Avian
influenza
(also known as bird flu) is a type of influenza virulent in birds. It
was first identified in Italy in the early 1900s and is now known to
exist worldwide.
Infection
The
causative
agent is the avian influenza (AI) virus. AI viruses all belong to the
influenza virus A genus of the Orthomyxoviridae family and are
negative-stranded, segmented RNA viruses.
Avian influenza
spreads in the air and in manure. Wild fowl often act as resistant
carriers, spreading it to more susceptible domestic stocks. It can also
be transmitted by contaminated feed, water, equipment and clothing;
however, there is no evidence that the virus can survive in well cooked
meat.
Cats are also
thought to be possible infection vectors for H5N1 strains of avian flu
(Kuiken et al, 2004).
The incubation
period is 3 to 5 days. Symptoms in animals vary, but virulent strains
can cause death within a few days.
Avian
influenza in humans
Of the 15
subtypes known, only subtypes H5 and H7 are known to be capable of
crossing the species barrier from birds to humans. It is feared that if
the avian influenza virus undergoes antigenic shift with a human
influenza virus, the new subtype created could be both highly
contagious and highly lethal in humans. Such a subtype could cause a
global influenza pandemic, similar to the Spanish Flu that killed over
20 million people in 1918 (though a variety of sources quote average
figures even higher, up to 40 million in some cases).
The first cases
of avian influenza passed from birds to humans was H5N1 in 1997 in Hong
Kong. 16 people were infected, of which 4 died. The outbreak was
limited to Hong Kong. All chickens in the territory were slaughtered.
In January 2004,
a major new outbreak of H5N1 avian influenza surfaced in Vietnam and
Thailand's poultry industry, and within weeks spread to ten countries
and regions in Asia, including Indonesia, South Korea, Japan and
mainland China. Intensive efforts were undertaken to slaughter
chickens, ducks and geese (over 40 million chickens alone were
slaughtered in high-infection areas), and the outbreak was contained by
March, but the total human death toll in Vietnam and Thailand was 23
people. In February 2004, avian influenza virus was detected in pigs in
Vietnam, increasing fears of the emergence of new variant strains.
Fresh outbreaks
in poultry were confirmed in Ayutthaya and Pathumthani provinces of
Thailand, and Chaohu city in Anhui, China, in July 2004.
In North America,
the presence of avian influenza was confirmed at several poultry farms
in British Columbia in February 2004. As of April 2004, 18 farms have
been quarantined to halt the spread of the virus. Two cases of humans
with avian influenza have been confirmed in that region.
In August 2004
avian flu was confirmed in Kampung Pasir, Kelantan, Malaysia. Two
chickens were confirmed to be carrying H5N1. As a result Singapore has
imposed a ban on the importation of chickens and poultry products.
Similarly the EU has imposed a ban on Malaysian poultry products. A
cull of all poultry has been ordered by the government within a 10km
radius of the location of this outbreak.
On September 25th
2004, the World Health Organization began to suspect human to human
transmission of the avian influenza virus; however, this has not yet
been confirmed by a laboratory test. However, recent observation and
documentation has recorded at least one case of human to human
transmission of a rare strain of the influenza virus. The case was
unusual, though, in that the original carrier, who received the disease
from a bird, was held by her mother for roughly 5 days as the young
girl died. Shortly afterwards, the mother became ill and perished as
well.
An outbreak of
avian influenza in January 2005 affected 33 out of 64 cities and
provinces in Vietnam, leading to the forced killing of nearly 1.2
million poultry. The 2005 outbreak caused 13 deaths in Vietnam and 1
death in Cambodia.
Prevention
and treatment
Avian
influenza
in humans can be detected reliably with standard influenza tests.
Antiviral drugs are sometimes effective in both preventing and treating
the disease, but no virus has ever been really cured in medical
history. Vaccines, however, take at least four months to produce and
must be prepared for each subtype. In July 2004 researchers, headed by H.
Deng of the Harbin Veterinary Research Institute, Harbin, China and
Professor Robert Webster of the St Jude Children's Research Hospital,
Memphis, Tennessee, reported results of experiments in which mice had
been exposed to 21 isolates of confirmed H5N1 strains obtained from
ducks in China between 1999 and 2002. They found "a clear temporal
pattern of progressively increasing pathogenicity".
Symptoms
In humans,
it has
been found that avian flu causes similar symptoms to other types of flu:
fever
cough
sore throat
muscle aches
conjunctivitis
in severe cases
of avian flu, it can cause severe breathing problems and pneumonia, and
can be fatal.
