Swine influenza
Swine influenza (also
swine flu) refers to influenza caused by any strain of the influenza virus endemic in pigs (swine). Strains endemic in swine are called
swine influenza virus (
SIV). Swine flu is common in swine and rare in humans.
SIV
can mutate into a form that allows it to pass from human to human. The
strain responsible for the 2009 swine flu outbreak is believed to have
undergone such a mutation.
[1] People who work with poultry
and swine, especially people with intense exposures, are at risk of
infection with influenza from these animals if the animals carry a
strain that is also able to infect humans. However, these strains rarely
are transmitted from human to human.
In humans, the symptoms of
swine flu 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 strain
responsible for the 2009 swine flu outbreak in most cases causes only
mild symptoms and the infected person recovers fully.
Of the three genera of human flu, two are endemic also in swine: Influenzavirus A (common) and Influenzavirus C (rare).
[2] Influenzavirus B has not been reported in swine. Within Influenzavirus A
and Influenzavirus C, the strains endemic to swine and humans are
largely distinct
BackgroundThe swine flu is a descendant of the infamous "Spanish flu" that caused a devastating pandemic in humans in 1918-1919.
[3] In less than a year, that pandemic killed more than 500,000 Americans
and some 20 million people worldwide - the greatest number ever killed
in so short a period by any natural or man-made catastrophe. It also
killed and sickened large numbers of hogs. Within a decade, the disease
stopped circulating among humans, but it has infected swine ever since.
Although hogs had initially caught the virus from humans, it has
undergone slight changes over the years, emerging occasionally to infect
individuals who work closely with pigs. However, there have only been
12 cases in the U.S. since 2005 in which humans caught swine flu after
being in contact with pigs and there is currently no requirement that
pigs be vaccinated against swine flu.
[4]The flu virus is
perhaps the trickiest known to medical science; it constantly changes
form to elude the protective antibodies that the body has developed in
response to previous exposures to influenza or to influenza vaccines.
Every two or three years the virus undergoes minor changes. Then, at
intervals of roughly a decade, after the bulk of the world's population
has developed some level of resistance to these minor changes, it
undergoes a major shift that enables it to tear off on yet another
pandemic sweep around the world, infecting hundreds of millions of
people who suddenly find their antibody defenses outflanked.
[5] Even during the Spanish flu pandemic, the initial wave of the disease
was relatively mild and the second wave was highly lethal.
[3]In
1957 there was an Asian flu pandemic that infected some 45 million
Americans and killed 70,000 of them. Eleven years later, lasting from
1968 to 1969, the Hong Kong pandemic afflicted 50 million Americans and
caused 33,000 deaths, costing approximately $3.9 billion. In 1976 about
500 soldiers became infected with swine flu over a period of a few
weeks. However, by the end of the month investigators found that the the
virus had "mysteriously disappeared" and there were no more signs of
swine flu anywhere on the post.
[3] There were isolated cases around the U.S. but those cases were supposedly to individuals who caught the virus from pigs.
Medical
researchers worldwide remain vigilant knowing that the swine flu virus
might again mutate into something as deadly as the Spanish flu. They are
carefully watching the latest 2009 outbreak of swine flu and making
contingency plans for a possible global pandemic
ClassificationSIV
strains isolated to date have been classified either as Influenzavirus C
or one of the various subtypes of the genus Influenzavirus A.
[6]Influenza ASwine influenza is known to be caused by influenza A subtypes H1N1,
[7] H1N2,
[7] H3N1,
[8] H3N2,
[7] and H2N3.
[9]In
swine, three influenza A virus subtypes (H1N1, H3N2, and H1N2) are
circulating throughout the world. In the United States, the H1N1 subtype
was exclusively prevalent among swine populations before 1998; however,
since late August 1998, H3N2 subtypes have been isolated from pigs. As
of 2004, H3N2 virus isolates in US swine and turkey stocks were triple
reassortants, containing genes from human (HA, NA, and PB1), swine (NS,
NP, and M), and avian (PB2 and PA) lineages.
[10]Interaction with H5N1Avian
influenza virus H3N2 is endemic in pigs in China and has been detected
in pigs in Vietnam, increasing fears of the emergence of new variant
strains.
[11] Health experts
[who?] say pigs
can carry human influenza viruses, which can combine (i.e. exchange
homologous genome sub-units by genetic reassortment) with H5N1, passing
genes and mutating into a form which can pass easily among humans.
[12] H3N2 evolved from H2N2 by antigenic shift.
[13] In August 2004, researchers in China found H5N1 in pigs.
[14]Nature magazine reported that Chairul Nidom, a virologist at Airlangga
University's tropical disease center in Surabaya, East Java, conducted
an independent research study in 2005. He tested the blood of 10
apparently healthy pigs housed near poultry farms in West Java where
avian flu had broken out. Five of the pig samples contained the H5N1
virus. The Indonesian government has since found similar results in the
same region. Additional tests of 150 pigs outside the area were
negative.
[15][16]Signs and symptomsMain symptoms of swine flu in humans.
[17]According
to the Centers for Disease Control and Prevention (CDC), in humans the
symptoms of swine flu are similar to those of influenza and of
influenza-like illness in general. Symptoms include fever, cough, sore
throat, body aches, headache, chills and fatigue. The 2009 outbreak has
shown an increased percentage of patients reporting diarrhea and
vomiting.
[18]Because these symptoms are not specific to swine flu, a differential diagnosis of
probable swine flu requires not only symptoms but also a high likelihood of
swine flu due to the person's recent history. For example, during the
2009 swine flu outbreak in the United States, CDC advised physicians to
"consider swine influenza infection in the differential diagnosis of
patients with acute febrile respiratory illness who have either been in
contact with persons with confirmed swine flu, or who were in one of the
five U.S. states that have reported swine flu cases or in Mexico during
the 7 days preceding their illness onset."
[19] A diagnosis of
confirmed swine flu requires laboratory testing of a respiratory sample (a simple nose and throat swab).
[19]PathophysiologyInfluenza
viruses bind through hemagglutinin onto sialic acid sugars on the
surfaces of epithelial cells; typically in the nose, throat and lungs of
mammals and intestines of birds (Stage 1 in infection figure).
[20]Swine flu in humansPeople
who work with poultry and swine, especially people with intense
exposures, are at increased risk of zoonotic infection with influenza
virus endemic in these animals, and constitute a population of human
hosts in which zoonosis and reassortment can co-occur.
[21] Transmission of influenza from swine to humans who work with swine was
documented in a small surveillance study performed in 2004 at the
University of Iowa.
[22] This study among others forms the
basis of a recommendation that people whose jobs involve handling
poultry and swine be the focus of increased public health surveillance.
[21] The 2009 swine flu outbreak is an apparent reassortment of several
strains of influenza A virus subtype H1N1, including a strain endemic in
humans and two strains endemic in pigs, as well as an avian influenza.
[23]The
CDC reports that the symptoms and transmission of the swine flu from
human to human is much like that of seasonal flu. Common symptoms
include fever, lethargy, lack of appetite and coughing, while runny
nose, sore throat, nausea, vomiting and diarrhea have also been
reported.
[24] It is believed to be spread between humans
through coughing or sneezing of infected people and touching something
with the virus on it and then touching their own nose or mouth.
[25] Swine flu cannot be spread by pork products, since the virus is not transmitted through food.
[25] The swine flu in humans is most contagious during the first five days
of the illness although some people, most commonly children, can remain
contagious for up to ten days. Diagnosis can be made by sending a
specimen, collected during the first five days, to the CDC for analysis.
[26]The
swine flu is susceptible to four drugs licensed in the United States,
amantadine, rimantadine, oseltamivir and zanamivir; however, for the
2009 outbreak it is recommended it be treated under medical advice only
with oseltamivir and zanamivir to avoid drug resistance.
[27] The vaccine for the human seasonal H1N1 flu does not protect against the
swine H1N1 flu, even if the virus strains are the same specific
variety, as they are antigenically very different.
[28] 
