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Avian influenza, more commonly known as bird flu is a contagious
viral infection which can affect all species of birds. This infectious
disease is caused by type A strain of the influenza virus. There are about
fifteen subtypes of influenza viruses. Influenza A (H5N1) is a subtype
of the Type A influenza virus. Wild waterfowl are the main carriers of
this virus and can be responsible for the primary introduction of infection
into domestic poultry. When avian influenza spreads to poultry or other
birds, it can cause more severe diseases. Avian influenza viruses that
cause no obvious disease in waterfowl can be highly pathogenic in domestic
poultry. Among domestic poultry species, turkeys are more commonly infected
than chickens. All birds are thought to be susceptible to infection with
avian influenza, though some species are more resistant to infection.
The virus was first isolated from birds (terns) in South
Africa in 1961. Outbreaks of avian influenza have been recognised in poultry
flocks in most countries of the world for many years. If the disease is
severe, outbreaks are eradicated by slaughtering all birds in affected
flocks. Avian influenza A (H5N1) was first recognised in 1997 in Hong
Kong; millions of chickens were slaughtered after the virus was found
to cause disease in people exposed to infected birds. This was the first
time that the avian influenza virus had ever been found to transmit directly
from birds to humans. During this outbreak, 18 people were affected, with
six deaths and the outbreak was halted in Hong Kong by slaughter of the
chickens. Prior to the above case, avian influenza usually caused
only mild symptoms, such as pink eye. In 2003, H7N7 avian influenza
affected poultry flocks in the Netherlands, leading to one human death
amongst 83 affected people. The outbreak was halted by culling affected
flocks.
H5N1 has recently re-emerged in many Asian countries
in slightly altered form. The disease have been confirmed among poultry
in Cambodia, China, Hong Kong, Indonesia, Japan, Laos, South Korea, Thailand
and Vietnam. Millions of poultry have been slaughtered. The disease has
also infected humans in Vietnam and Thailand and deaths have been reported.
Symptoms
Signs of the disease range from a mild infection with
no symptoms to a severe epidemic that kills up to 100 per cent of infected
birds. The symptoms can vary from a mild disease with little or no mortality
to a highly fatal, rapidly spreading epidemic (highly pathogenic avian
influenza) depending on the infecting virus strain, host factors, and
environmental stressors. Symptoms include coughing, sneezing, ruffled
feathers, swelling of the skin under the eyes, blisters on the combs,
swollen heads, nervous signs like depression, and diarrhea. Decreased
food consumption and drops in egg production are among some of the earliest
and most predictable signs of disease. In some cases, birds die rapidly
without clinical signs of disease.
Avian influenza A viruses do not usually infect humans;
only people who come into contact with birds are at risk. Subtypes of
the influenza A virus known as (H5N1) and (H9N2) have been known to infect
humans. The symptoms of avian influenza in humans are akin to those
of human influenza, fever, fatigue, malaise, myalgia, sore throat,
cough and in severe cases pneumonia. Conjunctivitis is seen in some patients.
Spreading
Certain water birds act as hosts of avian influenza virus
by carrying the virus in their intestinal tract and shedding it in their
feces. Infected birds shed virus in saliva, nasal secretions and feces.
Avian influenza viruses are transmitted to susceptible birds through inhalation
of influenza particles in nasal, respiratory and fecal material from infected
birds; however, fecal-to-oral transmission is the most common mode of
spread. Live bird markets have also played an important role in the
spread of epidemics.
The avian influenza virus can remain viable for long
periods of time at moderate temperatures, and can survive indefinitely
in frozen material. As a result, the disease can be spread through improper
disposal of infected carcasses, manure, or poultry by-products.
Transmission of Avian influenza A from birds to humans
is a rare event; but it may spread to humans, when they come into contact
with the droppings of infected birds. Although it was not possible to
establish how every case became infected, it is most likely that the individuals
who contracted avian flu did so directly through exposure to chickens.
However, if the virus were to mutate or combine with a human flu virus,
it could start spreading from person to person. The human flu virus can
spread more easily than the
SARS virus. There is no evidence that the virus can survive in
well cooked meat.
Prevention
Infected birds shed virus in the first two weeks of
infection. Four weeks after infection, virus can no longer be detected.
Hence, prevention is best accomplished by preventing contact between newly
infected and susceptible birds. Preventing direct contact with free-flying
birds and protecting domestic poultry from contact with the feces of wild
birds is an important way to prevent avian influenza. Countries across
Asia are battling to stop the spread of the disease by culling poultry.
The quarantining of infected farms and destruction of
infected or potentially exposed flocks are standard control measures aimed
at preventing the spread of the virus in a country’s poultry population.
Apart from being highly contagious, avian influenza viruses are readily
transmitted from farm to farm by mechanical means, such as by contaminated
equipment, vehicles, feed, cages, or clothing. Highly pathogenic viruses
can survive for long periods in the environment, especially when temperatures
are low. In the absence of prompt control measures backed by good surveillance,
the epidemic can last for years. Any object located on an infected poultry
farm must be considered contaminated and should be completely cleaned
and disinfected before it is moved from that premises.
Influenza viruses are very sensitive to most detergents
and disinfectants. They are readily inactivated by heating and drying.
However, flu viruses are well-protected from inactivation by organic material
and infectious virus can be recovered from manure for up to 105 days.
Complete removal of all organic material is part of any effective disinfection
procedure. All buildings should be cleaned and disinfected after an infected
flock is removed. The poultry litter should be composted before being
used as manure to cultivated lands.
Contaminated houses are to be heated for several
days to inactivate virus. Organic material should be removed followed
by complete cleaning and disinfection of all surfaces. Contaminated litter
and manure is problematic and should be composted to ensure that it does
not spread infectious virus.
Treatment
There is no effective treatment for avian influenza.
Good husbandry, proper nutrition and broad spectrum antibiotics may reduce
secondary infections. It must be remembered that recovered flocks continue
to intermittently shed the virus.
Diagnosis of avian influenza may be made on the basis
of symptoms and events leading to the disease. However, since the symptoms
and course of avian influenza are similar to other diseases, laboratory
diagnosis is essential.
Avian influenza in humans can be detected reliably with
standard influenza tests. Antiviral drugs are clinically effective in
both preventing and treating the disease. Recently
discovered anti-viral drug, Tamiflue is considered as the only possible
defense against an outbreak of human to human avian flue. A chemical
compound called shikimic acid is the basic material for making
Tamiflue drug. The acid is extracted from a spice named star anise,
which is harvested in China, North Vietnam and neighbouring countries. The drug has
proved effective against the lethal H5N1 strain of the bird flu. The drug will
not prevent the Avian flue but it can reduce the severity of the disease. Vaccines, however, take at least
four months to be made and must be prepared for each subtype. Once the
disease has been detected and reported, stringent disease control measures
must be taken.
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