English: Negative stain EM image of the swine influenza A/CA/4/09 (Photo credit: Wikipedia)
Swine flu or influenza is a condition caused by strains of subtypes of Influenza A virus called H1N1, H1N2, H3N1, H3N2, and H3N3. These viruses are common in pigs located in midwestern United States, Canada, South America, Mexico, Mainland China, Japan, Taiwan, Europe, and other Eastern Asian countries.
Transmission of influenza virus from swine to humans is relatively uncommon and does not always result to human influenza but often leads to production of antibodies in the blood. When properly cooked, pig meat does not have the potential of passing the virus. Transmission that leads to human influenza is called zoonotic swine flu.
People who work with pigs, particularly those with direct exposure, are at greater risk of being infected with swine flu. Towards the middle of the 20th century, identification of influenza subtypes became likely paving the way for an accurate diagnosis of transmission to humans. Since then, 50 infections have been confirmed and recorded.
It is rare for these strains of swine flu to be transmitted from one human to another. In humans, the symptoms of swine flu are similar to common influenza as well as influenza-like illness such as chills, sore throat, fever, coughing, muscular pains, severe headache, and general discomfort.
The 2009 swine flu outbreak in humans was caused by a new strain of influenza A virus subtype H1N1 which have genes that closely resemble swine influenza. The root of this new strain is not known. According to the World Health Organization for Animal Health, this new strain has not been isolated in pigs. It is capable of human-to-human transmission and manifests the normal symptoms of influenza.
Swine can be infected with human influenza such as the case of the 1918 flu pandemic and the 2009 flu outbreak. Swine flu was first proposed as a disease associated with humans during the 1918 flu pandemic. During that time, pigs became simultaneously sick with humans.
Influenza virus as a cause of disease in pigs was first identified in 1930. For the next six decades, strains of swine influenza became almost exclusively H1N1. From 1997 to 2002, new strains of three varied subtypes and 5 different genotypes were identified as the cause of influenza among pigs in North America.
From 1997-1998, H3N2 strains developed. They included genes acquired through reassortment of human, avian, and swine viruses and have been a principal cause of wine influenza in North America. Reassortment between H1N1 and H3N2 resulted to the development of H1N2. In Canada, a strain of H4N6 resulted from the reassortment of avian and swine flu but was isolated on a single farm.
The H1N1 variant of swine flu is one of the descendants of the strain that brought about the 1918 flu pandemic. While persisting in pigs, the descendants of the 1918 virus have also been transmitted throughout the 20th century which resulted to the usual seasonal influenza outbreaks.
It is interesting to note that direct infection from pigs to humans is rare, with only 12 confirmed cases in the United States since 2005.
However, since the influenza strains remain in the pigs after they have disappeared in the human population can make these pigs a reservoir where the influenza virus could survive and later on transmitted to humans as soon as their immunity to the strain is no longe effective.
Swine flu has been recorded as zoonosis in humans several times, oftentimes with limited distribution and rarely with massive distribution. Swine outbreaks are common and can lead to significant economic losses in the industry, mainly leading to stunting and expanded market time. Swine influenza, for instance, the British meat industry has experienced £65 million of losses annually.