What Is the Cause of Bird Flu?
Bird flu is caused by a types of influenza A virus (for example, H5N1 and H7N9). 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 seasonal 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 avian influenza mutated to be able to spread easily among people, it would likely cause a serious pandemic. Such a mutation occurred in the so-called "swine flu" virus (H1N1) in 2009 that triggered a pandemic.
Humans may get bird flu from contact with infected birds (chicken, for example) or their droppings or surfaces with infected droppings. Human-to-human spread of bird flu may occur but has been 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. The following section shows how in nature these influenza subtypes may undergo genetic changes that could enhance the viral subtype's contagiousness and/or pathogenicity.
Biology of the Flu Virus
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). There are many different types of hemagglutinin and neuraminidase proteins. A recent bird flu had type 5 hemagglutinin and type 1 neuraminidase. Thus, it is an "H5N1" influenza virus type. Another strain of bird flu has two different surface proteins and was termed H7N9. Also, H9 is another avian flu subtype.
A person's immune system learns to recognize these surface proteins either by being infected with a 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 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 avian influenza 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 scenarios are what concerns scientists about the bird flu and are shown below for 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 the antigenic shift and drift for bird flu strain H5N1 but represents the way genetic material is reassembled and altered in all influenza A viruses including the H7N9 bird flu.
Influenza A (bird flu H5N1): examples of antigenic shift and antigenic drift
If such a dangerous virus acquires the ability to spread easily among humans, it could cause a serious health 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 very rare. Rare transfer to humans is seen in other non-flu diseases such as mad cow disease. It is hoped that person-to-person transmission remains a rare event for any strain of bird flu.
Serious pandemic influenza is uncommon. The deadliest 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. The risk of 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.