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Commentary
Media Myths On Why
Swine Flu Is Called Swine Flu
Recombinomics
Commentary 17:43
September 14, 2009
The above comments represent a subset of media myths on swine flu. Swine flu is called swine flu because it is a flu virus that originated in swine. Virtually every statement above is incorrect. Pandemic H1N1 has been found in swine (in Canada, Argentina, Australia, Indonesia, and Singapore), people can get sick from food cross contaminated by H1N1 in swine (although the vast majority of human cases come from H1N1 in other humans), it is hog specific, although it has recently jumped to humans and turkeys and probably many more species, and the genetic make-up is not unique, it has previously been found in swine.
The first influenza virus that was isolated was from a pig in Iowa in 1930 and was H1N1. Three years later the first human influenza virus was isolated and it was also H1N1. However, the swine and human versions of H1N1 were easily distinguished from each other, but both had a relationship with the 1918 H1N1 pandemic strain, which was about a 50/50 mixture of human and swine H1N1. That mixture was generated by recombination between the swine and human flu genes because the swine virus was able to jump species to human and exchange genetic information, which is why the jump of swine H1N1 to humans in 2009 has created such concern.
The pandemic H1N1 is somewhat different than the swine H1N1 with 8 swine flu genes, which is called "classical swine" because the 1930 virus had 8 swine gene segments. However, in the early 1990's new versions of H1N1 and H3N2 were found in swine. These new isolates had acquired a human PB1 gene when a pig was infected with human and swine virus and the swine virus acquired the entire PB1 gene through another genetic exchange mechanism, reassortment. This new constellation (7 swine and 1 human) then acquired two avian genes (PB2 and PA) leading to a virus called a triple reassortant because the virus had 1 human, 2 avian, and 5 swine flu genes. However, although the constellation was new, the host was the same, it was swine and there have been hundreds of triple reassortants isolated in swine long before the 2009 swine triple reassortant jumped species and began to transmit in humans.
Initially, the pandemic H1N1 was called novel because in addition to the constellation of genes seen previously in swine, the pandemic strain also had a North American swine HA and an Eurasian swine NA. However, this combination had also been seen previously in swine in Thailand and Hong Kong, so the novel triple reassortant was not new and had also been isolated previously in swine.
Thus, the pandemic H1N1 was clearly a swine virus which is why it is called swine flu and all pandemic H1N1 is scientifically designated as "swl", which stands for swine-like.
Thus, the virus is clearly swine, but it is somewhat different than earlier swine H1N1, including the swine H1N1 in 1976 that led to the swine flu vaccine. That virus had also jumped species, but transmission was limited to soldiers at Fort Dix in NJ, and burned itself out long before the vaccine was ready.
In the three years prior to the emergence of pandemic H1N1, swine triple reassortants had cause 13 reported instances of human infections, but these infections were closely linked to swine, and did not spread beyond close contacts of the infected humans.
The pandemic strain was different because it efficiently transmitted from human to human, which had not happened since 1918. Like the 1918 strain, the virus attacked and killed previously health young adults, ad initial infections were relatively mild. However the current case fatality rate is about two orders of magnitude lower than the 1918 strain. However, the 1918 strain was extensively recombined, so each gene had a 50/50 mix of human and swine components, which could happen in 2009 because the swine virus is in humans.
However, the swine virus can still replicate in swine, and virus closely related to the swine strain have been reported in three province in Canada, two provinces in Australia, as well as additional outbreaks in Argentina, Indonesia, and Singapore. Moreover, the H1N1 has also jumped to turkeys in Chile, raising concerns that the virus can efficiently transmit in multiple species and can rapidly evolve via recombination with influenza from other species.
Thus, swine H1N1 is called swine H1N1 because it is a swine flu virus that has recently jumped species and now is transmitting in swine, human, and avian species. Moreover, most of these infections are asymptomatic or mild, allowing for significant silent spread.
The last time swine H1N1 jumped to humans and spread efficiently was 1918.
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