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Avian Influenza Viruses and Food Safety

Introduction

1. The recent outbreaks of avian influenza are caused by influenza A viruses. All birds are thought to be susceptible to the infection, though some species are more resistant than others. Infection causes a wide spectrum of symptoms in birds, ranging from mild illness to a highly contagious and rapidly fatal disease resulting in severe epidemics. The latter is known as "highly pathogenic avian influenza" (HPAI).

2. Influenza A viruses have 16 H subtypes and 9 N subtypes. The H subtypes are most important in that they take control of the ability of the virus in binding to and entering cells. The N subtypes control the release of newly formed virus from the cells. Although most of the subtypes of influenza virus A can infect birds, at present, only H5 and H7 subtypes viruses are known to cause the highly pathogenic form of the disease. However, some H5 and H7 subtypes viruses are also known to cause mild illness.

3. On the basis of the current understanding, H5 and H7 viruses are spread to poultry flocks in the low pathogenic form. However, the viruses can mutate into the highly pathogenic form within a few months if they are allowed to circulate in poultry flocks. This is why the appearance of an H5 or H7 virus in poultry is always a concern, even when the infection is mild.

4. Although human cases of avian flu are rare, cases have been reported in a number of countries and areas, including Hong Kong, around the world. According to the WHO, there were about 165 laboratory-confirmed H5N1 human cases recorded between 2003 and February of 2006. Direct contact with infected poultry, or surfaces and objects contaminated by their faeces, is presently considered the main route of human infections. Human to human transmission, if exists, is considered to be inefficient.

5. The recent outbreaks of avian influenza in poultry and migratory birds in many countries are related to H5N1 which belongs to HPAI. Although human cases of avian influenza are rare, people who become infected with the H5N1 virus can become seriously ill and may die. The initial symptoms of avian influenza are similar to those of other influenza viruses, including fever, headache, muscle pain, runny nose, cough and sore throat. However, it is more likely to result in high fever, chest infection, respiratory failure, multi-organ failure, and death.

6. Three other avian influenza viruses also reportedly caused illness in humans. An outbreak of H7N7 (HPAI in birds), which occurred in the Netherlands in February 2003, caused one death and mild illness in 83 other humans. Another three mild cases of H9N2 infection (not HPAI in birds) in children were reported in Hong Kong in 1999 and in 2003. In March 2004, 2 human cases of H7N3 were reported in British Columbia, Canada.

7. According to the case-control study done in Hong Kong during the H5N1 avian influenza outbreak in human in 1997, exposure to live poultry (by visiting either a retail poultry stall or a market selling live poultry) in the week before illness began was significantly associated with H5N1 disease. The WHO considered that direct contact with infected poultry is the principal source of infections. Exposure to an environment that may have been contaminated by faeces from infected birds is a second, though less common, source of human infection.


Survival of avian influenza viruses in environment


1. Environmental conditions have a marked effect on virus survival outside the bird. Avian influenza viruses could remain viable for long periods in tissues, faeces and also in water. Organic material, such as faeces, will protect the virus from inactivation. Low temperatures are also found to increase the stability of the viruses.

2. Studies have shown that the virus remain viable in faeces for at least 35 days at 4oC and for 6 days at 37oC. It is also reported that the virus is capable of surviving on contaminated surfaces such as the poultry house environment for several weeks.


Food Safety and public health significance


1. There were concerns over the possibility that avian influenza could spread to human through the handling and consumption of contaminated poultry products and eggs. From the limited studies available, it appears that almost all parts of an infected bird are contaminated with the viruses. The virus can also be found inside and on the surface of eggs.

2. On the basis of current scientific evidence available, WHO holds the view that to date there is no epidemiological information to suggest that the disease can be transmitted through properly cooked food (even if contaminated with the virus prior to cooking).

3. The avian influenza virus, however, can survive on contaminated raw poultry meat and can pass around through contaminated food products (e.g. frozen meat). Freezing and refrigeration is not effective in reducing the concentration or virulence of the virus on contaminated meat, but normal cooking (temperatures at or above 70oC) will inactivate the virus.

4. As precautionary advice and in order to avoid known risks of food poisoning from Salmonella and other organisms, WHO, as well as other health authorities such as the European Food Safety Authority, reiterates that, whilst it is unlikely that H5N1 could be passed onto humans by raw meat or eggs, cooking food thoroughly would inactivate the virus and eliminate this potential risk.


Advice to public


1. Although there is no epidemiological information to suggest that the disease can be transmitted through the consumption of contaminated poultry products and eggs, proper handling and cooking of poultry provides protection against H5N1, as it does against other viruses and pathogens such as Salmonella.

2. As a general precautionary measure, the public are advised to observe good personal, food and environmental hygiene at all times. Among which the following good hygienic practices are of particular importance in avoiding the spread of the virus through food.

  • Cook poultry meat and eggs thoroughly. Do not eat raw or undercooked eggs (including food items containing raw or undercooked eggs as ingredients).
  • Avoid contaminating cooked or ready-to-eat foods with raw meat (e.g. do not handle both raw and cooked poultry without washing your hands in between).
  • After handling poultry products or eggs, wash hands and food-contact surfaces thoroughly.
  • As a general rule, shell eggs need not be washed. However, if eggs are soiled with faecal matter, they can be washed (with household detergent if required). Washed eggs should be used immediately.


    Other site related to avian influenza virus:

    Prevention of Avian Influenza – Tips for Food Safety

    Risk Assessment Section
    February 2006

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