What is bird flu?
"Bird flu", or avian influenza, is an infectious disease of
animals caused by viruses that normally infect birds and, less
commonly, pigs. While all bird species are thought to be
susceptible to infection, domestic poultry flocks are especially
vulnerable to infections that can rapidly reach epidemic
proportions.
What are the types?
The disease in birds has two forms. The first causes mild illness,
sometimes expressed only as ruffled feathers or reduced egg
production. Of greater concern is the second form, known as
“highly pathogenic avian influenza”. This form is highly
infectious in birds and is rapidly fatal, with a mortality
approaching 100%. Birds can die on the same day that symptoms
first appear.
What are the symptoms?
The reported symptoms of avian influenza in humans have ranged
from typical influenza-like symptoms (e.g., fever, cough, sore
throat and muscle aches) to eye infections, pneumonia, acute
respiratory distress, viral pneumonia, and other severe and
life-threatening complications.
Most influenza viruses cause no symptoms, or only mild ones in
wild birds; however, the range of symptoms in birds vary greatly
depending on the strain of virus and the type of bird. Infection
with certain avian influenza A viruses (for example, some H5 and
H7 strains) can cause widespread disease and death among some
species of wild and especially domesticated birds such as chickens
and turkeys.
Certain water birds act as hosts of influenza viruses by carrying
the virus in their intestines and shedding it. Infected birds shed
virus in saliva, nasal secretions and faeces. Avian influenza
viruses spread among susceptible birds when they have contact with
contaminated nasal, respiratory and faecal material from infected
birds; however, faecal-to-oral transmission is the most common
mode of spread.
What are the control measures in birds?
The most important control measures are rapid destruction
(‘culling’ or ‘stamping out’) of all infected or exposed birds,
proper disposal of carcasses, and the quarantining and rigorous
disinfection of farms. The virus is killed by heat (56 degrees C
for 3 hours or 60 degrees C for 30 minutes) and common
disinfectants, such as formalin and iodine compounds. The virus
can survive, at cool temperatures, in contaminated manure for at
least three months. In water, the virus can survive for up to four
days at 22 degrees C and more than 30 days at 0 degrees C. For the
highly pathogenic form, studies have shown that a single gram of
contaminated manure can contain enough virus to infect 1 million
birds. Restrictions on the movement of live poultry, both within
and between countries, is another important control measure.
How does it spread within a country? Within a country, the disease spreads easily from farm
to farm. Large amounts of virus are secreted in bird droppings,
contaminating dust and soil. Airborne virus can spread the disease
from bird to bird, causing infection when the virus is inhaled.
Contaminated equipment, vehicles, feed, cages or clothing,
especially shoes, can carry the virus from farm to farm. The virus
can also be carried on the feet and bodies of animals, such as
rodents, which act as ‘mechanical vectors’ for spreading the
disease.
Droppings from infected wild birds can introduce the virus into
both commercial and backyard poultry flocks. The risk that
infection will be transmitted from wild birds to domestic poultry
is greatest where domestic birds roam freely, share a water supply
with wild birds, or use a water supply that might become
contaminated by droppings from infected wild-bird carriers. Also
the risk of transmission of bird flu is mainly for the animal
handlers and people in close vicinity. Eggs should not be taken in
a raw form, not only for bird flu but also to prevent salmonella
infections.
How does it spread from one country to
another?
The disease can spread from country to country through
international trade in live poultry. Migratory birds, including
wild waterfowl, sea birds, and shore birds, can carry the virus
for long distances and have, in the past, been implicated in the
international spread of highly pathogenic avian influenza.
Migratory waterfowl - most notably wild ducks are the natural
reservoir of bird flu viruses, and these birds are also the most
resistant to infection. They can carry the virus over great
distances, and excrete it in their droppings, yet develop only
mild and short-lived illness.
Domestic ducks, however, are susceptible to lethal infections, as
are turkeys, geese, and several other species raised on commercial
or backyard farms.
What is the present situation?
Since mid-December 2003, a growing number of Asian countries have
reported outbreaks of highly pathogenic avian influenza in
chickens and ducks. Infections in several species of wild birds
and in pigs have also been reported. India has not reported any
case of bird flu as yet.
Particularly alarming, in terms of risks for human health, is the
detection of a highly pathogenic strain, known as “H5N1”, as the
cause of most of these outbreaks. H5N1 has jumped the species
barrier, causing severe disease in humans, on two occasions in the
recent past and is now doing so again, in gradually growing
numbers, in Vietnam and Thailand.
Is there a vaccine effective against H5N1
in humans?
There are currently no available vaccines to protect against
disease caused by the H5N1 influenza virus strain in humans. WHO
is urgently working together with laboratories to develop a
prototype H5N1 virus for use by leading vaccine manufacturers.
Are there drugs available for prevention and treatment?
Two classes of drugs are available. These are the M2 inhibitors (amantadine
and rimantadine) and the neuraminidase inhibitors (oseltamivir and
zanimivir). These drugs have been licensed for the prevention and
treatment of human influenza in some countries, and are thought to
be effective regardless of the causative strain.
However, initial analyses of viruses isolated from the recently
fatal cases in Vietnam indicate that the viruses are invariably
resistant to the M2 inhibitors. Further testing is under way to
confirm the resistance of amantadine. Network laboratories are
also conducting studies to confirm the effectiveness of
neuraminidase inhibitors against the current H5N1 strains.
Are presently available vaccines useful?
Yes, but in a precisely targeted way. Current vaccines, when
administered to high-risk groups, such as poultry cullers, protect
against circulating human strains and thus reduce the risk that
humans at high risk of exposure to the bird flu virus might become
infected with human and avian viruses at the same time. Such dual
infections give the avian and human viruses an opportunity to
exchange genes, possibly resulting in a new influenza virus
subtype with pandemic potential.
Annual vaccines are produced for routine use in protecting humans
during seasonal epidemics of influenza. However, they offer no
protection against infection with the H5N1 avian virus. WHO has
issued guidelines for the vaccination of groups considered at high
risk of exposure, using the current trivalent influenza vaccine,
in countries experiencing outbreaks of highly pathogenic H5N1
avian influenza in poultry.
Hen inspection
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