Swine flu refers to influenza caused by those strains of influenza virus that usually infect pigs and are called swine influenza virus (SIV). Swine influenza is common in pigs in the M?d-West of the United States and some other states, Mexico, Canada, South America, Europe, Kenya, Mainland China, Taiwan, Japan and other parts of eastern Asia.

Transmission of swine influenza virus from pigs to humans is not common and properly-cooked pork poses no risk of infection. When transmitted, it does not always cause human influenza and often the only sign of infection is the presence of antibodies in the blood which are only detectable by laboratory tests. When transmission results in influenza in a human, it is called zoonotic swine flu. People who work with pigs, especially people with intense exposures, are at risk of catching swine flu. But, only about fifty such transmissions have been recorded since the mid-20th Century, when identification of influenza subtypes became possible. Rarely, these strains of swine flu can pass from human to human. In humans, the symptoms of swine flu 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.

The 2009 flu outbreak in humans, known as “swine flu”, is due to an apparently virulent new strain of influenza A virus subtype H1N1 that contained many genetic elements normally found in swine influenza. The origin of this new strain is unknown, and the World Organization for Animal Health (OIE) reports that this strain has not been isolated in pigs. It can be transmitted from human to human, an ability attributed to an as-yet unidentified mutation. This 2009 H1N1 strain causes the normal symptoms of influenza, such as fever, coughing and headache.

The 2009 flu outbreak is due to a new strain of influenza, an apparent reassortment of at least four strains of influenza A virus subtype H1N1, including one strain endemic in humans, one endemic in birds, and two endemic in swine. Although initial reports identified the new strain as swine influenza (ie, a zoonosis), its origin is unknown. Several countries took precautionary measures to reduce the chances for a global pandemic of the disease.

This new strain had not previously been reported in pigs. On May 2, 2009, H1N1 was reported in pigs at a farm in Alberta, Canada, with a link to the 2009 swine flu outbreak in Mexico. The pigs are suspected to have caught this new strain of virus from a farm worker who recently travelled to Mexico, then showed symptoms of an influenza-like illness. These are probable cases, pending confirmation by laboratory testing.

The time and location of the outbreak is still unknown, but it was first detected in two cases in Southern California in late March. When this novel virus was publicized, officials in Mexico suspected a link to an outbreak of late-season flu cases they were finding. Mexican news media report that the outbreak may have started in February near a Smithfield Foods pig plant amid complaints about its intensive farming practices. Within days, hundreds more suspected cases were learned in Mexico, with more cases also showing up in the U.S. and several other countries. By late April, officials from the U.N.’s World Health Organization (WHO), based in Switzerland, and the Centers for Disease Control and Prevention (CDC) in the U.S., were expressing serious concern about the flu outbreak, worried that it might become a worldwide flu pandemic.

The new strain has spread widely beyond Mexico and the U.S., with confirmed cases in eighteen countries and suspected cases in forty-two. Many countries have advised their inhabitants not to travel to infected areas. Countries including Australia, China, Iceland, India, Indonesia, Malaysia, Philippines, Singapore, South Korea and Thailand are monitoring visitors returning from flu-affected areas to identify people with fever and respiratory symptoms. Many countries have also issued warnings to visitors of flu-affected areas to contact a doctor immediately if they had flu-like symptoms.

Mexico’s schools, universities, and all public events will be closed from April 24, 2009 to May 6, 2009. By May 3, 2009, more than 400 schools in the U.S. closed due to confirmed or probable cases in students or staff, affecting 250,000 students

By April 28, the new strain was confirmed to have spread to Spain, the United Kingdom, New Zealand, and Israel, and the virus was suspected in many other nations. As a result, WHO raised its alert level to “Phase 5″ out of 6 possible, which it defines as a “signal that a pandemic is imminent” By the end of April, 300 schools had closed across the United States and the Mexican government ordered a multi-day shutdown of all non-essential activities in the government and private sector, amounting to a shutdown of most of the country’s economy. At the same time, but, many scientists were reaching a consensus that the epidemic was so far “relatively mild,” and believed that it could be less fatal than previous pandemics.

The new strain is an apparent reassortment of four strains of influenza A virus subtype H1N1. Analysis by the CDC identified the four component strains as one endemic in humans, one endemic in birds, and two endemic in pigs (swine). But, other scientists have stated that analyses of the 2009 swine flu (A/H1N1) viral genome “suggests that all segments are of swine origin”, “we are puzzled about sources of information that affirm that the virus is a reassortment of avian, human and swine viruses,” and “this preliminary analysis suggests at least two swine ancestors to the current H1N1, one of them related to the triple resorting viruses isolated in North America in 1998.” One swine strain was widespread in the United States, the other in Eurasia. Worldwide the common human H1N1 influenza virus affects millions of people every year, according to WHO officials, and “these annual epidemics result in about three to five million cases of severe illness, and about 250,000 to 500,000 deaths” annually. In industrialized countries most of these annual deaths occur in people aged 65 or older. By May 2, some pigs in Canada were diagnosed with H1N1. Although some influenza strains can spread between species, the influenza virus is killed by normal cooking procedures, so there is no risk of infection from consumption of well-cooked pork and pork products.

Recommendations to prevent infection by the virus consist of the standard personal precautions against influenza. This includes frequent washing of hands with soap and water or with alcohol-based hand sanitizers, especially after being out in public. The CDC advises not touching the mouth, nose or eyes, as these are primary modes of transmission. When coughing, they recommend coughing into a tissue and disposing of the tissue, then immediately washing the hands.

Of the available antiviral treatments for influenza, the WHO stated that the viruses obtained from the human cases with swine influenza in the United States were sensitive to oseltamivir (Tamiflu) and zanamivir (Relenza) but resistant to amantadine and rimantadine. Tamiflu and Relenza also have a preventative effect against Influenzavirus A. On April 27, the CDC recommended the use of Tamiflu and Relenza for both treatment and prevention of the new strain. Roche and the U.S. government had already extended the shelf life of federally stockpiled Tamiflu from the original five years to seven years because studies indicated that the medication continues to maintain its effectiveness.