Avian influenza ((H5N1) Bird Flu)
|Avian influenza virus usually refers to
influenza A viruses found chiefly in
birds, but infections can occur in
humans. The risk is generally low to
most people, because the viruses do
not usually infect humans. However,
confirmed cases of human infection
have been reported since 1997.…
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Avian influenza in birds
Avian influenza is an infection caused by avian (bird) influenza (flu) viruses. These influenza
viruses occur naturally among birds. Wild birds worldwide carry the viruses in their intestines,
but usually do not get sick from them. However, avian influenza is very contagious among
birds and can make some domesticated birds, including chickens, ducks, and turkeys, very
sick and kill them.
Infected birds shed influenza virus in their saliva, nasal secretions, and feces. Susceptible
birds become infected when they have contact with contaminated secretions or excretions or
with surfaces that are contaminated with secretions or excretions from infected birds.
Domesticated birds may become infected with avian influenza virus through direct contact
with infected waterfowl or other infected poultry, or through contact with surfaces (such as dirt
or cages) or materials (such as water or feed) that have been contaminated with the virus.
Infection with avian influenza viruses in domestic poultry causes two main forms of disease
that are distinguished by low and high extremes of virulence. The “low pathogenic” form may
go undetected and usually causes only mild symptoms (such as ruffled feathers and a drop in
egg production). However, the highly pathogenic form spreads more rapidly through flocks of
poultry. This form may cause disease that affects multiple internal organs and has a mortality
rate that can reach 90-100% often within 48 hours.
Human infection with avian influenza viruses
There are many different subtypes of type A influenza viruses. These subtypes differ
because of changes in certain proteins on the surface of the influenza A virus (hemagglutinin
[HA] and neuraminidase [NA] proteins). There are 16 known HA subtypes and 9 known NA
subtypes of influenza A viruses. Many different combinations of HA and NA proteins are
possible. Each combination represents a different subtype. All known subtypes of influenza
A viruses can be found in birds.
Usually, “avian influenza virus” refers to influenza A viruses found chiefly in birds, but
infections with these viruses can occur in humans. The risk from avian influenza is generally
low to most people, because the viruses do not usually infect humans. However, confirmed
cases of human infection from several subtypes of avian influenza infection have been
reported since 1997. Most cases of avian influenza infection in humans have resulted from
contact with infected poultry (e.g., domesticated chicken, ducks, and turkeys) or surfaces
contaminated with secretion/excretions from infected birds. The spread of avian influenza
viruses from one ill person to another has been reported very rarely, and has been limited,
inefficient and unsustained.
“Human influenza virus” usually refers to those subtypes that spread widely among humans.
There are only three known A subtypes of influenza viruses (H1N1, H1N2, and H3N2)
currently circulating among humans. It is likely that some genetic parts of current human
influenza A viruses came from birds originally. Influenza A viruses are constantly changing,
and they might adapt over time to infect and spread among humans.
During an outbreak of avian influenza among poultry, there is a possible risk to people who
have contact with infected birds or surfaces that have been contaminated with secretions or
excretions from infected birds.
Symptoms of avian influenza in humans have ranged from typical human influenza-like
symptoms (e.g., fever, cough, sore throat, and muscle aches) to eye infections, pneumonia,
severe respiratory diseases (such as acute respiratory distress), and other severe and life-
threatening complications. The symptoms of avian influenza may depend on which virus
caused the infection.
Studies done in laboratories suggest that some of the prescription medicines approved in
the United States for human influenza viruses should work in treating avian influenza infection
in humans. However, influenza viruses can become resistant to these drugs, so these
medications may not always work. Additional studies are needed to demonstrate the
effectiveness of these medicines.
Avian Influenza A (H5N1)
Influenza A (H5N1) virus – also called “H5N1 virus” – is an influenza A virus subtype that
occurs mainly in birds, is highly contagious among birds, and can be deadly to them. H5N1
virus does not usually infect people, but infections with these viruses have occurred in
humans. Most of these cases have resulted from people having direct or close contact with
H5N1-infected poultry or H5N1-contaminated surfaces.
Avian influenza A (H5N1) outbreaks
Outbreaks of highly pathogenic avian influenza A (H5N1) associated with illness and death in
humans have occurred in Asia since late 2003. In the United States, different avian influenza
viruses caused outbreaks among poultry in 2003 and 2004, and one human infection in
Human health risks during the H5N1 outbreak
Of the few avian influenza viruses that have crossed the species barrier to infect humans,
H5N1 has caused the largest number of detected cases of severe disease and death in
humans. However, it is possible that those cases in the most severely ill people are more
likely to be diagnosed and reported, while milder cases go unreported. For the most current
information about avian influenza and cumulative case numbers, see the World Health
Organization (WHO) avian influenza website.
Of the human cases associated with the ongoing H5N1 outbreaks in poultry and wild birds in
Asia and parts of Europe, the Near East and Africa, more than half of those people reported
infected with the virus have died. Most cases have occurred in previously healthy children
and young adults and have resulted from direct or close contact with H5N1-infected poultry or
H5N1-contaminated surfaces. In general, H5N1 remains a very rare disease in people. The
H5N1 virus does not infect humans easily, and if a person is infected, it is very difficult for the
virus to spread to another person.
While there has been some human-to-human spread of H5N1, it has been limited, inefficient
and unsustained. For example, in 2004 in Thailand, probable human-to-human spread in a
family resulting from prolonged and very close contact between an ill child and her mother
was reported. In June 2006, WHO reported evidence of human-to-human spread in
Indonesia. In this situation, 8 people in one family were infected. The first family member is
thought to have become ill through contact with infected poultry. This person then infected six
family members. One of those six people (a child) then infected another family member (his
father). No further spread outside of the exposed family was documented or suspected.
Treatment and vaccination for H5N1 virus in humans
The H5N1 virus that has caused human illness and death in Asia is resistant to amantadine
and rimantadine, two antiviral medications commonly used for influenza. Two other antiviral
medications, oseltamivir and zanamivir, would probably work to treat influenza caused by
H5N1 virus, but additional studies still need to be done to demonstrate their effectiveness.
INFORMATION Adopted on March 24 2009 from CDC.GOV