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Swine Flu: H1N1
Influenza A (H1N1) virus is a subtype of
influenza A virus and the most common cause of influenza
(flu) in humans. Some strains of H1N1 are endemic in humans
and cause a small fraction of all influenza-like illness and
a small fraction of all seasonal influenza. H1N1 strains
caused a few percent all human flu infections in 2004–2005.
Other strains of H1N1 are endemic in pigs (swine influenza)
and in birds (avian influenza).
In June 2009, the World Health Organization declared the new
strain of swine-origin H1N1 as a pandemic. This strain is
often called swine flu by the public media.
Swine Influenza
Swine influenza (also called swine flu, hog flu, or pig flu)
is an infection by any one of several types of swine
influenza virus. Swine influenza virus (SIV) is any strain
of the influenza family of viruses that is endemic in pigs.
As of 2009, the known SIV strains include influenza C and
the subtypes of influenza A known as H1N1, H1N2, H3N1, H3N2,
and H2N3.
Swine influenza virus is common throughout pig populations
worldwide. Transmission of the virus from pigs to humans is
not common and does not always lead to human influenza,
often resulting only in the production of antibodies in the
blood. If transmission does cause human influenza, it is
called zoonotic swine flu. People with regular exposure to
pigs are at increased risk of swine flu infection. The meat
of an infected animal poses no risk of infection when
properly cooked.
During the mid-20th century, identification of influenza
subtypes became possible, allowing accurate diagnosis of
transmission to humans. Since then, only 50 such
transmissions have been confirmed. These strains of swine
flu rarely pass from human to human. Symptoms of zoonotic
swine flu in humans are similar to those of influenza and of
influenza-like illness in general, namely chills, fever,
sore throat, muscle pains, severe headache, coughing,
weakness and general discomfort. The recommended time of
isolation is about five days.
In the 2009 flu pandemic, the virus
isolated from patients in the United States was found to be
made up of genetic elements from four different flu viruses
– North American swine influenza, North American avian
influenza, human influenza, and swine influenza virus
typically found in Asia and Europe – "an unusually
mongrelised mix of genetic sequences." This new strain
appears to be a result of reassortment of human influenza
and swine influenza viruses, in all four different strains
of subtype H1N1.
Preliminary genetic characterization found that the
hemagglutinin (HA) gene was similar to that of swine flu
viruses present in U.S. pigs since 1999, but the
neuraminidase (NA) and matrix protein (M) genes resembled
versions present in European swine flu isolates. The six
genes from American swine flu are themselves mixtures of
swine flu, bird flu, and human flu viruses. While viruses
with this genetic makeup had not previously been found to be
circulating in humans or pigs, there is no formal national
surveillance system to determine what viruses are
circulating in pigs in the U.S.
On June 11, 2009, the WHO declared an H1N1 pandemic, moving
the alert level to phase 6, marking the first global
pandemic since the 1968 Hong Kong flu.
On October 25, 2009 U.S. President Barack Obama officially
declared H1N1 a national emergency.
November 29, 2009 worldwide update by the U.N.'s World
Health Organization (WHO) states that "207 countries and
overseas territories/communities have reported laboratory
confirmed cases of pandemic influenza H1N1 2009, including
at least 8,768 deaths."
January 15, 2010 worldwide update by the U.N.'s World Health
Organization (WHO) states that "208 countries and overseas
territories or communities have reported laboratory
confirmed cases of pandemic influenza H1N1 2009, including
at least 13,554 deaths."
A study conducted in coordination with the University of
Michigan Health Service is scheduled for publication in the
December 2009 American Journal of Roentgenology warning that
H1N1 flu can cause pulmonary embolism, surmised as a leading
cause of death in this current pandemic. The study authors
suggest physician evaluation via contrast enhanced CT scans
for the presence of pulmonary emboli when caring for
patients diagnosed with respiratory complications from a
"severe" case of the H1N1 flu.
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