Bird Flu
2003-04 Avian
Influenza Outbreak
An outbreak of
avian influenza,
more commonly known as bird flu, is affecting bird populations in
countries throughout Asia. The outbreak is caused by the H5N1 subtype of
influenza A. Human cases also have been reported.
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In birds:
Outbreaks of avian
influenza A (H5N1) have been confirmed among
poultry in Cambodia, China, Hong Kong (in a single peregrine
falcon), Indonesia, Japan, Laos, South Korea, Thailand, and Vietnam.
Visit the World Organization for Animal Health website for more
information.
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In people:
The outbreak of bird flu has resulted in human cases of H5N1
infection in Vietnam and Thailand. Deaths have been reported. At
this time it is believed that these cases resulted from contact with
infected birds or surfaces contaminated with excretions from
infected birds. An investigation is ongoing to determine the source
of human infections.
Visit the World Health Organization
(WHO) website for more information.
Information on
Influenza A (H5N1)
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Background:
Influenza A (H5N1) is a subtype of the Type A
influenza virus. Wild
birds are the natural hosts of the virus, hence the name avian
influenza or bird
flu. The virus was first isolated from birds
(terns) in South Africa in 1961. The virus circulates among birds
worldwide. It is very contagious among birds and can be deadly to
birds, particularly domesticated birds like chickens.
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Infection:
The virus does not typically infect humans. In 1997, however, the
first instance of direct bird-to-human transmission of H5N1 was
documented during an outbreak of avian
influenza among poultry in
Hong Kong; the virus caused severe respiratory illness in 18 people,
of who 6 died. Since that time, there have been other instances of
H5N1 infection among humans. (See "Basic
Information about Avian Influenza.") But so far, H5N1 viruses
have not been capable of efficient human-to-human transmission. This
is something that is being watched carefully and is being
investigated during this outbreak.
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Spread:
Infected birds shed virus in saliva, nasal secretions and feces.
Avian influenza viruses spread among susceptible birds when they
have contact with contaminated excretions. It is believed that most
cases of H5N1 infection in humans have resulted from contact with
infected poultry or contaminated surfaces.
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Current H5N1 Strain
The H5N1 strain
implicated in the current outbreak has been genetically sequenced.
Following is a summary of what has been learned:
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All genes are of
bird origin. This means that the virus has not acquired genes from
human influenza viruses, a development that would make
person-to-person spread more likely.
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There are likely
different variations of H5N1 virus circulating at this time. Genetic
sequencing of virus samples from South Korea and Vietnam show that
the viruses in these two countries are slightly different.
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Genetic sequencing
of A(H5N1) virus samples from human cases in Vietnam and Thailand
show antiviral resistance to amantadine and rimantadine, two of the
antiviral drugs commonly used for influenza. The remaining two antivirals (oseltamavir and zanamavir) should still be effective
against this strain of H5N1.
Containment
Key to containing the
outbreak is the culling (killing) of sick and exposed birds. This was
done to contain the 1997 H5N1 outbreak in Hong Kong. Many experts
believe this was crucial to averting many more human cases. For the
current outbreak in Asia, governments are culling poultry to try to
contain the virus. Patients are being treated and isolated, and
investigations are underway to determine the source of infection.
What Will Happen
All
influenza viruses
can change. It is possible that an avian
influenza virus could change so
that it could infect humans and could spread easily from person to
person. Because these viruses commonly do not infect humans, there is
little or no immune protection against them in the human population. If
an avian influenza virus were able to infect people and gain the ability
to spread easily from person to person, an "influenza pandemic" could
begin.
CDC Response to
Outbreak
CDC is working closely
with WHO and other partners in an effort to address the outbreak. For
its part, CDC activities include the following:
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Activation of CDC’s
Emergency Operations Center to provide round-the-clock coordination
and response.
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Deployment of 6
staff members (epidemiologists, laboratorians and a data manager) as
part of the WHO investigation team in Vietnam working with Vietnam's
human and animal health authorities to assess the extent of the
outbreak among humans and animals, identify risk factors for human
infection, and determine the characteristics of
influenza A (H5N1)
viruses isolated from human and poultry cases.
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As 1 of 4 WHO
Collaborating Centers in the WHO Global
Influenza Network, CDC
laboratories are conducting extensive testing of specimens from Asia
to determine the characteristics of the recent H5N1 viruses compared
with other avian influenza viruses. This information is needed to
help develop an H5 vaccine for the current situation.
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CDC’s WHO
Collaborating Center is developing reagent kits that will be
distributed to laboratories throughout Asia, the U.S., and elsewhere
so that they can detect recent H5N1 viruses.
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CDC Recommendations
Travelers:
CDC advises that travelers to countries in Asia with documented H5N1
outbreaks should avoid poultry farms, contact with animals in live food
markets, and any surfaces that appear to be contaminated with feces from
poultry or other animals. (For more information, visit
CDC’s Travelers’
Health page
Health
Departments and Health Care Professionals: CDC has provided
U.S. health departments with interim guidance on enhanced surveillance
and laboratory testing to help identify possible cases of H5N1
influenza
that might be imported into the United States. (For more information
about these recommendations, please see the
CDC Health Update)
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