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.
Last
revision date: 10-02-2006