays strong and poultry and humans continue to reside next to each other, the opportunity increases for zoonotic transmission.
Kendra M Wu
Postgraduate student
London School of Hygiene & Tropical Medicine
kendra.m.wu@gmail.com
1. World Health Organization. Avian influenza (”bird flu”) - fact sheet. http://www.who.int/mediacentre/factsheets/avian_influenza/en/index.html
2. World Health Organization. H5N1 avian influenza: Timeline of major events. http://www.who.int/csr/disease/avian_influenza/Timeline_08_06_17.pdf
3. World Health Organization. Cumulative Number of Confirmed Human Cases of Avian Influenza A/ (H5N1) Reported to WHO. http://www.who.int/csr/disease/avian_influenza/country/cases_table_2008_06_19/en/index.html
4. Mounts AW, Kwong H, Izurieta HS, et al. Case-control of risk factors for avian influenza A (H5N1) disease, Hong Kong. Journal of Infectious Diseases. 1999; 180: 505-08.
5. The Government of the Hong Kong Special Administrative Region. Press Releases: SFH on measures to prevent avian influenza. http://www.info.gov.hk/gia/general/200806/28/P200806280150.htm
6. Shortridge KF, Gao P, Guan Y, Ito T, Kawaoka Y, Markwell D, et al. Interspecies transmission of influenza viruses: H5N1 virus and a Hong Kong SAR perspective. Veterinary Microbiology. 2000; 74(1): 141-147.
7. Horimoto T, Kawaoka Y. Pandemic Threat posted by avian influenza A viruses. Clinical Microbiology Reviews. 2001; 14(1): 129-149.
8. Claas CJ, Osterhaus DME, van Beek R, De Jong JC, Rimmelzwaan GF, Senne DA, et al. Human influenza A H5N1 virus related to a highly pathogenic avian influenza virus. The Lancet. 1998; 351(9101): 472-477.
9. Kawaoka Y, Krauss S, Webster RG. Avian-to-human transmission of the PB1 gene of influenza A virus in the 1957 and 1968 pandemics. Journal of Virology. 1989; 63: 4603-08.
10. Fielding R, Lam WWT, Ho EYY, Lam TH, Hedley AJ, Leung GM. Avian Influenza Risk Perception, Hong Kong. Emerging Infectious Diseases. 2005; 11(5): 677-682.
11. Starling A, Ho FCS, Luke L, Tso SC, Yu ECL. Plague, SARS and the story of medicine in Hong Kong. Hong Kong: Hong Kong University Press. 2006.
12. Sims LD, Ellis TM, Liu KK, Dyrting K, Wong H, Peiris M, et al. Avian influenza in Hong Kong 1997-2002. Avian Diseases. 2003; 47: 832-838.
13. Food and Environmental Hygiene Department, The Government of the Hong Kong Special Administrative Region. Prevention of avian influenza. http://www.fehd.gov.hk/safefood_2/avian_flu/guide.html
14. The Government of the Hong Kong Special Administrative Region. Press Releases: Samples in Po On Road Market tested positive of H5N1. http://www.info.gov.hk/gia/general/200806/07/P200806070406.htm
15. The Government of the Hong Kong Special Administrative Region. Press Releases: Three more markets found to have samples tested positive of H5N1 virus. http://www.info.gov.hk/gia/general/200806/11/P200806110373.htm
16. Webster RG. Wet markets-a continuing source of severe acute respiratory syndrome and influenza? Lancet. 2004; 363: 234-236.
17. The Government of the Hong Kong Special Administrative Region. Press Releases: Samples in Po On Road Market tested positive of H5N1. http://www.info.gov.hk/gia/general/200806/07/P200806070406.htm
18. Veto clearing live chickens daily. Three political parties oppose. Ming Pao News. 13 July 2008 (in Chinese).
19. Peiris JSM, Guan Y, Yuen KY. Severe acute respiratory syndrome. Nature Medicine. 2004; 10: S88-S97.
20. World Organization for Animal Health. Avian influenza: H5N1 Timeline. http://www.oie.int/eng/info_ev/en_AI_factoids_H5N1_Timeline.htm
21. Thanawongnuwech R, Amonsin A, Tantilertcharoen R, Damrongwatanapokin S, Theamboonlers A, Payungporn S, et al. Probable tiger-to-tiger transmission of avian influenza H5N1. Emerging infectious diseases. 2006; 11(6): 976.
22. Kuiken T, Rimmelzwaan G, van Riel D, van Amerongen G, Baars M, Fouchier R, et al. Avian H5N1 influenza in cats. Science. 2004; 306(5694): 241.
23. United Nations, Department of Economic and Social Affairs, Population Division. World population prospects: The 2006 revision, highlights, working paper No. ESA/P/WP.202. New York. 2007.
24. Guan Y. Planetary discussion on Transmission, Pathogenesis, Immunity and the Control of Influenza. International Symposium of Area of Excellence on Influenza. The University of Hong Kong, Hong Kong. 22 July 2008.
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