History

Swine influenza was first proposed to be a disease related to human influenza during the 1918 flu pandemic, when pigs became sick at the same time as humans.[16] The first identification of an influenza virus as a cause of disease in pigs occurred about ten years later, in 1930.[17] For the following 60 years, swine influenza strains were almost exclusively H1N1. Then, between 1997 and 2002, new strains of three different subtypes and five different genotypes emerged as causes of influenza among pigs in North America. In 1997-1998, H3N2 strains emerged. These strains, which include genes derived by reassortment from human, swine and avian viruses, have become a major cause of swine influenza in North America. Reassortment between H1N1 and H3N2 produced H1N2. In 1999 in Canada, a strain of H4N6 crossed the species barrier from birds to pigs, but was contained on a single farm.[17]
The H1N1 form of swine flu is one of the descendants of the strain that caused the 1918 flu pandemic.[18][19] As well as persisting in pigs, the descendants of the 1918 virus have also circulated in humans through the 20th century, contributing to the normal seasonal epidemics of influenza.[19] However, direct transmission from pigs to humans is rare, with only 12 cases in the U.S. since 2005.[20] Nevertheless, the retention of influenza strains in pigs after these strains have disappeared from the human population might make pigs a reservoir where influenza viruses could persist, later emerging to reinfect humans once human immunity to these strains has waned.[21]
Swine flu has been reported numerous times as a zoonosis in humans, usually with limited distribution, rarely with a widespread distribution. Outbreaks in swine are common and cause significant economic losses in industry, primarily by causing stunting and extended time to market. For example, this disease costs the British meat industry about £65 million every year.[22]

1918 pandemic in humans
The 1918 flu pandemic in humans was associated with H1N1 and influenza appearing in pigs;[19] this may reflect a zoonosis either from swine to humans, or from humans to swine. Although it is not certain in which direction the virus was transferred, some evidence suggests that, in this case, pigs caught the disease from humans.[16] For instance, swine influenza was only noted as a new disease of pigs in 1918, after the first large outbreaks of influenza amongst people.[16] Although a recent phylogenetic analysis of more recent strains of influenza in humans, birds, and swine suggests that the 1918 outbreak in humans followed a reassortment event within a mammal,[23] the exact origin of the 1918 strain remains elusive.[24] It is estimated that anywhere from 50 to 100 million people were killed worldwide.[19][25]

1976 U.S. outbreak
Main article: 1976 swine flu outbreak
On February 5, 1976, in the United States an army recruit at Fort Dix said he felt tired and weak. He died the next day and four of his fellow soldiers were later hospitalized. Two weeks after his death, health officials announced that the cause of death was a new strain of swine flu. The strain, a variant of H1N1, is known as A/New Jersey/1976 (H1N1). It was detected only from January 19 to February 9 and did not spread beyond Fort Dix.[26]

President Ford receives swine flu vaccination
This new strain appeared to be closely related to the strain involved in the 1918 flu pandemic. Moreover, the ensuing increased surveillance uncovered another strain in circulation in the U.S.: A/Victoria/75 (H3N2) spread simultaneously, also caused illness, and persisted until March.[26] Alarmed public-health officials decided action must be taken to head off another major pandemic, and urged President Gerald Ford that every person in the U.S. be vaccinated for the disease.[27]
The vaccination program was plagued by delays and public relations problems.[28] On October 1, 1976, the immunization program began. That same day, three senior citizens died soon after receiving their swine flu shots and there was a media outcry linking the deaths to the immunizations, despite the lack of positive proof. According to science writer Patrick Di Justo, however, by the time the truth was known—that the deaths were not proven to be related to the vaccine—it was too late. "The government had long feared mass panic about swine flu—now they feared mass panic about the swine flu vaccinations." This became a strong setback to the program.[29]
There were reports of Guillain-Barré syndrome, a paralyzing neuromuscular disorder, affecting some people who had received swine flu immunizations. This syndrome is a rare side-effect of modern influenza vaccines, with an incidence of about one case per million vaccinations.[30] As a result, Di Justo writes that "the public refused to trust a government-operated health program that killed old people and crippled young people." In total, 48,161,019 Americans, or just over 22% of the population, had been immunized by the time the National Influenza Immunization Program (NIIP) was effectively halted on December 16, 1976.[31] [32]
Overall, there were 1098 cases of Guillain-Barré Syndrome (GBS) recorded nationwide by CDC surveillance, 532 of which were linked to the NIIP vaccination, resulting in death from severe pulmonary complications for 25 people, which, according to Dr. P. Haber, were probably caused by an immunopathological reaction to the 1976 vaccine. Other influenza vaccines have not been linked to GBS, though caution is advised for certain individuals, particularly those with a history of GBS. [33] [34][35] Still, as observed by a participant in the immunization program, the vaccine killed more Americans than the disease did.[36]

1988 zoonosis
In September 1988, a swine flu virus killed one woman and infected others. 32-year old Barbara Ann Wieners was eight months pregnant when she and her husband, Ed, became ill after visiting the hog barn at a county fair in Walworth County, Wisconsin. Barbara died eight days later, after developing pneumonia.[37] The only pathogen identified was an H1N1 strain of swine influenza virus.[38] Doctors were able to induce labor and deliver a healthy daughter before she died. Her husband recovered from his symptoms.
Influenza-like illness (ILI) was reportedly widespread among the pigs exhibited at the fair. 76% of 25 swine exhibitors aged 9 to 19 tested positive for antibody to SIV, but no serious illnesses were detected among this group. Additional studies suggested between one and three health care personnel who had contact with the patient developed mild influenza-like illnesses with antibody evidence of swine flu infection. However, there was no community outbreak.[39][40]

1998 US outbreak in swine
In 1998, swine flu was found in pigs in four U.S. states. Within a year, it had spread through pig populations across the United States. Scientists found that this virus had originated in pigs as a recombinant form of flu strains from birds and humans. This outbreak confirmed that pigs can serve as a crucible where novel influenza viruses emerge as a result of the reassortment of genes from different strains.[41][42][43]