swine flu virus

موضوع: swine flu virus الإثنين 14 ديسمبر 2009, 2:08 pm

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What is swine flu (novel H1N1 influenza A swine flu)?

Swine flu
(swine influenza) is a respiratory disease caused by viruses (influenza viruses) that infect the respiratory tract of pigs and
result in nasal secretions, a barking-like cough, decreased appetite, and listless behavior. Swine flu
produces most of the same symptoms in pigs as human flu produces in people.
Swine flu can last about one to two weeks in pigs that survive. Swine influenza
virus was first isolated from pigs in 1930 in the U.S. and has been recognized
by pork producers and veterinarians to cause infections in pigs worldwide. In a
number of instances, people have developed the swine flu infection when they
are closely associated with pigs (for example, farmers, pork processors), and
likewise, pig populations have occasionally been infected with the human flu
infection. In most instances, the cross-species infections (swine virus to man;
human flu virus to pigs) have remained in local areas and have not caused
national or worldwide infections in either pigs or humans. Unfortunately, this
cross-species situation with influenza viruses has had the potential to change.
Investigators think the 2009 swine flu strain, first seen in Mexico, should be
termed novel H1N1 flu since it is mainly found infecting people and exhibits
two main surface antigens, H1 (hemagglutinin type 1) and N1 (neuraminidase
type1). Recent investigations show the eight RNA strands from novel H1N1 flu
have one strand derived from human flu strains, two from avian (bird) strains,
and five from swine strains.

*Why is swine flu (H1N1)
now infecting humans?

Many researchers now consider that two main series of
events can lead to swine flu (and also avian or bird
flu) becoming a major cause for influenza
illness in humans.

First,
the influenza viruses (types A, B, C) are enveloped RNA viruses with a
segmented genome; this means the viral RNA genetic code is not a single strand
of RNA but exists as eight different RNA segments in the influenza viruses. A
human (or bird) influenza virus can infect a pig respiratory cell at the same
time as a swine influenza virus; some of the replicating RNA strands from the
human virus can get mistakenly enclosed inside the enveloped swine influenza
virus. For example, one cell could contain eight swine flu and eight human flu
RNA segments. The total number of RNA types in one cell would be 16; four swine
and four human flu RNA segments could be incorporated into one particle, making
a viable eight RNA segmented flu virus from the 16 available segment types.
Various combinations of RNA segments can result in a new subtype of virus
(known as antigenic
shift) that may have the ability to preferentially infect humans but
still show characteristics unique to the swine influenza virus (see Figure 1).
It is even possible to include RNA strands from birds, swine, and human
influenza viruses into one virus if a cell becomes infected with all three
types of influenza (for example, two bird flu, three swine flu, and three human
flu RNA segments to produce a viable eight-segment new type of flu viral
genome). Formation of a new viral type is considered to be antigenic shift;
small changes in an individual RNA segment in flu viruses are termed antigenic
drift and result in minor changes in the
virus. However, these can accumulate over time to produce enough minor changes
that cumulatively change the virus' antigenic makeup over time (usually years).

Second,
pigs can play a unique role as an intermediary host to new flu types because
pig respiratory cells can be infected directly with bird, human, and other
mammalian flu viruses. Consequently, pig respiratory cells are able to be
infected with many types of flu and can function as a "mixing pot"
for flu RNA segments (see Figure 1). Bird flu viruses, which usually infect the
gastrointestinal cells of many bird species, are shed in bird feces. Pigs can
pick these viruses up from the environment and seem to be the major way that
bird flu virus RNA segments enter the mammalian flu virus population

*What are the
symptoms of swine flu (H1N1)?

Symptoms of swine flu are similar to most influenza infections: fever
(100F or greater), cough,
nasal secretions, fatigue,
and headache,
with fatigue being reported in most infected individuals. Some patients also
get nausea,
vomiting,
and diarrhea.
In Mexico, many of the patients are young adults, which made some investigators
speculate that a strong immune response may cause some collateral tissue
damage. Some patients develop severe respiratory symptoms and need respiratory
support (such as a ventilator to breathe for the patient). Patients can get pneumonia
(bacterial secondary infection) if the viral infection persists, and some can
develop seizures.
Death often occurs from secondary bacterial infection of the lungs; appropriate
antibiotics need to be used in these patients. The usual mortality (death) rate
for typical influenza A is about 0.1%, while the 1918 "Spanish flu" epidemic had an estimated
mortality rate ranging from 2%-20%. Swine flu in Mexico (as of April 2009) has
had about 160 deaths and about 2,500 confirmed cases, which would correspond to
a mortality rate of about 6%, but these initial data have been revised and the
mortality rate currently in Mexico is estimated to be much lower. By June 2009,
the virus had reached 74 different countries on every continent except
Antarctica, and by September 2009, the virus had been reported in most
countries in the world. Fortunately, the mortality rate as of October 2009 has
been low but higher than for the conventional flu (average conventional flu
mortality rate is about 36,000 per year; projected novel H1N1 flu mortality
rate is 90,000 per year in the U.S. as determined by the president's advisory
committee).

How is swine flu
(H1N1) diagnosed?*

Swine flu is presumptively diagnosed clinically by the
patient's history of association with people known to have the disease and
their symptoms listed above. Usually, a quick test (for example, nasopharyngeal
swab sample) is done to see if the patient is infected with influenza A or B
virus. Most of the tests can distinguish between A and B types. The test can be
negative (no flu infection) or positive for type A and B. If the test is
positive for type B, the flu is not likely to be swine flu (H1N1). If it is
positive for type A, the person could have a conventional flu strain or swine
flu (H1N1). However, the accuracy of these tests has been challenged, and the
U.S. Centers for Disease Control and Prevention (CDC) has not completed their
comparative studies of these tests. However, a new test developed by the CDC
and a commercial company reportedly can detect H1N1 reliably in about one hour;
as of October 2009, the test is only available to the military.

Swine
flu (H1N1) is definitively diagnosed by identifying the particular antigens
associated with the virus type. In general, this test is done in a specialized
laboratory and is not done by many doctors' offices or hospital laboratories.
However, doctors' offices are able to send specimens to specialized
laboratories if necessary. Because of the large number of novel H1N1 swine flu
cases (as of October 2009, the vast majority of flu cases [about 99%] are due
to novel H1N1 flu viruses), the CDC recommends only hospitalized patients' flu
virus strains be sent to reference labs to be identified

What is the
history of swine flu (H1N1) in humans?*

In
1976, there was an outbreak of swine flu at Fort Dix. This virus is not the
same as the 2009 outbreak, but it was similar insofar as it was an influenza A
virus that had similarities to the swine flu virus. There was one death at Fort
Dix. The government decided to produce a vaccine against this virus, but the
vaccine was associated with neurological complications (Guillain-Barré
syndrome) and was discontinued. Some individuals speculate that formalin,
used to inactivate the virus, may have played a role in the development of this
complication in 1976. There is no evidence that anyone who obtained this
vaccine would be protected against the 2009 swine flu. One of the reasons it
takes a few months to develop a new vaccine is to test the vaccine for safety
to avoid the complications seen in the 1976 vaccine. New vaccines against any
flu virus type are usually made by growing virus particles in eggs. A serious
side effect (allergic
reaction such as swelling of the airway) to vaccines can occur in
people who are allergic to
eggs; these people should not get flu vaccines.
Individuals with active infections or diseases of the nervous system are also
not recommended to get flu vaccines.

*Can novel H1N1 swine flu be prevented
with a vaccine?

The
best way to prevent novel H1N1 swine flu would be the same best way to prevent
other influenza infections, and that is vaccination.
The CDC has multiple recommendations for vaccination based on who should obtain
the first doses when the vaccine becomes available (to protect the most
susceptible populations) and according to age groups. The CDC based the
recommendations on data obtained from vaccine trials and infection reports
gathered over the last few months. The current (October 2009) vaccine
recommendations from the CDC say the following groups should get the vaccine as
soon as it is available:

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* pregnant women,

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* people who live
with or provide care for children younger than 6 months of age,

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*
health-care and emergency medical services personnel,

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* people
between 6 months and 24 years of age, and

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people from the ages of 25 through 64 who are at higher risk
because of chronic health disorders such as asthma, diabetes, or a weakened immune system.

Currently, the CDC is stating that
people ages 10 and above are likely to need only one vaccine shot to provide
protection against novel H1N1 swine flu and further suggest that these shots
will be effective in about 76% of people who obtain the vaccine. New vaccine
trial data showed that healthy adults produce protective antibodies in about
98% of people in 21 days. Unfortunately, the vaccine shot in children ages 6
months to 9 years of age is not as effective as it is in older children and
adults. Consequently, the CDC currently recommends that for ages 6 months up to
and including 9 years of age, the children obtain two shots of the novel H1N1
vaccine, the second shot 21 days after the first shot.

Pregnant women are strongly
suggested to get vaccinated as stated above. Although some vaccine preparations
(multidose vials) contain low levels of thimerosal
preservative (a mercury-containing preservative), the CDC still considers the
vaccine safe for the fetus and mother. However, some vaccine preparations that
are in single-dose vials will not have thimerosal preservative, so those
pregnant individuals who are concerned about thimerosal can get this vaccine
preparation when it is available.

Another type of vaccine (currently
named Influenza A [H1N1] 2009 Monovalent Vaccine Live, Intranasal) has been
made available during the first week in October 2009. It is a live attenuated
novel H1N1 flu vaccine
that contains no thimerosal, is produced by MedImmune, LLC, and is sprayed into
the nostrils. This vaccine is only for healthy people 2-49 years of age, and
some data suggest that it is less effective in generating an immune response in
adults than the vaccine injection. The dosing schedule is as follows:

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* Children 2-9 years
of age should receive two doses (0.1 ml in each nostril; total equals 0.2 ml per dose) -- the second dose should
be given the same way about one month after the first dose

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* Children, adolescents and adults, 10-49
years of age should receive one dose -- (0.1 ml in each nostril; total equals
0.2 ml per dose)

The CDC occasionally
makes changes and updates its information on vaccines and other recommendations
about the current flu pandemic. The CDC states, "for the most accurate
health information, visit http://www.cdc.gov or call 1-800-CDC-INFO,
24/7." Caregivers should check the vaccine package inserts for more
detailed information on the vaccines when they become available. This article
has an updated timeline for novel H1N1 swine flu attached (see below) and
provides the reader with current details about the pandemic. The following is a
list of the CDC-approved H1N1 vaccines and the companies that name and
manufacture them as of 10/29/09:

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*
Influenza A (H1N1) 2009 Monovalent Vaccine by Sanofi Pasteur

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*
Influenza A (H1N1) 2009 Monovalent Vaccine by Novartis

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*
Influenza A (H1N1) 2009 Monovalent Vaccine Live, Intranasal by MedImmune, LLC

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*
Influenza A (H1N1) 2009 Monovalent Vaccine by CSL Limited

The CDC says that a good
way to prevent any flu disease is to avoid exposure to the virus; this is done
by frequent hand washing, not touching your hands to your face (especially the
nose and mouth), and avoiding any close proximity to or touching any person who
may have flu symptoms. Since the virus can remain viable and infectious for
about 48 hours on many surfaces, good hygiene and cleaning with soap and water
or alcohol-based hand disinfectants are also recommended. Some physicians say face
masks may help prevent getting airborne flu viruses (for example, from a cough
or sneeze), but others think the better use for masks would be on those people
who have symptoms and sneeze or cough. The use of Tamiflu or Relenza may help
prevent the flu if taken before symptoms develop or reduce symptoms if taken
within about 48 hours after symptoms develop. Some investigators say that
administration of these drugs is still useful after 48 hours, especially in
high-risk patient populations .However, taking these drugs is not routinely
recommended for prevention for the healthy population because investigators
suggest that as occurs with most drugs, flu strains will develop resistance to
these medications. Recently, the CDC made further suggestions about the use of
these antiviral medications. Dr. Schuchat, a CDC official, indicated that three
modifications were being suggested (Sept. 8, 2009) to the interim guidelines
for use of Tamiflu and Relenza:

1. Patients with
high-risk factors should discuss flu symptoms and when to use antiviral
medications; doctors should provide a
prescription for the antiviral drug for the patient to use if the patient is
exposed to flu or develops flu-like
symptoms without having to go in to see the doctor.

2. "Watchful waiting" was
added as a response to taking antiviral medications, with the emphasis on the
fact that those people who develop fever and have a preexisting health
condition should then begin the antiviral medication.

3. The antiviral medications are the
first-line medicines for treatment of novel H1N1 swine flu, and most current
cases of flu are novel H1N1 and are, to date, susceptible to Tamiflu and
Relenza.

Your doctor should be
consulted before these drugs are prescribed.

In general, preventive
measures to prevent the spread of flu are often undertaken by those people who
have symptoms. Symptomatic people should stay at home, avoid crowds, and take
off from work or school until the disease is no longer transmittable (about two
to three weeks) or until medical help and advice is sought. Sneezing, coughing,
and nasal secretions need to be kept away from other people; simply using
tissues and disposing of them will help others. Quarantining patients is
usually not warranted, but such measures depend on the severity of the disease.
The CDC recommends that people who appear to have an influenza-like illness
upon arrival at work or school or become ill during the day be promptly
separated from other people and be advised to go home until at least 24 hours
after they are free of fever (100 F [37.8 C] or greater), or signs of a fever,
without the use of fever-reducing medications. The novel H1N1 swine flu disease
takes about seven to 10 days before fevers stop, but new research data (Sept.
14, 2009) suggests waiting until the cough is gone since many people are still
infectious about one week after fever is gone. The CDC has not yet extended
their recommendations to stay home for that extra week.

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