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Swine influenza commonly known as Swine Flu is an infection caused by the swine influenza viruses. These viruses cause pig influenza and their transmission from pigs to humans is uncommon. The immune system is perfectly capable of producing antibodies to fight the virus and thus does not lead to an infection. Zoonotic infection is common amongst people who are regularly in contact with the pigs.
It was in the mid-20th century that the subtypes of the swine flu virus were identified. Upon its identification, studies showed that the subtypes caused zoonotic infections in humans. These strains were not known to pass from human to human. Such incidences were of rare occurrence. It was in 2009 that H1N1 virus responsible for H1N1 infection or commonly known as swine flu was discovered. H1N1 is also known as swine flu as the symptoms are very close to those that are observed in pig influenza.

Signs and symptoms of swine flu
Below is an illustration of the symptoms seen in swine flu (Source: thefitindian).
[Image: Symptoms-of-Swine-Flu.jpg]
In humans, zoonotic infections are common. The symptoms of the H1N1 swine flu are much like any other influenza infections. The incubation period is 1-4 days and the infection can show symptoms up to 2 weeks after the infection unless it is severe.  The symptoms include fever; a cough, sore throat, watery eyes, body aches, shortness of breath, headache, weight loss, chills, sneezing, runny nose, coughing, dizziness, abdominal pain, lack of appetite and fatigue.
The outbreak in 2009 led to the identification of diarrhea and vomiting as the other symptoms of swine flu. The transfer of the H1N1virus is not zoonotic, but it is the aerosols from the infected people that brings about the transmission of this disease. The symptoms are not limited to swine flu, hence, differential diagnosis combined with laboratory tests are essential for the identification of this infection. Though swine flu on its own is not fatal, death can occur due to respiratory failure. Pneumonia, high fever, dehydration, electrolyte imbalance and kidney failure also can cause fatality in both children and adults.
It is more of a risk in pregnant women as they undergo hormonal changes, physical changes, and the immune system also changes in accordance to the developing fetus and an infection with H1N1 at this stage can cause complications. Hence, they are advised to take vaccine shots.  

The structure of the virus is illustrated in the image below (Source: medicaljournalonline)
The virus causing swine flu in humans has been identified to be a strain influenza A virus which is designated as H1N1. This influenza virion is roughly spherical. It is enveloped in an outer lipid membrane. This lipid membrane is taken from the host cells which the virus replicates. The lipid layer has “spikes” of proteins embedded in it. These spikes are glycoproteins - haemagglutinin and neuraminidase, which is why they are designated as H1N1, H1N2, etc. based on the type of H and N antigens expressed. Haemagglutinin is responsible for the clumping of blood cells and this binds the virus to the infected cells. Neuraminidase is a glycoside hydrolase enzyme which helps in the movement of virus particles across the infected cells and also aids in the budding of the virus from the host cell. The worrisome part is the ability of the virus to mutate during transmission from human to human.
The influenza viruses have segmented genome, i.e. the viral RNA code is not a complete strand but is fragmented into eight parts. All the eight parts are present in the virus. Avian or human influenza virus can infect a pig’s host cell at the same time as the swine flu virus. This can cause a mixing of the genes and thus can bring about the replicating human RNA viral genomes to be embedded in the swine flu virus. Thus, transfer of the human virus through pigs is possible. This creates room for more subtypes of the virus to come up due to the various combinations of the human and swine RNA fragments being present in the virus. This overall is known as antigenic shift. The characteristics of the virus would be that of a swine flu virus, but it can infect a human due to the human RNA fragments in the genome. Such small changes do not drastically affect us unless the changes accumulate and produce enough minor changes to alter the entire makeup of the virus.
Below is the illustration of antigenic shift and antigenic drift (Source: medicinenet).
[Image: swine_flu_h1_n1.jpg]
The reason pigs are considered to be the intermediate host is because they can host swine, avian and human flu types in their cells. This is because pigs feed on the feces shed by birds. If the birds in question carry the avian flu virus, this virus can enter the pig’s host cells and the fragments of avian flu RNA can be picked up by the swine flu virus. This way, the avian flu virus enters the mammalian population.

Diagnosis of swine flu 
The first thing to be considered while diagnosing for swine flu is the contact of the person in question with someone tested positive for swine flu infection. A nasopharyngeal swab is done to identify whether it is an influenza A or B virus. If the test is positive for influenza B virus, it is likely to be not swine flu. If tested positive for influenza A virus, it is taken as a confirmation of conventional flu or swine flu. Pathological tests on the tissue sample of the specimen lead to the identification of the virus type.
The definitive identification involves identifying the surface antigens present in the virus. The diagnosis is done using rt-PCR and certain influenza diagnostic tests. Centre for Disease Control (CDC) also advises antiviral susceptibility testing by pyrosequencing.  A rapid influenza diagnostic test is performed for the identification of an infection by influenza A virus. This is a test based on the detection of influenza viral nucleoprotein antigen. Serological tests are also performed where the sera of the patients infected for a week and more than 2 weeks are taken for research purposes. Not all labs are authorized for serological testing. The FDA approved assay is the CDC rt-PCR Swine Flu Assay.

Treatment for swine flu

Vaccines are available in the form of nasal sprays, injections and intradermal shots which prevent the onset of this infection. However, it is to be noted that the nasal spray has the live attenuated virus. People with weak immune system are advised against taking nasal spray based vaccine as it can lead to complications.
Apart from vaccines, antiviral agents like zanamivir and oseltamivir are also available in the market. They are known to reduce the symptoms of the flu. Nevertheless, the resistance of the virus to these drugs has already been seen. Thus, people with more than 48 hours of infection are not given these drugs. In non-responsive patients, IV delivery of the drug is possible provided the healthcare provider has given a clearance to do so.
Prevention of swine flu
Below is an illustration of the preventive measures which are to be followed (Source: funonthenet)
[Image: swine-flu-precautions.jpg]
Vaccine shots are the best way to prevent the spread of the disease. washing hands after coming in contact with an infected person, covering mouth while sneezing or coughing and limiting contact with people and household articles if infected are the best ways to ensure that the infection does not spread further.