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Modern Avian
Influenza H5N1
Avian influenza, sometimes Avian flu, and
commonly Bird flu, refers to "influenza caused by viruses
adapted to birds."
"Bird flu" is a phrase similar to "Swine flu," "Dog flu,"
"Horse flu," or "Human flu" in that it refers to an illness
caused by any of many different strains of influenza viruses
that have adapted to a specific host. All known viruses that
cause influenza in birds belong to the species: Influenza A
virus. All subtypes (but not all strains of all subtypes) of
Influenza A virus are adapted to birds, which is why for
many purposes avian flu virus is the Influenza A virus (note
that the "A" does not stand for "avian").
Adaptation is non-exclusive. Being adapted towards a
particular species does not preclude adaptations, or partial
adaptations, towards infecting different species. In this
way strains of influenza viruses are adapted to multiple
species, though may be preferential towards a particular
host. For example, viruses responsible for influenza
pandemics are adapted to both humans and birds. Recent
influenza research into the genes of the Spanish Flu virus
shows it to have genes adapted to both birds and humans;
with more of its genes from birds than less deadly later
pandemic strains.
Influenza A virus subtype H5N1, also known
as "bird flu," A(H5N1) or simply H5N1, is a subtype of the
Influenza A virus which can cause illness in humans and many
other animal species. A bird-adapted strain of H5N1, called
HPAI A(H5N1) for "highly pathogenic avian influenza virus of
type A of subtype H5N1", is the agent of H5N1 flu, commonly
known as "avian influenza" or "bird flu". It is enzootic in
many bird populations, especially in Southeast Asia. One
strain of HPAI A(H5N1) is spreading globally after first
appearing in Asia. It is epizootic (an epidemic in
nonhumans) and panzootic (affecting animals of many species,
especially over a wide area), killing tens of millions of
birds and spurring the culling of hundreds of millions of
others to stem its spread. Most references to "bird flu" and
H5N1 in the popular media refer to this strain.
Early stages of the Avian Flu outbreak
According to the FAO Avian Influenza Disease Emergency
Situation Update, H5N1 pathogenicity is continuing to
gradually rise in wild birds in endemic areas but the avian
influenza disease situation in farmed birds is being held in
check by vaccination. Eleven outbreaks of H5N1 were reported
worldwide in June 2008 in five countries (China, Egypt,
Indonesia, Pakistan and Vietnam) compared to 65 outbreaks in
June 2006 and 55 in June 2007. The "global HPAI situation
can be said to have improved markedly in the first half of
2008 [but] cases of HPAI are still underestimated and
underreported in many countries because of limitations in
country disease surveillance systems".
HPAI A(H5N1) is considered an avian
disease, although there is some evidence of limited
human-to-human transmission of the virus. A risk factor for
contracting the virus is handling of infected poultry, but
transmission of the virus from infected birds to humans is
inefficient. Still, around 60% of humans known to have been
infected with the current Asian strain of HPAI A(H5N1) have
died from it, and H5N1 may mutate or reassort into a strain
capable of efficient human-to-human transmission. In 2003,
world-renowned virologist Robert Webster published an
article titled "The world is teetering on the edge of a
pandemic that could kill a large fraction of the human
population" in American Scientist. He called for adequate
resources to fight what he sees as a major world threat to
possibly billions of lives. On September 29, 2005, David
Nabarro, the newly-appointed Senior United Nations System
Coordinator for Avian and Human Influenza, warned the world
that an outbreak of avian influenza could kill anywhere
between 5 million and 150 million people. Experts have
identified key events (creating new clades, infecting new
species, spreading to new areas) marking the progression of
an avian flu virus towards becoming pandemic, and many of
those key events have occurred more rapidly than expected.
Due to the high lethality and virulence of HPAI A(H5N1), its
endemic presence, its increasingly large host reservoir, and
its significant ongoing mutations, the H5N1 virus is the
world's largest current pandemic threat and billions of
dollars are being spent researching H5N1 and preparing for a
potential influenza pandemic. At least 12 companies and 17
governments are developing pre-pandemic influenza vaccines
in 28 different clinical trials that, if successful, could
turn a deadly pandemic infection into a nondeadly one.
Full-scale production of a vaccine that could prevent any
illness at all from the strain would require at least three
months after the virus's emergence to begin, but it is hoped
that vaccine production could increase until one billion
doses were produced by one year after the initial
identification of the virus.
Countries with poultry
or wild birds killed by H5N1.
Countries with humans, poultry
and wild birds killed by H5N1.
H5N1 may cause more than one influenza
pandemic as it is expected to continue mutating in birds
regardless of whether humans develop herd immunity to a
future pandemic strain. Influenza pandemics from its genetic
offspring may include influenza A virus subtypes other than
H5N1. While genetic analysis of the H5N1 virus shows that
influenza pandemics from its genetic offspring can easily be
far more lethal than the Spanish Flu pandemic, planning for
a future influenza pandemic is based on what can be done and
there is no higher Pandemic Severity Index level than a
Category 5 pandemic which, roughly speaking, is any pandemic
as bad as the Spanish flu or worse; and for which all
intervention measures are to be used.
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