Sunday, February 3, 2008
Is bird-flu really a threat to public health?On-going outbreaks of highly pathogenic bird flu (avian influenza) due to infection with H5N1 virus in poultry in Asia and, more recently, in some states of India, especially West Bengal have raised concerns about multiple sources of infection and the risk to humans from various exposures. On present evidence, with 353 confirmed cases including 221 deaths since 2003 till 24 January, 2008, the vast majority of human cases have acquired their infection following direct contact with infected live or dead poultry and the concern that the virus could also spread to humans through contact with contaminated poultry products.
To date, no epidemiological data suggest that the disease can be transmitted to humans through properly cooked food (even if contaminated with the virus prior to cooking). However, in a few instances, cases have been linked to consumption of dishes made of raw contaminated poultry blood.
The epidemic of the Avian influenza (H5N1) virus among poultry (birds) that started affecting domestic and wild birds and humans in South-East Asia in mid-2003, and has since spread to the rest of Asia, Africa and Europe, is the largest and most severe outbreak. Before the outbreaks in Hong Kong (1997) and in the Netherlands (2003), human infection with avian influenza viruses were rarely reported and usually resulted in mild disease.
The widespread persistence of H5N1 in poultry populations poses two main risks for human health: (1) Sporadic human infections with the H5N1 avian influenza and (2) emergence of a pandemic (global) influenza strain.
Of the few avian influenza viruses that have crossed the species barrier to infect humans, H5N1 has caused the largest number of cases of severe disease and death in humans. Unlike normal seasonal influenza, where infection causes self-limited respiratory symptoms in most people, the disease caused by H5N1 follows an unusually aggressive clinical course, with rapid deterioration and high fatality.
A second risk, of even greater global concern, is that the virus – if given enough opportunities – could change into a form that is highly infectious for humans and spreads easily from person to person. Such a change could mark the start of a global outbreak (a pandemic). Thus, preventing the human pandemic requires control of the disease in animals and sensible precautionary measures to prevent human infection.
To make bird flu more comprehensive to our esteemed readers, here are some basic facts about the disease, its transmission to humans and apprehensions about getting it through consumption of poultry products.
¨ How do people become infected with Avian Influenza A/H5N1?
Direct contact with infected poultry, or surfaces and objects contaminated by their faeces, is presently considered the main route of human infection. To date, most human cases have occurred in rural or peri-urban areas where many households keep small poultry flocks, which often roam freely, sometimes entering homes or sharing outdoor areas where children play. As infected birds shed large quantities of virus in their faeces, opportunities for exposure to infected droppings or to environments contaminated by the virus are abundant under such conditions. Moreover, because households in many countries depend on poultry for income and food, many families sell or slaughter and consume birds when signs of illness appear in a flock, and this practice has proved difficult to change. Exposure is considered most likely during slaughter, de-feathering, butchering, and preparation of poultry for cooking. Ducks and other aquatic birds may present a special risk, they may be infected without showing any signs of disease.
¨ Does the Avian Influenza A/H5N1 virus spread easily from birds to humans?
No. Despite the extension and duration of the outbreaks in animals presenting vast opportunities for animal to human exposure (in particular in areas where backyard flocks are common), the number of human H5N1 avian influenza cases remains very small. It is not presently understood why some people, and not others, become infected following similar ‘high risk’ exposures. Family genetic predisposition might play a role as a blood relationship has been found in most of the clusters of cases.
¨ What bird species are the main carriers of avian influenza?
Many wild bird species, especially those in wetlands and aquatic environments, harbour influenza viruses. Ducks, geese, swans, gulls, terns and wadres constitute the major natural reservoir for “low pathogenic” avian influenza viruses. Transmission of avian influenza viruses between shore birds and wild ducks may occur when their breeding grounds overlap, providing an opportunity for the mixing and recombination of different avian influenza virus subtypes. Avian influenza viruses are less common in birds more closely associated with human environments such as domestic chickens, turkeys, pheasants, pigeons and parrots.
¨ Do migratory birds spread highly pathogenic avian influenza viruses to poultry?
Wild aquatic birds are considered the natural reservoir of all “low pathogenic” avian influenza viruses. Wild birds have probably carried influenza viruses, with no apparent harm, for centuries. Considerable circumstantial evidence suggests that migratory birds can introduce low pathogenic H5 and H7 viruses to poultry flocks. In some cases these viruses may then mutate in poultry to the highly pathogenic form. Recent events suggest that in some cases, migratory birds are now directly spreading the Avian influenza A/H5N1 virus in its highly pathogenic form to regions not previously affected. However, there is currently no scientific basis for culling migratory and wild birds to control the outbreaks and prevent possible spread of Avian influenza A/H5N1. This measure should therefore be strongly discouraged and more emphasis should be put on further investigating other mechanisms for spread such as through legal or illegal trade of birds and poultry products.
¨ Can migratory and wild birds transmit Avian influenza A/H5N1 to humans?
Avian influenza A/H5N1 is first and foremost a disease of poultry. Most human cases of H5N1 avian influenza have occurred in rural or peri-urban areas where many households keep small domestic poultry flocks. However, de-feathering or butchering of dead wild birds, especially waterfowl, is particularly hazardous in areas where Avian influenza A/H5N1 virus has been reported or is likely to occur, such as along migratory routes. The public should be advised to report, and avoid contact with, wild birds found dead.
¨ Do pigeons carry and spread avian influenza viruses in nature?
The H5N1 avian influenza virus was isolated from one dead pigeon in Hong Kong in 2001. In 2003, various avian influenza viruses were isolated from 0.5% of the pigeons sampled in south central China. In 2006, a total of six individual pigeons were found infected with H5N1 avian influenza virus in Romania, Turkey and the Ukraine. These findings suggest that pigeons have played a minimal role in the spread of the virus. However, the latest studies conducted with the H5N1 avian influenza virus, which emerged in Asia in 2004, demonstrated an increased susceptibility of pigeons to this virus compared to the 1997 Hong Kong virus. Thus, the general public should try to avoid unnecessary close contact with pigeons, especially in places where pigeons congregate in large numbers.
Food safety and
¨ Is it safe to slaughter chicken and handle dead chicken in outbreak areas?
In backyard production settings, the system of marketing live birds and the practices of home slaughtering, de-feathering and eviscerating, create opportunities for extensive human exposure to potentially contaminated parts of poultry. Therefore, the wearing of protective gear, and practicing measures to prevent personal contamination, is essential. A large number of confirmed human cases are believed to have acquired their infection during the slaughtering or subsequent handling of diseased or dead birds prior to cooking. For this reason, such practices involving obviously diseased or birds found dead must be stopped. In general, birds found in a diseased state or dead should never be used for human consumption.
The H5N1 avian influenza virus spreads to virtually all parts of an infected bird, including blood, meat and bones. Avian influenza viruses survive in contaminated raw poultry meat and therefore can be spread through the marketing and distribution of contaminated food products, such as fresh or frozen meat. In general the viability of the avian influenza virus is maintained at low temperatures. The H5N1 avian influenza virus can survive in faeces for at least 35 days at 4 °C and at least six days at 37 °C. The virus has also been shown to survive on surfaces for several weeks at ambient temperatures.
In outbreak areas, some poultry species (such as domestic ducks) can be asymptomatic carriers of the virus. Vaccinated poultry can also carry the virus without showing symptoms. In these areas, it is important to effectively monitor the poultry population. In the absence of such monitoring systems, it is recommended that home-slaughtering be avoided. In non-outbreak areas, the likelihood of the virus being present in the poultry population is very low. Therefore, the likelihood of infected poultry being marketed and eventually handled by a consumer or a restaurant worker is considered to be very low. In this case, the public health risk related to avian influenza is negligible.
¨ Is it safe to eat chicken?
Yes, though certain precautions should be followed in countries currently experiencing outbreaks. In areas free of the disease, poultry and poultry products can be prepared and consumed as usual following good hygienic practices and proper cooking, with no fear of acquiring infection with the H5N1 virus.
In areas experiencing outbreaks, poultry and poultry products can also be safely consumed provided these items are properly cooked and properly handled during food preparation. The virus is inactivated at temperatures reached during conventional cooking (70 °C in all parts of the food - “piping” hot - no “pink” parts). To date, there is no epidemiological evidence that anyone has become infected following the consumption of properly cooked poultry or poultry products. There have been reports of a few human cases potentially linked to the consumption of raw poultry parts (e.g. raw blood-based dishes). It should therefore be emphasized that the consumption of any raw poultry parts must be considered a high-risk practice and discouraged. In areas affected by Avian influenza A/H5N1 virus, handling of frozen or thawed raw infected poultry meat prior to cooking may be hazardous, if good hygienic practices are not observed.
¨ Standard hygienic handling practices should be used to prevent cross contamination:
Separate raw meat from cooked or ready-to-eat foods to avoid contamination: Do not use the same chopping board or the same knife for raw meat and other foods. Do not handle both raw and cooked foods without washing your hands in between and do not place cooked meat back on the same plate or surface it was on before cooking. Do not use raw or soft-boiled eggs in food preparations that will not be heat treated or cooked.
Keep clean and wash your hands: After handling frozen or thawed raw chicken or eggs, wash your hands thoroughly with soap. Wash and disinfect all surfaces and utensils that have been in contact with the raw meat.
Cook thoroughly: Thorough cooking of poultry meat will inactivate the virus. Either ensure that the poultry meat reaches 70 °C at the centre of the product (“piping” hot) or that the meat is not pink in any part.
¨ Is it safe to eat eggs?
Avian influenza A/H5N1 virus can be found inside and on the surface of eggs laid by infected birds. There is no epidemiological evidence to suggest that people have been infected with avian influenza through the consumption of eggs or egg products. Only proper cooking will inactivate virus present inside the egg. Eggs from areas with outbreaks in poultry should not be consumed raw or partially cooked (runny yolk) and the eggs should not be used as ingredients in foods which will not be cooked. Pasteurization or cooking of eggs will also significantly decrease the potential for transmission of other infections; (e.g. salmonellosis).
So there is no apprehension of contracting bird-flu with consumption of properly and thoroughly cooked poultry or poultry products including eggs or egg products. But precautions should be taken as exposure is considered most likely during slaughter, de-feathering, butchering, and improper (incomplete) preparation of poultry for cooking. And importantly the public is advised to report, and avoid contact with wild, migratory and poultry birds found dead.
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