Swine influenza (also called
Pig influenza, swine flu, hog flu and 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.
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 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[6] 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.
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.
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.
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 swine. Although it is not certain in which direction the virus was
transferred, some evidence suggests that, in this case, pigs caught the disease
from humans. For instance, swine influenza was only noted as a new disease
of pigs in 1918, after the first large outbreaks of influenza amongst people.
Although a recent phylogenetic analysis of more recent strains of influenza
in humans, birds, and swine suggests that the 1918 outbreak in humans followed
a reassortment event within a mammal, the exact origin of the 1918 strain
remains elusive. It is estimated that anywhere from 50 to 100 million people
were killed worldwide.
Main article: 1976 swine flu outbreak
On February 5, 1976, in the United States an army recruit at Fort Dix said
he felt tired and weak. He died the next day and four of his fellow soldiers
were later hospitalized. Two weeks after his death, health officials announced
that the cause of death was a new strain of swine flu. The strain, a variant
of H1N1, is known as A/New Jersey/1976 (H1N1). It was detected only from January
19 to February 9 and did not spread beyond Fort Dix.
President Ford receives swine flu vaccination
This new strain appeared to be closely related to the strain involved in the
1918 flu pandemic. Moreover, the ensuing increased surveillance uncovered
another strain in circulation in the U.S.: A/Victoria/75 (H3N2) spread simultaneously,
also caused illness, and persisted until March. Alarmed public-health officials
decided action must be taken to head off another major pandemic, and urged
President Gerald Ford that every person in the U.S. be vaccinated for the
disease
The vaccination program was plagued by delays and public relations problems.
On October 1, 1976, immunizations began and three senior citizens died soon
after receiving their injections. This resulted in a media outcry that linked
these deaths to the immunizations, despite the lack of any proof that the
vaccine was the cause. According to science writer Patrick Di Justo, however,
by the time the truth was known—that the deaths were not proven to be
related to the vaccine—it was too late. "The government had long
feared mass panic about swine flu—now they feared mass panic about the
swine flu vaccinations." This became a strong setback to the program.
There were reports of Guillain-Barré syndrome, a paralyzing neuromuscular
disorder, affecting some people who had received swine flu immunizations.
Although if a link exists is still not clear, this syndrome may be a rare
side-effect of influenza vaccines. As a result, Di Justo writes that "the
public refused to trust a government-operated health program that killed old
people and crippled young people." In total, 48,161,019 Americans, or
just over 22% of the population, had been immunized by the time the National
Influenza Immunization Program (NIIP) was effectively halted on December 16,
1976.
Overall, there were 1098 cases of Guillain-Barré Syndrome (GBS) recorded
nationwide by CDC surveillance, 532 of which occurred after vaccination and
543 before vaccination. There are about one to two cases of GBS per 100,000
people every year, whether or not people have been vaccinated. The vaccination
program seems to have increased this normal risk of developing GBS by about
to one extra case per 100,000 vaccinations.[33] The CDC states that most studies
on modern influenza vaccines have seen no link with GBS, Although one review
gives an incidence of about one case per million vaccinations.
In September 1988, a swine flu virus killed one woman and infected others.
32-year old Barbara Ann Wieners was eight months pregnant when she and her
husband, Ed, became ill after visiting the hog barn at a county fair in Walworth
County, Wisconsin. Barbara died eight days later, after developing pneumonia.
The only pathogen identified was an H1N1 strain of swine influenza virus.
Doctors were able to induce labor and deliver a healthy daughter before she
died. Her husband recovered from his symptoms.
Influenza-like illness (ILI) was reportedly widespread among the pigs exhibited
at the fair. Of the 25 swine exhibitors aged 9 to 19 at the fair, 19 tested
positive for antibodies to SIV, but no serious illnesses were seen. The virus
was able to spread between people, since 1-3 health care personnel who had
cared for the pregnant woman developed mild influenza-like illnesses, and
antibody tests suggested that they had been infected with swine flu. However,
there was no community 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.
Please help improve this article by expanding it. Further
information might be found on the talk page. (April 2009)
On August 20, 2007 Department of Agriculture officers investigated the outbreak
(epizootic) of swine flu in Nueva Ecija and Central Luzon, Philippines. The
mortality rate is less than 10% for swine flu, unless there are complications
like hog cholera. On July 27, 2007, the Philippine National Meat Inspection
Service (NMIS) raised a hog cholera "red alert" warning over Metro
Manila and 5 regions of Luzon after the disease spread to backyard pig farms
in Bulacan and Pampanga, even if these tested negative for the swine flu virus.
The H1N1 viral strain implicated in the 2009 flu pandemic among humans often
is called "swine flu" because initial testing showed many of the
genes in the virus were similar to influenza viruses normally occurring in
North American swine. Further research has shown that three-quarters or six
out of the eight gene segments of the 2009 virus arose from the 1998 North
American swine flu strains which emerged from the first-ever reported triple-hybrid
virus of 1998.
In late April, Margaret Chan, the World Health Organization's director-general,
declared a "public health emergency of international concern" under
the rules of the WHO's new International Health Regulations when the first
two cases of the H1N1 virus were reported in the United States, followed by
hundreds of cases in Mexico.Following the initial cases in the USA and Mexico,
on May 2, 2009, it was reported in pigs at a farm in Alberta, Canada, with
a link to the outbreak in Mexico. The pigs are suspected to have caught this
new strain of virus from a farm worker who recently returned from Mexico,
then showed symptoms of an influenza-like illness.These are probable cases,
pending confirmation by laboratory testing.
The new strain was initially described as an apparent reassortment of at least
four strains of influenza A virus subtype H1N1, including one strain endemic
in humans, one endemic in birds, and two endemic in swine.Subsequent analysis
suggested it was a reassortment of just two strains, both found in swine.
Although initial reports identified the new strain as swine influenza (i.e.,
a zoonosis originating in swine), its genetic origin was only later revealed
to have been mostly a descendant of the triple-reassortment virus which emerged
in factory farms in the United States in 1998. Several countries took precautionary
measures to reduce the chances for a global pandemic of the disease. The 2009
swine flu has been compared to other similar types of influenza virus in terms
of mortality: "in the US it appears that for every 1000 people who get
infected, about 40 people need admission to hospital and about one person
dies." There are fears that swine flu will become a major global pandemic
at the end of the year (coinciding with the Northern Hemisphere winter months),
with many countries planning major vaccination campaigns.
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.
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.
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 genes, 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.
Main symptoms of swine flu in swine.
These H5N1 infections may be quite common: in a survey 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.
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 humans
Main symptoms of swine flu in humans
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
According to the Centers for Disease Control and Prevention (CDC), in humans
the symptoms of the 2009 "swine flu" H1N1 virus 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.
The 2009 H1N1 virus is not zoonotic swine flu, as it is not transmitted from
pigs to humans, but from person to person.
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." A diagnosis of confirmed swine flu requires laboratory testing
of a respiratory sample (a simple nose and throat swab).
The most common cause of death is respiratory failure. Other causes of death
are pneumonia (leading to sepsis), high fever (leading to neurological problems),
dehydration (from excessive vomiting and diarrhea) and electrolyte imbalance.
Fatalities are more likely in young children and the elderly.
Thermal scanning of passengers arriving at Singapore Changi airport.
The CDC recommends real time RT-PCR as the method of choice for diagnosing
H1N1. This method allows a specific diagnosis of novel influenza (H1N1) as
opposed to seasonal influenza. Near-patient point of care tests are in development.
Prevention of
swine influenza has three components: prevention in swine, prevention of transmission
to humans, and prevention of its spread among humans.
Methods of preventing the spread of influenza among swine include facility
management, herd management, and vaccination (ATCvet code: QI09AA03). Because
much of the illness and death associated with swine flu involves secondary
infection by other pathogens, control strategies that rely on vaccination
may be insufficient.
Control of swine influenza by vaccination has become more difficult in recent
decades, as the evolution of the virus has resulted in inconsistent responses
to traditional vaccines. Standard commercial swine flu vaccines are effective
in controlling the infection when the virus strains match enough to have significant
cross-protection, and custom (autogenous) vaccines made from the specific
viruses isolated are created and used in the more difficult cases. Present
vaccination strategies for SIV control and prevention in swine farms typically
include the use of one of several bivalent SIV vaccines commercially available
in the United States. Of the 97 recent H3N2 isolates examined, only 41 isolates
had strong serologic cross-reactions with antiserum to three commercial SIV
vaccines. Since the protective ability of influenza vaccines depends primarily
on the closeness of the match between the vaccine virus and the epidemic virus,
the presence of nonreactive H3N2 SIV variants suggests that current commercial
vaccines might not effectively protect pigs from infection with a majority
of H3N2 viruses. The United States Department of Agriculture researchers say
that while pig vaccination keeps pigs from getting sick, it does not block
infection or shedding of the virus.
Facility management includes using disinfectants and ambient temperature to
control virus in the environment. The virus is unlikely to survive outside
living cells for more than two weeks, except in cold (but above freezing)
conditions, and it is readily inactivated by disinfectants. Herd management
includes not adding pigs carrying influenza to herds that have not been exposed
to the virus. The virus survives in healthy carrier pigs for up to 3 months
and can be recovered from them between outbreaks. Carrier pigs are usually
responsible for the introduction of SIV into previously uninfected herds and
countries, so new animals should be quarantined. After an outbreak, as immunity
in exposed pigs wanes, new outbreaks of the same strain can occur.
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.
Prevention of human to human transmission
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.
Thermal imaging camera & screen, photographed in an airport terminal in
Greece. Thermal imaging can detect elevated body temperature, one of the signs
of the virus N1H1 (Swine influenza).
Recommendations to prevent spread of the virus among humans include using
standard infection control against influenza. This includes frequent washing
of hands with soap and water or with alcohol-based hand sanitizers, especially
after being out in public. Chance of transmission is also reduced by disinfecting
household surfaces, which can be done effectively with a diluted chlorine
bleach solution.
Experts agree that hand-washing can help prevent viral infections, including
ordinary influenza and the swine flu virus. Also avoiding touching eyes, nose
and mouth with hands prevents flu.[90] Influenza can spread in coughs or sneezes,
but an increasing body of evidence shows small droplets containing the virus
can linger on tabletops, telephones and other surfaces and be transferred
via the fingers to the mouth, nose or eyes. Alcohol-based gel or foam hand
sanitizers work well to destroy viruses and bacteria. Anyone with flu-like
symptoms such as a sudden fever, cough or muscle aches should stay away from
work or public transportation and should contact a doctor for advice.
Social distancing is another tactic. It means staying away from other people
who might be infected and can include avoiding large gatherings, spreading
out a little at work, or perhaps staying home and lying low if an infection
is spreading in a community. Public health and other responsible authorities
have action plans which may request or require social distancing actions depending
on the severity of the outbreak.
Vaccination
Vaccines are available for different kinds of swine flu. The U.S. Food and
Drug Administration (FDA) approved the new swine flu vaccine for use in the
United States on September 15, 2009.Studies by the National Institutes of
Health (NIH), show that a single dose creates enough antibodies to protect
against the virus within about 10 days.
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.
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