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About Avian Influenza and Its Transmission to Humans
Oct. 17, 2005 - The following information was prepared by the World Health Organization in response to
the heightened interest in the avian (bird) flu that many see is the
threat of a worldwide pandemic. This information was current on October
14, 2005. They also have released "Avian Influenza Frequently Asked Questions,"
which you can read - click here, and
"Pandemic Influenza: Ten Things You Need To Know," -
click here.
The disease in birds: impact and control
measures
Avian influenza is an infectious disease of birds
caused by type A strains of the influenza virus. The disease, which was
first identified in Italy more than 100 years ago, occurs worldwide.
All birds are thought to be susceptible to
infection with avian influenza, though some species are more resistant
to infection than others. Infection causes a wide spectrum of symptoms
in birds, ranging from mild illness to a highly contagious and rapidly
fatal disease resulting in severe epidemics. The latter is known as
“highly pathogenic avian influenza”. This form is characterized by
sudden onset, severe illness, and rapid death, with a mortality that can
approach 100%.
Fifteen subtypes of influenza virus are known to
infect birds, thus providing an extensive reservoir of influenza viruses
potentially circulating in bird populations. To date, all outbreaks of
the highly pathogenic form have been caused by influenza A viruses of
subtypes H5 and H7.
Migratory waterfowl – most notably wild ducks – are
the natural reservoir of avian influenza viruses, and these birds are
also the most resistant to infection. Domestic poultry, including
chickens and turkeys, are particularly susceptible to epidemics of
rapidly fatal influenza.
Direct or indirect contact of domestic flocks with
wild migratory waterfowl has been implicated as a frequent cause of
epidemics. Live bird markets have also played an important role in the
spread of epidemics.
Recent research has shown that viruses of low
pathogenicity can, after circulation for sometimes short periods in a
poultry population, mutate into highly pathogenic viruses. During a
1983–1984 epidemic in the United States of America, the H5N2 virus
initially caused low mortality, but within six months became highly
pathogenic, with a mortality approaching 90%. Control of the outbreak
required destruction of more than 17 million birds at a cost of nearly
US$ 65 million. During a 1999–2001 epidemic in Italy, the H7N1 virus,
initially of low pathogenicity, mutated within 9 months to a highly
pathogenic form. More than 13 million birds died or were destroyed.
The quarantining of infected farms and destruction
of infected or potentially exposed flocks are standard control measures
aimed at preventing spread to other farms and eventual establishment 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.
Stringent sanitary measures on farms can, however, confer some degree of
protection.
In the absence of prompt control measures backed by
good surveillance, epidemics can last for years. For example, an
epidemic of H5N2 avian influenza, which began in Mexico in 1992, started
with low pathogenicity, evolved to the highly fatal form, and was not
controlled until 1995.
A constantly mutating virus: two consequences
All type A influenza viruses, including those that
regularly cause seasonal epidemics of influenza in humans, are
genetically labile and well adapted to elude host defenses. Influenza
viruses lack mechanisms for the “proofreading” and repair of errors that
occur during replication. As a result of these uncorrected errors, the
genetic composition of the viruses changes as they replicate in humans
and animals, and the existing strain is replaced with a new antigenic
variant. These constant, permanent and usually small changes in the
antigenic composition of influenza A viruses are known as antigenic
“drift”.
The tendency of influenza viruses to undergo
frequent and permanent antigenic changes necessitates constant
monitoring of the global influenza situation and annual adjustments in
the composition of influenza vaccines. Both activities have been a
cornerstone of the
WHO Global Influenza Programme since its inception in 1947.
Influenza viruses have a second characteristic of
great public health concern: influenza A viruses, including subtypes
from different species, can swap or “reassort” genetic materials and
merge. This reassortment process, known as antigenic “shift”, results in
a novel subtype different from both parent viruses. As populations will
have no immunity to the new subtype, and as no existing vaccines can
confer protection, antigenic shift has historically resulted in highly
lethal pandemics. For this to happen, the novel subtype needs to have
genes from human influenza viruses that make it readily transmissible
from person to person for a sustainable period.
Conditions favourable for the emergence of
antigenic shift have long been thought to involve humans living in close
proximity to domestic poultry and pigs. Because pigs are susceptible to
infection with both avian and mammalian viruses, including human
strains, they can serve as a “mixing vessel” for the scrambling of
genetic material from human and avian viruses, resulting in the
emergence of a novel subtype. Recent events, however, have identified a
second possible mechanism. Evidence is mounting that, for at least some
of the 15 avian influenza virus subtypes circulating in bird
populations, humans themselves can serve as the “mixing vessel”.
Human infection with avian influenza viruses: a
timeline
Avian influenza viruses do not normally infect
species other than birds and pigs. The first documented infection of
humans with an avian influenza virus occurred in Hong Kong in 1997, when
the H5N1 strain caused severe respiratory disease in 18 humans, of whom
6 died. The infection of humans coincided with an epidemic of highly
pathogenic avian influenza, caused by the same strain, in Hong Kong’s
poultry population.
Extensive investigation of that outbreak determined
that close contact with live infected poultry was the source of human
infection. Studies at the genetic level further determined that the
virus had jumped directly from birds to humans. Limited transmission to
health care workers occurred, but did not cause severe disease.
Rapid destruction – within three days – of Hong
Kong’s entire poultry population, estimated at around 1.5 million birds,
reduced opportunities for further direct transmission to humans, and may
have averted a pandemic.
That event alarmed public health authorities, as it
marked the first time that an avian influenza virus was transmitted
directly to humans and caused severe illness with high mortality. Alarm
mounted again in February 2003, when an outbreak of H5N1 avian influenza
in Hong Kong caused 2 cases and 1 death in members of a family who had
recently travelled to southern China. Another child in the family died
during that visit, but the cause of death is not known.
Two other avian influenza viruses have recently
caused illness in humans. An outbreak of highly pathogenic H7N7 avian
influenza, which began in the Netherlands in February 2003, caused the
death of one veterinarian two months later, and mild illness in 83 other
humans. Mild cases of avian influenza H9N2 in children occurred in Hong
Kong in 1999 (two cases) and in mid-December 2003 (one case). H9N2 is
not highly pathogenic in birds.
The most recent cause for alarm occurred in January
2004, when laboratory tests confirmed the presence of H5N1 avian
influenza virus in human cases of severe respiratory disease in the
northern part of Viet Nam.
Why H5N1 is of particular concern
Of the 15 avian influenza virus subtypes, H5N1 is
of particular concern for several reasons. H5N1 mutates rapidly and has
a documented propensity to acquire genes from viruses infecting other
animal species. Its ability to cause severe disease in humans has now
been documented on two occasions. In addition, laboratory studies have
demonstrated that isolates from this virus have a high pathogenicity and
can cause severe disease in humans. Birds that survive infection excrete
virus for at least 10 days, orally and in faeces, thus facilitating
further spread at live poultry markets and by migratory birds.
The epidemic of highly pathogenic avian influenza
caused by H5N1, which began in mid-December 2003 in the Republic of
Korea and is now being seen in other Asian countries, is therefore of
particular public health concern. H5N1 variants demonstrated a capacity
to directly infect humans in 1997, and have done so again in Viet Nam in
January 2004. The spread of infection in birds increases the
opportunities for direct infection of humans. If more humans become
infected over time, the likelihood also increases that humans, if
concurrently infected with human and avian influenza strains, could
serve as the “mixing vessel” for the emergence of a novel subtype with
sufficient human genes to be easily transmitted from person to person.
Such an event would mark the start of an influenza pandemic.
Influenza pandemics: can they be averted?
Based on historical patterns, influenza pandemics
can be expected to occur, on average, three to four times each century
when new virus subtypes emerge and are readily transmitted from person
to person. However, the occurrence of influenza pandemics is
unpredictable. In the 20th century, the great influenza pandemic of
1918–1919, which caused an estimated 40 to 50 million deaths worldwide,
was followed by pandemics in 1957–1958 and 1968–1969.
Experts agree that another influenza pandemic is
inevitable and possibly imminent.
Most influenza experts also agree that the prompt
culling of Hong Kong’s entire poultry population in 1997 probably
averted a pandemic.
Several measures can help minimize the global
public health risks that could arise from large outbreaks of highly
pathogenic H5N1 avian influenza in birds. An immediate priority is to
halt further spread of epidemics in poultry populations. This strategy
works to reduce opportunities for human exposure to the virus.
Vaccination of persons at high risk of exposure to infected poultry,
using existing vaccines effective against currently circulating human
influenza strains, can reduce the likelihood of co-infection of humans
with avian and influenza strains, and thus reduce the risk that genes
will be exchanged. Workers involved in the culling of poultry flocks
must be protected, by proper clothing and equipment, against infection.
These workers should also receive antiviral drugs as a prophylactic
measure.
When cases of avian influenza in humans occur,
information on the extent of influenza infection in animals as well as
humans and on circulating influenza viruses is urgently needed to aid
the assessment of risks to public health and to guide the best
protective measures. Thorough investigation of each case is also
essential. While WHO and the members of its global influenza network,
together with other international agencies, can assist with many of
these activities, the successful containment of public health risks also
depends on the epidemiological and laboratory capacity of affected
countries and the adequacy of surveillance systems already in place.
While all these activities can reduce the
likelihood that a pandemic strain will emerge, the question of whether
another influenza pandemic can be averted cannot be answered with
certainty.
Clinical course and treatment of human cases of
H5N1 avian influenza
Published information about the clinical course of
human infection with H5N1 avian influenza is limited to studies of cases
in the 1997 Hong Kong outbreak. In that outbreak, patients developed
symptoms of fever, sore throat, cough and, in several of the fatal
cases, severe respiratory distress secondary to viral pneumonia.
Previously healthy adults and children, and some with chronic medical
conditions, were affected.
Tests for diagnosing all influenza strains of
animals and humans are rapid and reliable. Many laboratories in the WHO
global influenza network have the necessary high-security facilities and
reagents for performing these tests as well as considerable experience.
Rapid bedside tests for the diagnosis of human influenza are also
available, but do not have the precision of the more extensive
laboratory testing that is currently needed to fully understand the most
recent cases and determine whether human infection is spreading, either
directly from birds or from person to person.
Antiviral drugs, some of which can be used for both
treatment and prevention, are clinically effective against influenza A
virus strains in otherwise healthy adults and children, but have some
limitations. Some of these drugs are also expensive and supplies are
limited.
Experience in the production of influenza vaccines
is also considerable, particularly as vaccine composition changes each
year to match changes in circulating virus due to antigenic drift.
However, at least four months would be needed to produce a new vaccine,
in significant quantities, capable of conferring protection against a
new virus subtype.
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