Swine Flu (cont.)
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Swine Flu Cause
Influenza viruses are named according to the types of proteins on the outer surface of the virus. The two main proteins are hemagglutinin (H) and neuraminidase (N). The swine influenza virus in the 2009 outbreak was an H1N1 virus. In fact, although the term swine flu is often used to describe the outbreak, the official term for the 2009 virus is novel H1N1 influenza.
It is important to realize that the influenza virus changes (mutates) constantly so that there are many strains of H1N1 that differ subtly from each other. Swine flu is caused by one strain of H1N1, but there are many other strains. Some H1N1 strains only infect pigs. Others infect humans, pigs, and birds. These subtle differences matter because the human body makes antibodies that are tailored to a single strain of influenza. If a person recovers from novel H1N1 (swine) flu, they are probably protected against infection from the same swine flu strain but are not protected against infections from variations on the strain or from other strains of influenza.
The 2009 novel H1N1 swine influenza strain appeared to be a result of genetic shift, meaning that it contains pieces of influenza from many different sources. The 2009 virus included genes that come from bird influenza viruses, swine influenza viruses, and human influenza viruses. This strain had not previously caused infections in humans or pigs. Thus, it was unlikely that most humans had preexisting immunity to this new strain. The diagram below shows how the gene assortment can happen.
Although the diagram shows the genetic shift and drift of various flu virus genes, this diagram represents the mechanisms used by all influenza A viruses that result in "new" viral antigenic types, such as the newest swine flu virus, H3N2v, that is being transmitted from pigs to humans. This new type was first detected in 2011 with an M gene acquired from the H1N1 virus.
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