What Is Avian Influenza?
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 avian
influenza virus in their intestines, but usually do not get sick from it. The condition can be highly contagious among birds and can make some domesticated birds, including chickens, ducks, and turkeys, sick and even kill them. Infected birds shed the virus in their saliva, nasal secretions, and feces.
Susceptible birds become infected when they have contact with contaminated secretions, 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
- Contact with infected surfaces (such as dirt or cages)
- Materials (such as water or feed) that have been contaminated with the virus.
Infection with the avian influenza virus in domestic poultry causes two main forms of the 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 to 100 percent, often within 48 hours.
One strain of avian influenza, the
H5N1 virus, is endemic in much of Asia and has recently spread into Europe. Avian
H5N1 infections have recently killed poultry and other birds in a number of countries.
Avian Influenza in Humans
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, which includes 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, and all known subtypes of influenza A viruses can be found in birds.
Usually, the term "avian
influenza virus" refers to influenza A viruses found chiefly in birds, but infections with these viruses can occur in humans. The risk from the disease is generally low in 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 infection in humans have resulted from contact with infected poultry (domesticated chicken, ducks, and turkeys) or with surfaces contaminated with secretions/excretions from infected birds. The spread of the avian influenza virus from one ill person to another has been reported only rarely, and transmission has not been observed to continue beyond one person.
During an outbreak of avian influenza among poultry, there is a possible risk to people who have contact with infected birds or with surfaces that have been contaminated with secretions or excretions from infected birds. Avian influenza
H5N1 in humans is currently limited and not a
pandemic flu.
Human H5N1 influenza infection was first recognized in 1997, when this virus infected 18 people in Hong Kong, causing six deaths. Since 2003, more than 100 human H5N1 flu cases have been diagnosed in Thailand, Vietnam, Cambodia, Indonesia, and China. Of those cases, more than half have died as a result of avian influenza.
(Click Pandemic Flu for more information about flu pandemics.)
Pandemic Versus Seasonal Outbreak of Avian Influenza
Scientists are worried that avian
influenza could become an
influenza pandemic. An influenza
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 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.
Human Symptoms of Avian Influenza
Symptoms of avian influenza in humans have ranged from typical influenza-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 may depend on which virus caused the infection.
Prevention and Treatment of Avian Influenza
Vaccines to protect humans against the
H5N1 virus currently are under development. In addition, research is underway on methods to make large quantities of avian
influenza vaccine more quickly. Studies done in laboratories suggest that the prescription medicines approved in the United States for human influenza viruses should work in treating avian influenza infection in humans. However, the 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 in the long term.
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