What causes swine flu?
The cause of the 2009 swine flu was an influenza A virus type designated
as H1N1. In 2011, a new swine flu virus was detected. The new strain
was named influenza A (H3N2)v. Only a few people (mainly children) were
first infected, but CDC officials reported increased numbers of people
infected in the 2012-13 flu season. Currently (fall 2013), there are not
large numbers of people infected with H3N2v. Unfortunately, another
virus termed H3N2 (note no "v" in its name) has been detected and caused
flu, but this strain is different from H3N2v. In general, all of the
influenza A viruses have a structure similar to the H1N1 virus; each
type has a somewhat different H and/or N structure.
Why is swine flu now infecting humans?
Many researchers now consider that two main series of events can lead to swine flu (and also
avian or bird flu) becoming a major cause for influenza illness in humans.
First, the influenza viruses (types A, B, C) are enveloped RNA
viruses with a segmented genome; this means the viral RNA genetic code
is not a single strand of RNA but exists as
eight different RNA segments in the influenza viruses. A human (or bird)
influenza virus can infect a pig respiratory cell at the same time as a
swine influenza virus; some of the replicating RNA strands from the
human virus can get mistakenly enclosed inside the enveloped swine
influenza virus. For example, one cell could contain
eight swine flu and eight human flu RNA segments. The total number of
RNA types in one cell would be 16;
four swine and four human flu RNA segments could be incorporated into
one particle, making a viable
eight RNA-segmented flu virus from the 16 available segment types.
Various combinations of RNA segments can result in a new subtype of
virus (this
process is known as
antigenic shift)
that may have the ability to preferentially infect humans but still
show characteristics unique to the swine influenza virus (see Figure 1).
It is even possible to include RNA strands from birds, swine, and human
influenza viruses into one virus if a
single cell becomes infected with all three types of influenza (for
example,
two bird flu, three swine flu, and three human flu RNA segments to
produce a viable
eight-segment new type of flu viral genome). Formation of a new viral
type is considered to be antigenic shift; small changes
within an individual RNA segment in flu viruses
are termed
antigenic drift
and result in minor changes in the virus. However, these can accumulate
over time to produce enough minor changes that cumulatively change the
virus' antigenic makeup over time (usually years).
Second, pigs can play a unique role as an intermediary host to new flu
types because pig respiratory cells can be infected directly with bird,
human, and other mammalian flu viruses. Consequently, pig respiratory
cells are able to be infected with many types of flu and can function as
a "mixing pot" for flu RNA segments (see Figure 1). Bird flu viruses,
which usually infect the gastrointestinal cells of many bird species,
are shed in bird feces. Pigs can pick these viruses up from the
environment, and this seems to be the major way that bird flu virus RNA
segments enter the mammalian flu virus population. Figure 1 shows this
process in H1N1, but the figure represents the genetic process for all
flu viruses, including human, swine, and avian strains.
