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What is avian influenza?
Avian influenza, or “bird flu”, is a
contagious disease caused by viruses that
normally infect only birds and, less commonly, pigs and
other animals.
Avian influenza viruses are highly species-specific, but
have, on rare occasions, crossed the species barrier to
infect humans.
In domestic poultry, infection with
avian influenza viruses causes two main forms of
disease, distinguished by low and high extremes of
virulence. The so-called “low pathogenic” form commonly
causes only mild symptoms (ruffled feathers, a drop in
egg production) and may easily go undetected. The highly
pathogenic form is far more dramatic. It spreads very
rapidly through poultry flocks, causes disease affecting
multiple internal organs, and has a mortality that can
approach 100%, often within 48 hours.
Do migratory birds spread
highly pathogenic avian influenza viruses?
The role of migratory birds in the
spread of highly pathogenic avian influenza is not fully
understood. Wild waterfowl are considered the natural
reservoir of all influenza A viruses. They have probably
carried influenza viruses, with no apparent harm, for
centuries. They are known to carry viruses of the H5 and
H7 subtypes, but usually in the low pathogenic form.
Considerable circumstantial evidence suggests that
migratory birds can introduce low pathogenic H5 and H7
viruses to poultry flocks, which then mutate to the
highly pathogenic form.
In the past, highly pathogenic
viruses have been isolated from migratory birds on very
rare occasions involving a few birds, usually found dead
within the flight range of a poultry outbreak. This
finding long suggested that wild waterfowl are not
agents for the onward transmission of these viruses.
Recent events make it likely that
some migratory birds are now directly spreading the H5N1
virus in its highly pathogenic form. Further spread to
new areas is expected.
What is special about the
current outbreaks in poultry?
The current outbreaks of highly
pathogenic avian influenza, which began in South-East
Asia in mid-2003, are the largest and most severe on
record. Never before in the history of this disease have
so many countries been simultaneously affected,
resulting in the loss of so many birds.
The causative agent, the H5N1 virus,
has proved to be especially tenacious. Despite the death
or destruction of an estimated 150 million birds, the
virus is now considered endemic in many parts of
Indonesia and Viet Nam and in some parts of Cambodia,
China, Thailand, and possibly also the Lao People’s
Democratic Republic. Control of the disease in poultry
is expected to take several years.
The H5N1 virus is also of particular
concern for human health, as explained below.
Which countries have been
affected by outbreaks in poultry?
From mid-December 2003 through early
February 2004, poultry outbreaks caused by the H5N1
virus were reported in eight Asian nations (listed in
order of reporting): the Republic of Korea, Viet Nam,
Japan, Thailand, Cambodia, Lao People’s Democratic
Republic, Indonesia, and China. Most of these countries
had never before experienced an outbreak of highly
pathogenic avian influenza in their histories.
In early August 2004, Malaysia
reported its first outbreak of H5N1 in poultry, becoming
the ninth Asian nation affected. Russia reported its
first H5N1 outbreak in poultry in late July 2005,
followed by reports of disease in adjacent parts of
Kazakhstan in early August. Deaths of wild birds from
highly pathogenic H5N1 were reported in both countries.
Almost simultaneously, Mongolia reported the detection
of H5N1 in dead migratory birds. In October 2005, H5N1
was confirmed in poultry in Turkey and Romania.
Outbreaks in wild and domestic birds are under
investigation elsewhere.
Japan, the Republic of Korea, and
Malaysia have announced control of their poultry
outbreaks and are now considered free of the disease. In
the other affected areas, outbreaks are continuing with
varying degrees of severity.
What are the implications
for human health?
The widespread persistence of H5N1 in
poultry populations poses two main risks for human
health.
The first is the risk of direct
infection when the virus passes from poultry to humans,
resulting in very severe disease. 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 only
mild respiratory symptoms in most people, the disease
caused by H5N1 follows an unusually aggressive clinical
course, with rapid deterioration and high fatality.
Primary viral pneumonia and multi-organ failure are
common. In the present outbreak, more than half of those
infected with the virus have died. Most cases have
occurred in previously healthy children and young
adults.
A second risk, of even greater
concern, is that the virus – if given enough
opportunities – will 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).
Where have human cases
occurred?
In the current outbreak,
laboratory-confirmed human cases have been reported in
four countries: Cambodia, Indonesia, Thailand, and Viet
Nam.
Hong Kong has experienced two
outbreaks in the past. In 1997, in the first recorded
instance of human infection with H5N1, the virus
infected 18 people and killed 6 of them. In early 2003,
the virus caused two infections, with one death, in a
Hong Kong family with a recent travel history to
southern China.
How do people become
infected?
Direct contact with infected poultry,
or surfaces and objects contaminated by their feces, is
presently considered the main route of human infection.
To date, most human cases have occurred in rural or
periurban 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
feces, opportunities for exposure to infected droppings
or to environments contaminated by the virus are
abundant under such conditions. Moreover, because many
households in Asia 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, defeathering,
butchering, and preparation of poultry for cooking.
Is it safe to eat poultry
and poultry products?
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 H5N1
virus is sensitive to heat. Normal temperatures used for
cooking (70oC in all parts of the food) will
kill the virus. Consumers need to be sure that all parts
of the poultry are fully cooked (no “pink” parts) and
that eggs, too, are properly cooked (no “runny” yolks).
Consumers should also be aware of the
risk of cross-contamination. Juices from raw poultry and
poultry products should never be allowed, during food
preparation, to touch or mix with items eaten raw. When
handling raw poultry or raw poultry products, persons
involved in food preparation should wash their hands
thoroughly and clean and disinfect surfaces in contact
with the poultry products. Soap and hot water are
sufficient for this purpose.
In areas experiencing outbreaks in
poultry, raw eggs should not be used in foods that will
not be further heat-treated as, for example by cooking
or baking.
Avian influenza is not transmitted
through cooked food. To date, no evidence indicates that
anyone has become infected following the consumption of
properly cooked poultry or poultry products, even when
these foods were contaminated with the H5N1 virus.
Does the virus spread easily
from birds to humans?
No. Though more than 100 human cases
have occurred in the current outbreak, this is a small
number compared with the huge number of birds affected
and the numerous associated opportunities for human
exposure, especially in areas where backyard flocks are
common. It is not presently understood why some people,
and not others, become infected following similar
exposures.
What about the pandemic
risk?
A pandemic can start when three
conditions have been met: a new influenza virus subtype
emerges; it infects humans, causing serious illness; and
it spreads easily and sustainably among humans. The H5N1
virus amply meets the first two conditions: it is a new
virus for humans (H5N1 viruses have never circulated
widely among people), and it has infected more than 100
humans, killing over half of them. No one will have
immunity should an H5N1-like pandemic virus emerge.
All prerequisites for the start of a
pandemic have therefore been met save one: the
establishment of efficient and sustained human-to-human
transmission of the virus. The risk that the H5N1 virus
will acquire this ability will persist as long as
opportunities for human infections occur. These
opportunities, in turn, will persist as long as the
virus continues to circulate in birds, and this
situation could endure for some years to come.
What changes are needed for
H5N1 to become a pandemic virus?
The virus can improve its
transmissibility among humans via two principal
mechanisms. The first is a “reassortment” event, in
which genetic material is exchanged between human and
avian viruses during co-infection of a human or pig.
Reassortment could result in a fully transmissible
pandemic virus, announced by a sudden surge of cases
with explosive spread.
The second mechanism is a more
gradual process of adaptive mutation, whereby the
capability of the virus to bind to human cells increases
during subsequent infections of humans. Adaptive
mutation, expressed initially as small clusters of human
cases with some evidence of human-to-human transmission,
would probably give the world some time to take
defensive action.
What is the significance of
limited human-to-human transmission?
Though rare, instances of limited
human-to-human transmission of H5N1 and other avian
influenza viruses have occurred in association with
outbreaks in poultry and should not be a cause for
alarm. In no instance has the virus spread beyond a
first generation of close contacts or caused illness in
the general community. Data from these incidents suggest
that transmission requires very close contact with an
ill person. Such incidents must be thoroughly
investigated but – provided the investigation indicates
that transmission from person to person is very limited
– such incidents will not change the WHO overall
assessment of the pandemic risk. There have been a
number of instances of avian influenza infection
occurring among close family members. It is often
impossible to determine if human-to-human transmission
has occurred since the family members are exposed to the
same animal and environmental sources as well as to one
another.
How serious is the current
pandemic risk?
The risk of pandemic influenza is
serious. With the H5N1 virus now firmly entrenched in
large parts of Asia, the risk that more human cases will
occur will persist. Each additional human case gives the
virus an opportunity to improve its transmissibility in
humans, and thus develop into a pandemic strain. The
recent spread of the virus to poultry and wild birds in
new areas further broadens opportunities for human cases
to occur. While neither the timing nor the severity of
the next pandemic can be predicted, the probability that
a pandemic will occur has increased.
Why are pandemics such dreaded
events?
Influenza pandemics are remarkable
events that can rapidly infect virtually all countries.
Once international spread begins, pandemics are
considered unstoppable, caused as they are by a virus
that spreads very rapidly by coughing or sneezing. The
fact that infected people can shed virus before symptoms
appear adds to the risk of international spread via
asymptomatic air travellers.
The severity of disease and the
number of deaths caused by a pandemic virus vary
greatly, and cannot be known prior to the emergence of
the virus. During past pandemics, attack rates reached
25-35% of the total population. Under the best
circumstances, assuming that the new virus causes mild
disease, the world could still experience an estimated 2
million to 7.4 million deaths (projected from data
obtained during the 1957 pandemic). Projections for a
more virulent virus are much higher. The 1918 pandemic,
which was exceptional, killed at least 40 million
people. In the USA, the mortality rate during that
pandemic was around 2.5%.
Pandemics can cause large surges in
the numbers of people requiring or seeking medical or
hospital treatment, temporarily overwhelming health
services. High rates of worker absenteeism can also
interrupt other essential services, such as law
enforcement, transportation, and communications. Because
populations will be fully susceptible to an H5N1-like
virus, rates of illness could peak fairly rapidly within
a given community. This means that local social and
economic disruptions may be temporary. They may,
however, be amplified in today’s closely interrelated
and interdependent systems of trade and commerce. Based
on past experience, a second wave of global spread
should be anticipated within a year.
As all countries are likely to
experience emergency conditions during a pandemic,
opportunities for inter-country assistance, as seen
during natural disasters or localized disease outbreaks,
may be curtailed once international spread has begun and
governments focus on protecting domestic populations.
What is the status of vaccine
development and production?
Vaccines effective against a pandemic
virus are not yet available. Vaccines are produced each
year for seasonal influenza but will not protect against
pandemic influenza. Although a vaccine against the H5N1
virus is under development in several countries, no
vaccine is ready for commercial production and no
vaccines are expected to be widely available until
several months after the start of a pandemic.
Some clinical trials are now under
way to test whether experimental vaccines will be fully
protective and to determine whether different
formulations can economize on the amount of antigen
required, thus boosting production capacity. Because the
vaccine needs to closely match the pandemic virus,
large-scale commercial production will not start until
the new virus has emerged and a pandemic has been
declared. Current global production capacity falls far
short of the demand expected during a pandemic.
Is the world adequately
prepared?
No. Despite an advance warning that
has lasted almost two years, the world is ill-prepared
to defend itself during a pandemic. WHO has urged all
countries to develop preparedness plans, but only around
40 have done so. WHO has further urged countries with
adequate resources to stockpile antiviral drugs
nationally for use at the start of a pandemic. Around 30
countries are purchasing large quantities of these
drugs, but the manufacturer has no capacity to fill
these orders immediately. On present trends, most
developing countries will have no access to vaccines and
antiviral drugs throughout the duration of a pandemic. |
