Avian influenza -- commonly called "bird flu" or "
avian flu" -- is an infection caused by
influenza viruses that occur naturally in birds. Wild birds can carry the bird flu viruses, but usually do not get sick from them. However, some domesticated birds, including chickens, ducks, and turkeys, can become infected with the avian
flu virus and die.
Wild birds often carry the bird flu virus in their intestines, but usually do not get sick from it. Bird flu is highly contagious among birds and can make some domesticated birds, including chickens, ducks, and turkeys, very sick and even kill them.
Transmission of Bird Flu in Birds
Infected birds shed bird flu 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 bird flu virus through:
- Direct contact with infected waterfowl or other infected poultry
- Contact with surfaces (such as dirt or cages)
- Materials (such as water or feed) that have been contaminated with the virus.
Types of Bird Flu in Birds
Infection with bird flu viruses in domestic poultry causes two main forms of disease that are distinguished by low and high extremes of virulence. The "low pathogenic" form of bird flu may go undetected and usually causes only mild symptoms (such as ruffled feathers and a drop in egg production).
However, the highly pathogenic form of bird flu 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 percent to 100 percent, often within 48 hours.
One strain of bird flu, the
H5N1 virus, is endemic in much of Asia and has recently spread into Europe.
H5N1 infections have recently killed poultry and other birds in a number of countries. Strains of H5N1 bird flu may infect various types of animals, including wild birds, pigs, and tigers. Symptoms of H5N1 in birds and other animals vary, but virulent strains can cause death within a few days.
H5N1 Bird Flu in Humans
H5N1
bird flu in humans is currently limited and not considered a
pandemic. H5N1
influenza infection in humans was first recognized in 1997, when this virus infected 18 people in Hong Kong, causing 6 deaths. Since 2003, more than 100 human H5N1 cases have been diagnosed in:
- Thailand
- Vietnam
- Cambodia
- Indonesia
- China.
Of those cases, more than half have died as a result of the bird
flu virus.
Transmission of Bird Flu in Humans
Currently, close contact with infected poultry has been the primary source of human infection with bird flu. Though rare, there have been isolated reports of human-to-human transmission of the disease. Genetic studies confirm that the influenza A virus H5N1 mutates rapidly, which means that should it adapt to allow easy human-to-human transmission, a pandemic could ensue (it has not done so to date). At this time, it is uncertain whether the currently circulating H5N1 virus will lead to a global outbreak of the disease in humans -- a pandemic.
Pandemic Versus Seasonal Outbreak of Bird Flu
A flu
pandemic is different from a seasonal outbreak or "epidemic" of
influenza. Seasonal outbreaks are caused by subtypes of influenza viruses that already circulate among people, whereas
influenza pandemic outbreaks are caused by:
- New subtypes
- Subtypes that have never circulated among people
- Subtypes that have not circulated among people for a long time.
Past influenza pandemics have led to high levels of illness, death, social disruption, and economic loss.
(Click Pandemic Flu for more information about influenza pandemics.)
The reported
symptoms of bird flu in humans have ranged from typical influenza-like symptoms (fever, cough, sore throat, and muscle aches) to eye infections (conjunctivitis), acute respiratory distress, viral pneumonia, and other severe, life-threatening complications.
Vaccines to protect humans against
H5N1 bird flu currently are under development. In addition, research is underway on methods to make large quantities of
bird flu vaccine more quickly.
Current Treatment for Bird Flu
So far, research suggests that two antiviral medicines, oseltamavir (
Tamiflu®) and zanamavir (
Relenza®), may be useful treatments for H5N1 bird flu. However, H5N1 viruses are generally resistant to two other available antiviral medications,
amantadine and
rimantadine, so they cannot be used to treat bird flu.
