H1N1 influenza

What is swine flu (novel H1N1 influenza A swine flu)?

Swine flu (swine influenza) is a respiratory disease
caused
by viruses (influenza
viruses) that infect the respiratory tract
of pigs and result in nasal secretions, a
barking-like

cough, decreased
appetite,
 and listless behavior. Swine flu produces
 most of the same symptoms in pigs as
 human flu produces in people. Swine flu

can last about one to two weeks in pigs that survive.

Swine influenza virus was first isolated from pigs in
1930 in the
 U.S. and has been recognized by
 pork producers and veterinarians to cause
 infections in pigs worldwide. In a number

of instances, people have developed the swine flu
infection
 when they are closely associated with pigs (for
example, farmers, pork processors), and likewise,
 pig populations have occasionally been infected
with the human
 flu infection. In most instances, the
cross-species infections (swine virus to man;
human flu virus to pigs) have remained in local areas
and have not caused national or
worldwide infections in either pigs
or humans. Unfortunately, this cross-species situation
with influenza viruses has had the potential to
change. Investigators think the 2009
swine flu strain, first seen in Mexico,
should be termed novel H1N1 flu since it is mainly
found infecting people
and exhibits two main surface antigens, H1
(hemagglutinin type 1) and N1
(neuraminidase type1). Recent investigations show the
eight RNA strands from novel H1N1 flu have
 one strand derived from human flu strains, two
from avian (bird) s
trains, and five from swine strains.

Why is swine flu (H1N1) 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 (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 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 in 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 seem to be
the major way
that bird flu virus RNA segments enter the mammalian
flu virus population.

Figure 1.