Bird Flu

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• Bird Flu Overview
• Bird Flu Causes
• Bird Flu Symptoms in Humans
• When to Seek Medical Care
• Exams and Tests
• Bird Flu Treatment
• Medications
• Next Steps
• For More Information
• Synonyms and Keywords
• Authors and Editors

Bird Flu Overview

Bird flu (avian influenza) is caused by a virus that primarily affects birds and is not easily able to infect people. However, in the late 1990s, a new strain of bird flu arose that was remarkable for its ability to cause severe disease and death in domesticated birds such as ducks, chickens, or turkeys. As a result, this strain was called "highly pathogenic" (meaning very severe) avian influenza. The first human case of illness from highly pathogenic avian influenza was identified in 1997, and more than 400 cases have been identified in the ensuing years. Human infection with bird flu is fatal in more than half of cases.

First, some definitions to put the bird flu threat into perspective:

• Pandemic: A pandemic is a global outbreak of disease. This could occur if a new virus (mutated from a bird flu virus) that causes serious illness were to emerge among humans (not birds) with the ability to spread easily from one person to another. A pandemic is caused by a new subtype that has never (or not recently) occurred in humans.



• Epidemic: A fast-spreading, seasonal, or regional outbreak of flu among humans is called an epidemic.

Birds have been affected with avian influenza in Asia, Europe, the Near East, and Africa, and the outbreak has killed millions of poultry. Bird flu from the highly pathogenic strain is not currently found in the United States. Human cases of bird flu have largely been confined to Southeast Asia and Africa. However, mutations (changes in the genetic material of the virus) often occur in the virus, and it is possible that some mutations could create a more contagious virus that could cause a worldwide pandemic of flu among humans. Fortunately, the mutations that have occurred to date have not made the virus more contagious.

The virus spreads from bird to bird as infected birds shed flu virus in their saliva, nasal secretions, and droppings. Healthy birds get infected when they come into contact with contaminated secretions or feces from infected birds. Contact with contaminated surfaces such as cages might also allow the virus to transfer from bird to bird.

Bird Flu Causes

Bird flu is caused by a type of influenza virus. There are many types of influenza viruses, and most prefer to live in a limited number of animal hosts. Thus, "swine flu" primarily infects swine, and bird flu primarily infects birds. The human influenza virus is best adapted to humans. A few rare cases may occur in an "accidental" host, as when people who have extensive contact with sick birds get the "bird flu." Sometimes, a species-specific flu virus will change (mutate) in a way that makes it easily able to infect other species. If bird flu mutated to be able to spread easily among people, it would likely cause a serious pandemic.

Humans may get bird flu from contact with infected birds (chickens, for example) or their droppings or surfaces with infected droppings. Human-to-human spread of bird flu may occur, but is very rare so far.

Biology of the Flu Virus

Influenza viruses have two proteins that can be recognized and attacked by the body's defenses (immune system). The proteins are called hemagglutinin (H) and neuraminidase (N). There are many different types of hemagglutinin and neuraminidase proteins. The current bird flu has type 5 hemagglutinin and type 1 neuraminidase. Thus, it is an "H5N1" influenza virus.

A person's immune system learns to recognize these surface proteins either by being infected with the flu virus and recovering or by getting a vaccine (flu shot) that contains similar H and N proteins. After that, any virus that infects with the identical H and N on its surface will usually be quickly recognized and stopped, causing either a mild illness or no illness. This type of defense is known as immunity.

Minor changes in the H or N components can allow the virus to evade a person's immune defense. These minor changes are so common that they are almost routine. This is why a person can get influenza infections year after year. If the new virus is very similar to older viruses, the immune system may still be of some help in reducing the severity of disease. This is sometimes referred to as "partial" immunity.

Major changes in H and N viral proteins are more serious because people have no immune defense at all against the new virus. If the new virus spreads easily from person to person, there is a risk of a worldwide pandemic with a very large percentage of people becoming infected and ill from the flu. One way the bird flu virus could make such a dramatic change is if it picked up proteins from human viruses through a process known as "recombination." Another way would be spontaneous changes (mutations) in the bird flu virus itself that would make it more infectious. These scenarios are what concerns scientists about the bird flu. If such a dangerous virus acquires the ability to spread easily among humans, it could cause a serious pandemic. Fortunately, this has not happened to date. Although the highly pathogenic strain of bird flu has changed over time, person-to-person transmission of bird flu remains very rare.

Serious pandemic influenza is uncommon. The most deadly pandemic in modern history was the 1918 influenza, also known as the "Spanish flu" (although it did not originate in Spain). The 1918 virus spread rapidly and killed tens of millions of people worldwide. Mortality (death rate) was especially high in healthy young adults. Although the 1918 virus was a human influenza virus, it had many genes that suggest it had an avian ancestor.