Influenza
(Flu)
of several types of swine influenza virus. Swine influenza
virus (SIV) or S-OIV (swine-origin influenza virus) is
any strain of the influenza family of viruses that is endemic
in pigs.[2]
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
Classification
Of the three genera of
influenza viruses that cause human flu, two also cause influenza in pigs,
with influenza A being common in pigs and influenza C
being rare. Influenza B has not been reported in pigs.
Within influenza A and influenza C, the strains found in pigs and humans are
largely distinct, although because of reassortment
there have been transfers of genes among strains crossing swine, avian, and
human species boundaries.
Influenza C
Influenza C viruses
infect both humans and pigs, but do not infect birds. Transmission between pigs
and humans have occurred in the past For example, influenza C caused small
outbreaks of a mild form of influenza amongst children in Japan and California.
Because of its limited host range and the lack of genetic diversity in
influenza C, this form of influenza does not cause pandemics in humans.
Influenza A
Swine influenza is known
to be caused by influenza A subtypes H1N1, H1N2,
H2N3, H3N1, and H3N2. In pigs, three
influenza A virus subtypes (H1N1, H1N2, and H3N2) are the most common strains
worldwide. 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.
Surveillance
Although there is no
formal national surveillance system in the United
States to determine what viruses are circulating in pigs,
there is an informal surveillance network in the United States that is part of a
world surveillance network.
Veterinary medical pathologist, Tracey
McNamara, set up a national disease surveillance system in zoos because the
zoos do active disease surveillance and many of the exotic animals housed there
have broad susceptibilities. Many species fall below the radar of any federal
agencies (including dogs, cats, pet prairie dogs, zoo animals, and urban
wildlife), even though they may be important in the early detection of human
disease outbreaks.
History
Swine influenza was
first proposed to be a disease related to human influenza during the 1918 flu
pandemic, when pigs became sick at the same time as humans. The first
identification of an influenza virus as a cause of disease in pigs occurred
about ten years later, in 1930. For the following 60 years, swine influenza
strains were almost exclusively H1N1. Then, between 1997 and 2002, new strains of three different
subtypes and five different genotypes emerged as causes of influenza among pigs
in North America. In 1997–1998, H3N2 strains emerged.
These strains, which include genes derived by reassortment
from human, swine and avian viruses, have become a major cause of swine
influenza in North America. Reassortment
between H1N1
and H3N2
produced H1N2.
In 1999 in Canada,
a strain of H4N6 crossed the species barrier from
birds to pigs, but was contained on a single farm.
The H1N1 form of swine flu is
one of the descendants of the strain that caused the 1918 flu
pandemic. As well as persisting in pigs, the descendants of the 1918
virus have also circulated in humans through the 20th century, contributing to
the normal seasonal epidemics of influenza. However, direct transmission from
pigs to humans is rare, with only 12 cases in the U.S. since 2005. Nevertheless, the
retention of influenza strains in pigs after these strains have disappeared
from the human population might make pigs a reservoir where influenza viruses
could persist, later emerging to reinfect humans once human immunity to these
strains has waned.
Swine flu has been
reported numerous times as a zoonosis in humans, usually with limited distribution, rarely
with a widespread distribution. Outbreaks in swine are common and cause
significant economic losses in industry, primarily by causing stunting and
extended time to market. For example, this disease costs the British meat
industry about £65 million every year
The 1918 flu
pandemic in humans was associated with H1N1 and influenza appearing
in pigs; this may reflect a zoonosis either from swine to humans, or from
humans to
1976 U.S. outbreak In
1998, swine flu was found in pigs in four U.S. states. Within a year, it had
spread through pig populations
across the United States.
Scientists found that this virus had originated in pigs as a recombinant form
of flu strains from birds and humans. This outbreak confirmed that pigs can
serve as a crucible where novel influenza viruses emerge as a result of the
reassortment of genes from different strains. Genetic components of these 1998
triple-hybrid stains would later form six out of the eight viral gene segments
in the 2009 flu outbreak.2007 Philippine
outbreak in swine
Transmission between pigs
Influenza is quite
common in pigs, with about half of breeding pigs having been exposed to the
virus in the US Antibodies to the virus are also common in pigs in other
countries.
The main route of
transmission is through direct contact between infected and uninfected animals.
These close contacts are particularly common during animal transport. Intensive
farming may also increase the risk of transmission, as the pigs are
raised in very close proximity to each other. The direct transfer of the virus
probably occurs either by pigs touching noses, or through dried mucus. Airborne
transmission through the aerosols produced by pigs coughing or sneezing are
also an important means of infection. The virus usually spreads quickly through
a herd, infecting all the pigs within just a few days. Transmission may also
occur through wild animals, such as wild boar,
which can spread the disease between farms.
Transmission to humans
People 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. Vaccination of these workers against influenza and surveillance
for new influenza strains among this population may therefore be an important
public health measure. 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. 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. Other professions at particular risk of infection are
veterinarians and meat processing workers, although the risk of infection for
both of these groups is lower than that of farm workers
Interaction with avian
H5N1 in pigs In swine Pigs are unusual as they can be infected with
influenza strains that usually infect three different species: pigs, birds and
humans This makes pigs a host where influenza viruses might exchange producing
new and dangerous strains Avian 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. H3N2
evolved from H2N2
by antigenic shift. In August 2004, researchers in
China
found H5N1
in pigs.
In pigs influenza infection produces fever, lethargy,
sneezing,
coughing,
difficulty
breathing and decreased appetite In some cases the infection can
cause abortion.
Although mortality is usually low (around 1–4%), the virus can produce weight loss
and poor growth,
causing economic loss to farmers. Infected pigs can lose up to 12 pounds
of body weight over a 3 to 4 week period.
In human
Main symptoms of swine flu in human
Direct transmission of a
swine flu virus from pigs to humans is occasionally possible (called zoonotic
swine flu). In all, 50 cases are known to have occurred since the first report
in medical literature in 1958, which have resulted in a total of six deaths. Of
these six people, one was pregnant, one had leukemia,
one had Hodgkin disease and two were known to be
previously healthy. Despite these apparently low numbers of infections, the
true rate of infection may be higher, since most cases only cause a very mild
disease, and will probably never be reported or diagnosed.
Prevention of pig to human transmission
Swine can be infected by
both avian and human influenza strains of influenza, and therefore are hosts
where the antigenic shifts can occur that create new
influenza strains.
The transmission from
swine to human is believed to occur mainly in swine farms where farmers are in
close contact with live pigs. Although strains of swine influenza are usually
not able to infect humans this may occasionally happen, so farmers and
veterinarians are encouraged to use a face mask
when dealing with infected animals. The use of vaccines on swine to prevent
their infection is a major method of limiting swine to human transmission. Risk
factors that may contribute to swine-to-human transmission include smoking and
not wearing gloves when working with sick animals.
Influenza
spreads between humans through coughing or sneezing and people touching
something with the virus on it and then touching their own nose or mouth Swine
flu cannot be spread by pork products, since the virus is not transmitted through
food. 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 for analysis.
Treatment
In swine
As swine influenza is rarely fatal to pigs, little treatment beyond
rest and supportive care is required Instead veterinary efforts are focused on
preventing the spread of the virus throughout the farm, or to other farms.
Vaccination and animal management techniques are most important in these
efforts. Antibiotics are also used to treat this disease, which although they
have no effect against the influenza virus, do help prevent bacterial pneumonia
and other secondary infections in influenza-weakened
herds.
In humans
If a person becomes sick with swine flu, antiviral drugs can make
the illness milder and make the patient feel better faster. They may also
prevent serious flu complications. For treatment, antiviral drugs work best if
started soon after getting sick (within 2 days of symptoms). Beside antivirals,
supportive care at home or in hospital, focuses on controlling fevers,
relieving pain and maintaining fluid balance, as well as identifying and
treating any secondary infections or other medical problems. The U.S. Centers for Disease Control and
Prevention recommends the use of Tamiflu (oseltamivir)
or Relenza (zanamivir)
for the treatment and/or prevention of infection with swine influenza viruses;
however, the majority of people infected with the virus make a full recovery without
requiring medical attention or antiviral drugs. The virus isolates in the 2009
outbreak have been found resistant to amantadine
and rimantadine.
In the U.S., on April 27, 2009, the FDA issued Emergency Use Authorizations to make
available Relenza
and Tamiflu
antiviral
drugs to treat the swine influenza virus in cases for which they are
currently unapproved. The agency issued these EUAs to allow treatment of
patients younger than the current approval allows and to allow the widespread
distribution of the drugs, including by non-licensed volunteers.
(Flu)
of several types of swine influenza virus. Swine influenza
virus (SIV) or S-OIV (swine-origin influenza virus) is
any strain of the influenza family of viruses that is endemic
in pigs.[2]
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
Classification
Of the three genera of
influenza viruses that cause human flu, two also cause influenza in pigs,
with influenza A being common in pigs and influenza C
being rare. Influenza B has not been reported in pigs.
Within influenza A and influenza C, the strains found in pigs and humans are
largely distinct, although because of reassortment
there have been transfers of genes among strains crossing swine, avian, and
human species boundaries.
Influenza C
Influenza C viruses
infect both humans and pigs, but do not infect birds. Transmission between pigs
and humans have occurred in the past For example, influenza C caused small
outbreaks of a mild form of influenza amongst children in Japan and California.
Because of its limited host range and the lack of genetic diversity in
influenza C, this form of influenza does not cause pandemics in humans.
Influenza A
Swine influenza is known
to be caused by influenza A subtypes H1N1, H1N2,
H2N3, H3N1, and H3N2. In pigs, three
influenza A virus subtypes (H1N1, H1N2, and H3N2) are the most common strains
worldwide. 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.
Surveillance
Although there is no
formal national surveillance system in the United
States to determine what viruses are circulating in pigs,
there is an informal surveillance network in the United States that is part of a
world surveillance network.
Veterinary medical pathologist, Tracey
McNamara, set up a national disease surveillance system in zoos because the
zoos do active disease surveillance and many of the exotic animals housed there
have broad susceptibilities. Many species fall below the radar of any federal
agencies (including dogs, cats, pet prairie dogs, zoo animals, and urban
wildlife), even though they may be important in the early detection of human
disease outbreaks.
History
Swine influenza was
first proposed to be a disease related to human influenza during the 1918 flu
pandemic, when pigs became sick at the same time as humans. The first
identification of an influenza virus as a cause of disease in pigs occurred
about ten years later, in 1930. For the following 60 years, swine influenza
strains were almost exclusively H1N1. Then, between 1997 and 2002, new strains of three different
subtypes and five different genotypes emerged as causes of influenza among pigs
in North America. In 1997–1998, H3N2 strains emerged.
These strains, which include genes derived by reassortment
from human, swine and avian viruses, have become a major cause of swine
influenza in North America. Reassortment
between H1N1
and H3N2
produced H1N2.
In 1999 in Canada,
a strain of H4N6 crossed the species barrier from
birds to pigs, but was contained on a single farm.
The H1N1 form of swine flu is
one of the descendants of the strain that caused the 1918 flu
pandemic. As well as persisting in pigs, the descendants of the 1918
virus have also circulated in humans through the 20th century, contributing to
the normal seasonal epidemics of influenza. However, direct transmission from
pigs to humans is rare, with only 12 cases in the U.S. since 2005. Nevertheless, the
retention of influenza strains in pigs after these strains have disappeared
from the human population might make pigs a reservoir where influenza viruses
could persist, later emerging to reinfect humans once human immunity to these
strains has waned.
Swine flu has been
reported numerous times as a zoonosis in humans, usually with limited distribution, rarely
with a widespread distribution. Outbreaks in swine are common and cause
significant economic losses in industry, primarily by causing stunting and
extended time to market. For example, this disease costs the British meat
industry about £65 million every year
The 1918 flu
pandemic in humans was associated with H1N1 and influenza appearing
in pigs; this may reflect a zoonosis either from swine to humans, or from
humans to
1976 U.S. outbreak In
1998, swine flu was found in pigs in four U.S. states. Within a year, it had
spread through pig populations
across the United States.
Scientists found that this virus had originated in pigs as a recombinant form
of flu strains from birds and humans. This outbreak confirmed that pigs can
serve as a crucible where novel influenza viruses emerge as a result of the
reassortment of genes from different strains. Genetic components of these 1998
triple-hybrid stains would later form six out of the eight viral gene segments
in the 2009 flu outbreak.2007 Philippine
outbreak in swine
Transmission between pigs
Influenza is quite
common in pigs, with about half of breeding pigs having been exposed to the
virus in the US Antibodies to the virus are also common in pigs in other
countries.
The main route of
transmission is through direct contact between infected and uninfected animals.
These close contacts are particularly common during animal transport. Intensive
farming may also increase the risk of transmission, as the pigs are
raised in very close proximity to each other. The direct transfer of the virus
probably occurs either by pigs touching noses, or through dried mucus. Airborne
transmission through the aerosols produced by pigs coughing or sneezing are
also an important means of infection. The virus usually spreads quickly through
a herd, infecting all the pigs within just a few days. Transmission may also
occur through wild animals, such as wild boar,
which can spread the disease between farms.
Transmission to humans
People 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. Vaccination of these workers against influenza and surveillance
for new influenza strains among this population may therefore be an important
public health measure. 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. 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. Other professions at particular risk of infection are
veterinarians and meat processing workers, although the risk of infection for
both of these groups is lower than that of farm workers
Interaction with avian
H5N1 in pigs In swine Pigs are unusual as they can be infected with
influenza strains that usually infect three different species: pigs, birds and
humans This makes pigs a host where influenza viruses might exchange producing
new and dangerous strains Avian 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. H3N2
evolved from H2N2
by antigenic shift. In August 2004, researchers in
China
found H5N1
in pigs.
In pigs influenza infection produces fever, lethargy,
sneezing,
coughing,
difficulty
breathing and decreased appetite In some cases the infection can
cause abortion.
Although mortality is usually low (around 1–4%), the virus can produce weight loss
and poor growth,
causing economic loss to farmers. Infected pigs can lose up to 12 pounds
of body weight over a 3 to 4 week period.
In human
Main symptoms of swine flu in human
Direct transmission of a
swine flu virus from pigs to humans is occasionally possible (called zoonotic
swine flu). In all, 50 cases are known to have occurred since the first report
in medical literature in 1958, which have resulted in a total of six deaths. Of
these six people, one was pregnant, one had leukemia,
one had Hodgkin disease and two were known to be
previously healthy. Despite these apparently low numbers of infections, the
true rate of infection may be higher, since most cases only cause a very mild
disease, and will probably never be reported or diagnosed.
Prevention of pig to human transmission
Swine can be infected by
both avian and human influenza strains of influenza, and therefore are hosts
where the antigenic shifts can occur that create new
influenza strains.
The transmission from
swine to human is believed to occur mainly in swine farms where farmers are in
close contact with live pigs. Although strains of swine influenza are usually
not able to infect humans this may occasionally happen, so farmers and
veterinarians are encouraged to use a face mask
when dealing with infected animals. The use of vaccines on swine to prevent
their infection is a major method of limiting swine to human transmission. Risk
factors that may contribute to swine-to-human transmission include smoking and
not wearing gloves when working with sick animals.
Influenza
spreads between humans through coughing or sneezing and people touching
something with the virus on it and then touching their own nose or mouth Swine
flu cannot be spread by pork products, since the virus is not transmitted through
food. 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 for analysis.
As swine influenza is rarely fatal to pigs, little treatment beyond
rest and supportive care is required Instead veterinary efforts are focused on
preventing the spread of the virus throughout the farm, or to other farms.
Vaccination and animal management techniques are most important in these
efforts. Antibiotics are also used to treat this disease, which although they
have no effect against the influenza virus, do help prevent bacterial pneumonia
and other secondary infections in influenza-weakened
herds.
If a person becomes sick with swine flu, antiviral drugs can make
the illness milder and make the patient feel better faster. They may also
prevent serious flu complications. For treatment, antiviral drugs work best if
started soon after getting sick (within 2 days of symptoms). Beside antivirals,
supportive care at home or in hospital, focuses on controlling fevers,
relieving pain and maintaining fluid balance, as well as identifying and
treating any secondary infections or other medical problems. The U.S. Centers for Disease Control and
Prevention recommends the use of Tamiflu (oseltamivir)
or Relenza (zanamivir)
for the treatment and/or prevention of infection with swine influenza viruses;
however, the majority of people infected with the virus make a full recovery without
requiring medical attention or antiviral drugs. The virus isolates in the 2009
outbreak have been found resistant to amantadine
and rimantadine.
In the U.S., on April 27, 2009, the FDA issued Emergency Use Authorizations to make
available Relenza
and Tamiflu
antiviral
drugs to treat the swine influenza virus in cases for which they are
currently unapproved. The agency issued these EUAs to allow treatment of
patients younger than the current approval allows and to allow the widespread
distribution of the drugs, including by non-licensed volunteers.