Swine influenza, also called pig influenza, swine flu, hog flu and pig flu, is an infection caused by any one of several types of swine influenza viruses. Swine influenza virus (SIV) or swine-origin influenza virus (S-OIV) is any strain of the influenza family of viruses that is endemic in pigs.[2]As of 2009, the known SIV strains include influenza C and the subtypes of influenza A known as H1N1, H1N2, H2N1, H3N1, H3N2, and H2N3.
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The Swine flu was initially seen in humans in Mexico in 2009, where the strand of the particular virus was a marriage of 3 types of strands.[3] Six of the genes are very similar to the H1N2 influenza virus that was found in pigs around 2000.[3]
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 flu, often resulting only in the production of antibodies in the blood. If transmission does cause human flu, it is called zoonotic swine flu. People with regular exposure to pigs are at increased risk of swine flu infection.
Around 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.
In August 2010, the World Health Organization declared the swine flu pandemic officially over.[citation needed]
Cases of swine flu have been reported in India, with over 31,156 positive test cases and 1,841 deaths up to March 2015.[4]
Signs and symptoms
Humans
Direct transmission of a swine flu virus from pigs to humans is occasionally possible (zoonotic swine flu).[citation needed] 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.[8] Of these six people, one was pregnant, one had leukemia, one had Hodgkin's lymphoma and two were known to be previously healthy.[8] 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.[8]
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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, watery eyes, body aches, shortness of breath, headache, weight loss, chills, sneezing, runny nose, coughing, dizziness, abdominal pain, lack of appetite and fatigue. The 2009 outbreak has shown an increased percentage of patients reporting diarrhea and vomiting as well. The 2009 H1N1 virus is not zoonotic swine flu, as it is not transmitted from pigs to humans, but from person to person through airborne droplets.
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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 and past medical history. For example, during the 2009 swine flu outbreak in the United States, the 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 seven days preceding their illness onset."[10] A diagnosis of confirmed swine flu requires laboratory testing of a respiratory sample (a simple nose and throat swab).[10]
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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), electrolyte imbalance and kidney failure.[12] Fatalities are more likely in young children and the elderly.
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Transmission
To humans
People who work with poultry and swine, especially those 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.[17] Vaccination of these workers against influenza and surveillance for new influenza strains among this population may therefore be an important public health measure.[18] 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.[19]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.[17] 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.[20]
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Structure
The influenza virion is roughly spherical. It is an enveloped virus; the outer layer is a lipid membrane which is taken from the host cell in which the virus multiplies. Inserted into the lipid membrane are "spikes", which are proteins—actually glycoproteins, because they consist of protein linked to sugars—known as HA (hemagglutinin) and NA (neuraminidase). These are the proteins that determine the subtype of influenza virus (A/H1N1, for example). The HA and NA are important in the immune response against the virus; antibodies (proteins made to combat infection) against these spikes may protect against infection. The NA protein is the target of the antiviral drugs Relenza and Tamiflu. Also embedded in the lipid membrane is the M2 protein, which is the target of the antiviral adamantanesamantadine and rimantadine.
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Classification
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.[27] 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 re assortment there have been transfers of genes among strains crossing swine, avian, and human species boundaries.
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Influenza C
Influenza viruses infect both humans and pigs, but do not infect birds.[28] Transmission between pigs and humans have occurred in the past.[29] For example, influenza C caused small outbreaks of a mild form of influenza amongst children in Japan[30] and California.[30] 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.[31]
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Influenza A
Swine influenza is caused by influenza A subtypes H1N1,[32] H1N2,[32] H2N3,[33] H3N1,[34] and H3N2.[32] In pigs, four influenza A virus subtypes (H1N1, H1N2,H3N2 and H7N9) are the most common strains worldwide.[5] 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.[35] In August 2012, the Center for Disease Control and Prevention confirmed 145 human cases (113 in Indiana, 30 in Ohio, one in Hawaii and one in Illinois) of H3N2v since July 2012.[36] The death of a 61-year-old Madison County, Ohio woman is the first in the nation associated with a new swine flu strain. She contracted the illness after having contact with hogs at the Ross County Fair.[37]
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Diagnosis
The CDC recommends real-time PCR as the method of choice for diagnosing H1N1.[38] The oral or nasal fluid collection and RNA virus preserving filter paper card is commercially available.[39] This method allows a specific diagnosis of novel influenza (H1N1) as opposed to seasonal influenza. Near-patient point-of-care tests are in development.[40]
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