Swine Flu
(Swine Influenza A [H1N1] Virus)
Medical Author: Charles Davis, MD, PhD
Medical Editor: Melissa Conrad Stöppler, MD
What to Do if You Think You Have H1N1 Swine Flu Virus
Medical Author: Melissa Conrad Stöppler, MD
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
 If you've got fever, cough, or one of the other symptoms of the flu, you may
be wondering if you have contracted the H1N1 swine flu virus. The reality is
that it isn't possible to know unless specialized testing is ordered, and for
uncomplicated cases of the flu in non-hospitalized patients, routine testing for
the H1N1 virus is not being carried out.
Experts recommend that people who suspect that they have H1N1 infection stay
home and avoid contact with other people. The only time you should leave home is
to access medical care if needed. That said, it is important to remember that
the vast majority of flu cases (even H1N1 cases) produce only a mild illness for
which doctor's visits and/or antiviral drugs are not necessary. So, the presence
of cough and fever in an individual who is not at high risk for complications
(see below) and who does not have warning signs of a medical emergency should
not be a reason to visit an ER. The emergency department should be used for the
treatment of people who are very sick or who have life-threatening emergencies
(listed below). If you're in doubt, a call to your health-care practitioner can
help you decide whether or not you need to access medical care.
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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.
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Next: What are the symptoms of swine flu
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Flu (Influenza) »
What is influenza?
Influenza, commonly called "the flu," is an illness caused by RNA viruses that infect the respiratory tract of many animals, birds, and humans. In most people, the infection results in the person getting fever, cough, headache, and malaise (tired, no energy); some people also may develop a sore throat, nausea, vomiting, and diarrhea. The majority of individuals has symptoms for about one to two weeks and then recovers with no problems. However, compared with most other viral respiratory infections, such as the common cold, influenza (flu) infection can cause a more severe illness with a mortality rate (death rate) of about 0.1% of people who are infected with the virus.
The above is the usual situation for the yearly occurring "conventional" or "seasonal" flu strains. However, there are situations in which some flu outbreaks are severe. These severe outbreaks occur when the human population is exposed to a flu strain against which the populati...
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