Avian Influenza (Bird Flu)
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
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
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 Institute's Health Websites.