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 change
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, tens of cases have been reported in a number
of countries and areas, including Hong Kong, around
the world. 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. Two 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.
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 opined that direct
contact with infected poultry, or surfaces and objects
contaminated by their faeces, is presently considered
the main route of human infections.
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 contaminated
food or that products shipped from affected areas have
been the source of infection in humans.
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.
