Swine
Flu: A Review
Arti Mohan
Dibrugarh University, Dibrugarh, Assam. India
INTRODUCTION
Swine influenza (also swine flu, hog flu, and pig flu(1))
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).(1)
Of the three genera of
human flu, two are endemic also in swine: Influenza A virus is common and
Influenza C is rare.(2) Influenza B has not been reported in swine.
Within Influenzavirus A and Influenzavirus C, the strains endemic to swine and
humans are largely distinct.Swine flu is common in swine in the midwestern
United States (and occasionally in other states), Mexico, Canada, South
America, Europe (including the United Kingdom, Sweden, and Italy), Kenya,
China, Japan, Taiwan, and other parts of eastern Asia.(1)
Swine flu is rare in
humans. People who work with swine, especially people with intense exposures,
are at risk of catching swine influenza if the swine carry a strain able to
infect humans. However, these strains infrequently circulate between humans as
SIV rarely mutates into a form able to pass easily from human to human. 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 In
humans, the symptoms of swine flu are similar to those of inflenza and of
influenza like illness in general, namely chills,fever,sore throat,muscle
pains, severe headache,coughing,weakness and general discomfort. The World
Health Organization has stated that Swine flu symptoms may even be less severe
than any seasonal flu symptoms.
The 2009 flu outbreak in
humans that is widely known as "swine flu" (3)apparently
is not solely due to a swine influenza virus. It is due to a new strain of
influenza A virus subtype H1N1 that derives from one strain of human influenza
virus, , one strain of avian influenza virus, and two separate strains of swine
influenza virus. The origins of this new strain are unknown, and the World Organization
for Animal Health (OIE) reports that this strain has not been isolated in
swine. It passes with apparent ease from human to human, an ability attributed
to an as-yet unidentified mutation. The strain in most cases causes only mild
symptoms and the infected person makes a full recovery without requiring
medical attention and without the use of antiviral medicines..
In swine, three influenza
A virus subtypes (H1N1, H3N2, and H1N2) are circulating throughout the world.
The
H1N1 form of swine flu is one of the descendants of the Spanish flu that caused
a devastating pandemic in humans in 1918–1919.(7) 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
The influenza virus constantly changes form, thereby eluding the protective
antibodies that people may have developed in response to previous exposures to
influenza or to influenza vaccines.
Every
two or three years the virus undergoes minor changes. But 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 change that
enables it to easily infect populations around the world, often infecting
hundreds of millions of people whose antibody defenses are unable to resist it.
The influenza virus has also been known to change form over a much shorter period
of time. For instance, during the Spanish flu pandemic, the initial wave of the
disease was relatively mild, while the second wave of the disease a year later
was highly lethal.
In 1957,
an Asian flu pandemic infected some 45 million Americans and killed 70,000. It
caused about 2 million deaths globally. Eleven years later, lasting from 1968
to 1969, the Hong Kong flu 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 virus had "mysteriously
disappeared. In the U.S. during an average year, there are approximately 50
million cases of "normal" flu leading to around 36,000 deaths, mostly
to extremely young, old, or frail persons, with a large percentage of those due
to complications such as pneumonia.
Medical
researchers worldwide, recognizing that the swine flu virus might again mutate
into something as deadly as the Spanish flu, are carefully watching the latest
2009 outbreak of swine flu and making contingency plans for a possible global
pandemic. Several countries took precautionary measures to reduce the chances
for a global pandemic of the disease.
In pigs
influenza infection produces fever, lethargy, sneezing, coughing, difficulty
breathing and decreased appetite.(8) 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.
According to the Centre 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.(9)
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. A diagnosis of confirmed swine flu
requires laboratory testing of a respiratory sample (a simple nose and throat
swab).(10)
The most
common cause of death is respiratory failure, other causes of death are
pneumonia (leading to sepsis)(11) 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
Prevention
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. 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.(12)(13) 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 >2 wk 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. After an outbreak, as immunity in exposed pigs wanes, new
outbreaks of the same strain can occur.
The
transmission from swine to human is believe 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.
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.(14)
Swine flu cannot be spread by pork products, since the virus is not transmitted
through food.(14) 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.
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. Although the current trivalent influenza vaccine is unlikely to provide
protection against the new 2009 H1N1 strain, vaccines against the new strain
are being developed and could be ready as early as June 2009.
Experts
agree that hand-washing can help prevent viral infections, including ordinary
influenza and the swine flu virus. 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 see a doctor to be tested.
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 authorites have action plans
which social distancing actions to request or require depending on the severity
of the outbreak.
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, palliative care,
at home or in the hospitals, focuses on controlling fevers and maintaining
fluid balance. The U.S. Centers for Disease Control and Prevention recommends
the use of Tamiflu (oseltamir)) 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.(15) The virus isolates in the 2009
outbreak have been found resistant to amantadine and rimantadine.(16)
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.(17)
REFERENCES:
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May 2, 2009. Retrieved on 2009-05-03.
Received on 25.09.2009
Accepted on 30.09.2009
© A&V Publication all right reserved
Research J. Pharmacology and
Pharmacodynamics 2009; 1(2): 56-58
