Avian Influenza (Bird Flu)
• What You Should Know About Avian Flu
• Key Facts
• Infections in Humans
• Questions & Answers
• Current Situation
This fact sheet provides general information about avian
influenza (bird flu) and information about one type of bird
flu, called avian influenza A (H5N1), that has caused infections
in birds in Asia and Europe and in humans in Asia. Also
see Questions and Answers on the CDC website and Frequently
Asked Questions (FAQs) on the World Health Organization
(WHO) website.
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 transmission
has not been observed to continue beyond one person.
“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
For current information about avian influenza A (H5N1) outbreaks,
see our Outbreaks page.
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. In the current outbreaks in Asia and Europe more
than half of those infected with the virus have died. Most
cases have occurred in previously healthy children and young
adults. However, it is possible that the only cases currently
being reported are those in the most severely ill people,
and that the full range of illness caused by the H5N1 virus
has not yet been defined. For the most current information
about avian influenza and cumulative case numbers, see the
World Health Organization (WHO) avian influenza website.
So far, the spread of H5N1 virus from person to person has
been limited and has not continued beyond one person. Nonetheless,
because all influenza viruses have the ability to change,
scientists are concerned that H5N1 virus one day could be
able to infect humans and spread easily from one person
to another. Because these viruses do not commonly infect
humans, there is little or no immune protection against
them in the human population. If H5N1 virus were to gain
the capacity to spread easily from person to person, an
influenza pandemic (worldwide outbreak of disease) could
begin. For more information about influenza pandemics, see
the CDC Pandemic Influenza website and PandemicFlu.gov.
No one can predict when a pandemic might occur. However,
experts from around the world are watching the H5N1 situation
in Asia and Europe very closely and are preparing for the
possibility that the virus may begin to spread more easily
and widely from person to person.
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, oseltamavir and zanamavir, would probably work
to treat influenza caused by H5N1 virus, but additional
studies still need to be done to demonstrate their effectiveness.
There currently is no commercially available vaccine to
protect humans against H5N1 virus that is being seen in
Asia and Europe. However, vaccine development efforts are
taking place. Research studies to test a vaccine to protect
humans against H5N1 virus began in April 2005, and a series
of clinical trials is under way. For more information about
H5N1 vaccine development process, visit the National Institutes
of Health website.
You’ve Got 48 Hours !!
If you or your loved ones are showing signs of the flu,
call your doctor or pediatrician immediately. Why is it
urgent to act quickly? Because the antiviral medications
that can stop the flu episode need to be prescribed and
treatment begun within 48 hours of the first symptoms.
The antiviral medication TAMIFLU was prescribed more often
than any other during last year’s flu season. It is indicated
for the treatment of influenza in patients aged 1 year and
older who have had symptoms for no more than 2 days.
TAMIFLU attacks the influenza virus and stops it from spreading
inside your body. TAMIFLU treats flu at its source by attacking
the virus that causes the flu, rather than simply masking
symptoms.
TAMIFLU is also indicated for the prevention of influenza
in adults and adolescents aged 13 years and older.
Call your doctor now. Ask if TAMIFLU is right for you for
the treatment or prevention of flu.
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