Avian
Bird
Flu
CONTENTS
What is avian influenza (bird flu)?
Avian 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.
The Avian
Influenza A (H5N1) virus – also called “H5N1 virus” – is an
influenza A virus subtype that occurs mainly in birds. Like all bird
flu viruses, H5N1 virus circulates among birds worldwide, is very
contagious among birds, and can be deadly.
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 . Mongolia and Croatia have reported outbreaks of H5N1
in wild, migratory birds. Human infections of influenza A (H5N1)
have been reported in Cambodia , China , Indonesia , Thailand , and
Vietnam.
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 [HA] and neuraminidase [NA]
proteins). There are 16 different HA subtypes and 9 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.
Avian flu viruses do not usually infect humans, but more than 100
confirmed cases of human infection with bird flu viruses have
occurred since 1997. For example, the World Health Organization
(WHO) maintains situation updates and cumulative reports of human
cases of avian influenza A (H5N1).
Symptoms of avian flu in humans have ranged from typical flu-like
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 avian flu may depend on which virus
caused the infection.
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 avian 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.
Studies done in laboratories suggest that the prescription medicines
approved for human flu viruses should work in preventing avian 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 medicines. The H5N1 virus currently infecting birds in Asia that has caused
human illness and death 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 flu caused by the H5N1 virus, but additional studies
still need to be done to prove their effectiveness.
There currently is no commercially available vaccine to protect
humans against the 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 underway.
The risk from avian 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 avian 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 (H5N1) among poultry in Asia and
Europe (see below) is an example of a avian 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,
and transmission has not been observed to continue beyond one
person.
The virus caused severe respiratory illness
in 18 people, 6 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 World
Health Organization (WHO) 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. So far, spread of H5N1 virus from person to person has been rare and
has not continued beyond one person. However, because all influenza
viruses have the ability to change, scientists are concerned that
the 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 the H5N1 virus were able to
infect people and spread easily from person to person, an influenza
pandemic (worldwide outbreak of disease) could begin. No one can
predict when a pandemic might occur. However, experts from around
the world are watching the H5N1 situation in Asia very closely and
are preparing for the possibility that the virus may begin to spread
more easily and widely from person to person.
The current risk to Americans from the H5N1 bird flu outbreak in
Asia is low. The strain of H5N1 virus found in Asia and Europe has
not been found in the United States . There have been no human cases
of H5N1 flu in the United States . It is possible that travelers
returning from affected countries in Asia could be infected if they
were exposed to the virus. Since February 2004, medical and public
health personnel have been watching closely to find any such cases. |
