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What is the swine flu?
Swine flu (swine influenza) is a respiratory disease caused
by viruses (influenza viruses) that infect the respiratory tract of pigs,
resulting in nasal secretions, a barking 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 decided the
2009 so-called "swine flu" strain, first seen in Mexico, should be
termed novel H1N1 flu since it was mainly found infecting people and exhibits
two main surface antigens, H1 (hemagglutinin type 1) and N1 (neuraminidase
type1). The eight RNA strands from novel H1N1 flu have one strand derived from
human flu strains, two from avian (bird) strains, and five from swine strains.
Swine flu is transmitted from person to person by inhalation
or ingestion of droplets containing virus from people sneezing or coughing; it
is not transmitted by eating cooked pork products. The newest swine flu virus
that has caused swine flu is influenza A H3N2v (commonly termed H3N2v) that
began as an outbreak in 2011. The "v" in the name means the virus is
a variant that normally infects only pigs but has begun to infect humans. There
have been small outbreaks of H1N1 since the pandemic; a recent one is in India
where at least three people have died.
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 officials from the U.S. Centers for Disease Control and
Prevention (CDC) reported increased numbers of people infected in the 2012-2013
flu season. Currently, 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 (see figure 1) and result in minor changes in the virus. However, these
small genetic changes can accumulate over time to produce enough minor changes
that cumulatively alter the virus' 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.
What are the symptoms of swine flu?
Symptoms of swine flu are similar to most influenza infections:
fever (100 F or greater), cough, nasal secretions, fatigue, and headache, with
fatigue being reported in most infected individuals. Some patients may also get
a sore throat, rash, body aches, headaches, chills, nausea, vomiting, and
diarrhea. In Mexico, many of the initial patients infected with H1N1 influenza
were young adults, which made some investigators speculate that a strong immune
response, as seen in young people, may cause some collateral tissue damage.The
incubation period from exposure to first symptoms is about one to four days,
with an average of two days. The symptoms last about one to two weeks and can
last longer if the person has a severe infection.
Some patients develop severe respiratory symptoms and need
respiratory support (such as a ventilator to breathe for the patient). Patients
can get pneumonia (bacterial secondary infection) if the viral infection
persists, and some can develop seizures. Death often occurs from secondary
bacterial infection of the lungs; appropriate antibiotics need to be used in
these patients. The usual mortality (death) rate for typical influenza A is
about 0.1%, while the 1918 "Spanish flu" epidemic had an estimated
mortality rate ranging from 2%-20%. Swine (H1N1) flu in Mexico had about 160 deaths
and about 2,500 confirmed cases, which would correspond to a mortality rate of
about 6%, but these initial data were revised and the mortality rate worldwide
was estimated to be much lower. Fortunately, the mortality rate of H1N1
remained low and similar to that of the conventional flu (average conventional
flu mortality rate is about 36,000 per year; projected H1N1 flu mortality rate
was 90,000 per year in the U.S. as determined by the president's advisory
committee, but it never approached that high number).
Fortunately, although H1N1 developed into a pandemic
(worldwide) flu strain, the mortality rate in the U.S. and many other countries
only approximated the usual numbers of flu deaths worldwide. Speculation about
why the mortality rate remained much lower than predicted includes increased
public awareness and action that produced an increase in hygiene (especially
hand washing), a fairly rapid development of a new vaccine, and patient
self-isolation if symptoms developed.
Article Credit : http://www.medicinenet.com/