Avian Flu


Avian Flu, also known as bird flu, an infectious disease of wild and domestic birds, caused by a range of viruses known as Type A influenza viruses. Variants of avian influenza viruses have also infected humans and a number of other mammals.

Avian influenza viruses exist in wild populations of seabirds, shorebirds, and other wildfowl, but do not usually cause illness in wild bird species. When wild birds contaminate ponds and fields with fecal droppings containing the virus, however, domesticated birds such as chickens, turkeys, and ducks can be infected. For these species, avian influenza is often fatal, afflicting the respiratory system and nervous system, and opening the way for dangerous bacterial infections. With their nasal and fecal secretions, sick individuals can rapidly spread illness to other poultry in the close confines of a farm enclosure or live animal market.

Avian influenza was not known to directly infect humans until 1997, when an outbreak in Hong Kong, China, caused by infected poultry, sickened 18 people, killing 6 of them. Death was caused by pneumonia or other respiratory ailments, kidney failure, or related complications. Symptoms of avian flu resemble those of other influenzas: fever, cough, sore throat, and muscle aches. Although humans have a degree of immunity to the influenza subtypes that circulate during the winter flu season, the human immune system is unaccustomed to recognizing and fighting off avian influenza. This makes the avian viral strains all the more dangerous. After the 1997 Hong Kong episode, other outbreaks of avian influenza followed.

Further confirmation that avian influenza can directly infect humans came in 2005 when scientists succeeded in reconstructing the infamous 1918 influenza virus, known as the Spanish flu, that killed from 20 million to 50 million people worldwide in the worst-known influenza pandemic. Two teams of United States government and university scientists succeeded in assembling the entire genetic code of the 1918 virus after discovering viral samples in the tissues of three victims of the disease, including a woman buried in Alaska’s permafrost whose body remained frozen. The scientists injected the reconstructed virus into fertilized bird eggs. The eggs died, confirming that the virus had an avian rather than human origin because a human influenza virus will not kill bird eggs.

Scientists identify the various strains of avian flu and other varieties of Type A influenza by categorizing them according to the differences in two key proteins found on the surface of the virus. The two proteins are Hemagglutinin (H) and Neuraminidase (N). There are 15 major subtypes of H and 9 major subtypes of N. The virus that caused the 1997 Hong Kong outbreak was designated H5N1 because the key proteins on the surface of the virus were subtype H5 and subtype N1. Tests determined that strains related to H5N1 were behind the deadly Asian outbreak that began in 2003. Some poultry farms in Europe and the eastern United States, meanwhile, suffered outbreaks in 2003 and 2004 of subtypes of H7, an avian strain that is currently believed to be less dangerous to humans.

Avian influenza appears to spread from birds to humans through direct, close contact with sick birds or with fecal-contaminated surfaces. As yet there is no confirmed evidence that current avian influenza viruses spread from person to person. Influenza viruses, however, mutate (change) easily. Scientists and public health experts fear that an avian flu strain might strike a person who is already infected with a human variant of influenza. The two variants could swap, or combine, their viral components in the infected person before spreading to other people. This combination of virus components could even take place in a susceptible mammal, such as a pig.

The result could be a novel virus strain completely unknown to the human immune system. It could be especially virulent and cause death in a high percentage of infected individuals, passing easily from person to person. Such a virus could touch off a global epidemic, or pandemic, of influenza that could kill millions of people. The grim benchmark for such a catastrophe is the “Spanish flu” outbreak of 1918.

Currently, the most effective means of fighting avian influenza is the destruction of infected birds or those at risk of infection, often millions at a time when outbreaks occur. In August 2005 the U.S. National Institutes of Health reported that the first trials of an avian flu vaccine were effective in humans. However, public health officials expressed concern that the vaccine did not exist in sufficient amounts to respond to a pandemic. Drugs, such as Tamiflu, can be used to treat avian flu, but some studies suggest that Tamiflu may not be fully effective against the H5N1 virus. Public health officials say there is no danger to the public from eating poultry or eggs as long as they are well cooked.

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