What is avian influenza (bird flu)?
Bird flu is an infection caused by avian (bird) influenza (flu)
viruses. These flu viruses occur naturally among birds. Wild birds
worldwide carry the viruses in their intestines but usually do not
get sick from them. However, bird flu is very contagious among birds
and can make some domesticated birds -- including chickens, ducks
and turkeys -- very sick and kill them.
Do bird flu viruses infect humans?
Bird flu viruses do not usually infect humans, but several cases
of human infection with bird flu viruses have occurred since 1997.
How are bird flu viruses different from human flu viruses?
There are many different subtypes of type A influenza viruses.
These subtypes differ because of certain proteins on the surface of
the influenza A virus -- hemagglutinin, or HA, protein and
neuraminidase, or NA, protein. There are 16 different HA subtypes
and nine different NA subtypes of flu A viruses. Many different
combinations of HA and NA proteins are possible. Each combination is
a different subtype. All known subtypes of flu A viruses can be
found in birds. However, when we talk about "bird flu" viruses, we
are referring to influenza A subtypes chiefly found in birds. They
do not usually infect humans, even though we know they can. When we
talk about "human flu viruses" we are referring to those subtypes
that occur widely in humans. There are only three known A subtypes
of human flu viruses: H1N1, H1N2 and H3N2. 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.
What are the symptoms of bird flu in humans?
Symptoms of bird flu in humans have ranged from typical flulike
symptoms -- 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 bird flu may depend on which virus
caused the infection.
How does bird flu spread?
Infected birds shed flu virus in their saliva, nasal secretions
and feces. Susceptible birds become infected when they have contact
with contaminated excretions or surfaces that are contaminated with
excretions. It is believed that most cases of bird flu infection in
humans have resulted from contact with infected poultry or
contaminated surfaces. 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.
How is bird flu in humans treated?
Studies done in laboratories suggest that the prescription
medicines approved for human flu viruses should work in preventing
bird flu infection in humans. However, flu viruses can become
resistant to these drugs, so these medications may not always work.
Additional studies are needed to prove the effectiveness of these
What is the risk to humans from bird flu?
The risk from bird flu is generally low to most people because
the viruses occur mainly among birds and do not usually infect
humans. However, during an outbreak of bird flu among poultry --
domesticated chicken, ducks, turkeys -- there is a possible risk to
people who have contact with infected birds or surfaces that have
been contaminated with excretions from infected birds. The current
outbreak of avian influenza A, or H5N1, among poultry in Asia and
Europe (see below) is an example of a bird flu outbreak that has
caused human infections and deaths. In such situations, people
should avoid contact with infected birds or contaminated surfaces
and should be careful when handling and cooking poultry. In rare
instances, limited human-to-human spread of H5N1 virus has occurred,
but transmission has not been observed to continue beyond one
For more information about avian influenza and food safety
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What is an avian influenza A (H5N1) virus?
Influenza A virus, also called H5N1 virus, is an influenza A
virus subtype that occurs mainly in birds. It was first isolated
from terns in South Africa in 1961. Like all bird flu viruses, H5N1
virus circulates among birds worldwide, is very contagious among
birds and can be deadly.
What is the H5N1 bird flu that has been reported in Asia and
Outbreaks of influenza H5N1 occurred among poultry in eight
countries in Asia -- Cambodia, China, Indonesia, Japan, Laos, South
Korea, Thailand and Vietnam -- during late 2003 and early 2004. At
that time, more than 100 million birds in the affected countries
either died from the disease or were killed in order to try to
control the outbreak. By March 2004, the outbreak was reported to be
under control. Beginning in late June 2004, however, new outbreaks
of influenza H5N1 among poultry were reported by several countries
in Asia: Cambodia, China (Tibet), Indonesia, Kazakhastan, Malaysia,
Mongolia, Russia (Siberia), Thailand and Vietnam. It is believed
that these outbreaks are ongoing. Most recently, influenza H5N1 has
been reported among poultry in Turkey and Romania. Human infections
of influenza A have been reported in Cambodia, Indonesia, Thailand
What is the risk to humans from the H5N1 virus in Asia and
The H5N1 virus does not usually infect humans. In 1997, however,
the first case of spread from a bird to a human was seen during an
outbreak of bird flu in poultry in Hong Kong, Special Administrative
Region. The virus caused severe respiratory illness in 18 people,
six of whom died. Since that time, there have been other cases of
H5N1 infection among humans. Recent human cases of H5N1 infection
that have occurred in Cambodia, Thailand and Vietnam have coincided
with large H5N1 outbreaks in poultry. The
Health Organization also has reported human cases in Indonesia.
Most of these cases have occurred from contact with infected poultry
or contaminated surfaces; however, it is thought that a few cases of
human-to-human spread of H5N1 have occurred.
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