Bird Flu (cont.)

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What causes bird flu?

Bird flu is caused by strains of influenza A virus. There are many different strains of influenza viruses, and most strains prefer to live in a limited number of animal hosts. Thus, swine flu primarily infects swine, bird flu primarily infects birds, and human influenza virus is best adapted to infecting 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 hosts. If bird flu mutated to be able to spread easily among people, it would likely cause a serious pandemic (worldwide infections). Such a mutation occurred in the swine flu virus (H1N1) that triggered a pandemic.

Humans may get bird flu from contact with infected birds (chickens, for example), their droppings, or from surfaces with infected droppings. Human-to-human spread of bird flu may occur but is very rare so far. However, if the highly pathogenic strains of bird flu (H5N1, H7N9) mutate to allow them to be easily transmitted from human to human, investigators are concerned that a lethal pandemic could occur in humans because the mortality (death) rate could be about 60%.

Influenza viruses have two surface proteins that can be recognized and attacked by the human body's defenses (immune system). The proteins are called hemagglutinin (H) and neuraminidase (N). Many different types of hemagglutinin and neuraminidase proteins exist in influenza A virus types. Recent bird flu had type 5 hemagglutinin and type 1 neuraminidase. Thus, it is an "H5N1" influenza virus type; the newest strain of bird flu has two different surface proteins, thus it is termed H7N9.

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 (to a specific viral type). Unfortunately, immunity to one viral type often does not protect against other viral types.

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 routinely detected each year. 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 provide some protection by 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 most 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 of genetic material that results in an "antigenic shift." Another way would be spontaneous changes (mutations) in the bird flu virus itself that would make it more infectious; this results in "antigenic drift." These changes are what concerns scientists about the bird flu.

Shown below are schematics of bird, human, and swine flu RNA genomes; these influenza A viruses all follow the same genetic methods that result in new influenza viral types. The schematic below shows an example of how the antigenic shift and drift for bird flu strain H5N1 occurs. However, it represents the way genetic material is reassembled and altered in all influenza A viruses, including the new H7N9 bird flu.

Picture of influenza A (bird flu H5N1): examples of antigenic shift and 
antigenic drift
Figure 1: Influenza A (bird flu H5N1): examples of antigenic shift and antigenic drift

If a lethal 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 repeatedly changed over time, person-to-person transmission of bird flu remains rare. Rare transfer to humans is seen in other diseases such as mad cow disease and SARS; it is hoped that person-to-person transmission remains a rare event for any strain of bird flu.

Serious pandemic influenza is uncommon but can be devastating. 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, recent genetic studies showed it had many genes that suggest it had an avian ancestor.

Medically Reviewed by a Doctor on 1/10/2014