In 1957 and in 1968, reassortment events led to new viruses that resulted in pandemic influenza. The 1957 influenza virus (Asian influenza, an H2N2 virus) acquired three genetic segments from an avian species (a hemagglutinin, a neuraminidase, and a polymerase gene, PB1), and the 1968 influenza virus (Hong Kong influenza, an H3N2 virus) acquired two genetic segments from an avian species (hemagglutinin and PB1). Future pandemic strains could arise through either mechanism.
In both 1957 and 1968, a new influenza virus emerged because of reassortment events involving two influenza viruses. The segmented genome allows each influenza A virus to exchange genetic material with other influenza A viruses. In 1957, dual infection of an individual animal — probably a human, but possibly another species, such as a pig — with an avian H2N2 influenza and a human H1N1 influenza resulted in the emergence of a new influenza virus containing the hemagglutinin, the neuraminidase, and the gene for one of the polymerase proteins (PB1) from the avian virus, along with the remaining five genetic segments from the human H1N1 influenza virus.
The new reassortant virus circulated in humans until 1968, when it was replaced by another reassortant virus, the H3N2 Hong Kong virus — created by the replacement of the hemagglutinin (H2) and polymerase (PB1) genes of the H2N2 virus with two new avian genes, H3 and a new PB1. Today, the descendants of this virus continue to cause the majority of influenza infections in humans (see diagram). Five of the genes of today's H3N2 influenza virus have their origin in the 1918 pandemic.
http://content.nejm.org/cgi/content/full/353/21/2209
"Swine flu showed signs of posing a human pandemic threat more than a decade ago - but the danger was largely ignored, it has been claimed.
In 1976 there was an outbreak of swine flu in humans at a military camp in New Jersey with one death. The virus was not able to spread and soon "fizzled out".
But in 1998 the H1N1 strain combined with both human and bird viruses to produce "triple reassortants" that surfaced in Minnesota, Iowa and Texas.
This was a much more aggressive form of flu that should have set alarm bells ringing, according to New Scientist.
The bird element - two genes for the RNA polymerase enzyme - allowed the viruses to replicate rapidly and become more virulent. By 1999 they had become the dominant flu strain in North American pigs and were actively evolving, the report said.
Many different versions evolved, making the creation of a strain that could infect humans more likely. And by switching surface proteins, the viruses adapted to evade the pig immune system.
Today there are so many kinds of swine flu that outbreaks at pig farms are no longer seasonal, New Scientist said. One in five US pig producers were said to make their own vaccines because the vaccine industry could not keep up with the changes.
One US expert, Richard Webby from St Jude's Children's Research Hospital in Memphis, Tennessee, warned in 2004 that pigs in the US were "an increasingly important reservoir of viruses with human pandemic potential", the report said.
A similar warning was issued last year by Amy Vincent, of the US Department of Agriculture, it was claimed. She said the rapid evolution of swine flu posed the "potential for pandemic influenza emergence in North America".
The Mexican virus strain causing panic today has pig surface proteins the human immune system cannot recognise.
It also harbours bird flu genes similar to those that have made other strains lethal to humans. The flu virus that killed around 50 million people in 1918 is thought to have originated in birds.
Despite the warnings from veterinary scientists, medical researchers focused on human flu viruses failed to see the threat, said New Scientist.
New Scientist said: "All the evidence suggests that swine flu was a disaster waiting to happen. But it got little research attention, perhaps because it caused mild infections in people which didn't spread. Now one swine flu virus has stopped being so well behaved."
In both 1957 and 1968, a new influenza virus emerged because of reassortment events involving two influenza viruses. The segmented genome allows each influenza A virus to exchange genetic material with other influenza A viruses. In 1957, dual infection of an individual animal — probably a human, but possibly another species, such as a pig — with an avian H2N2 influenza and a human H1N1 influenza resulted in the emergence of a new influenza virus containing the hemagglutinin, the neuraminidase, and the gene for one of the polymerase proteins (PB1) from the avian virus, along with the remaining five genetic segments from the human H1N1 influenza virus.
The new reassortant virus circulated in humans until 1968, when it was replaced by another reassortant virus, the H3N2 Hong Kong virus — created by the replacement of the hemagglutinin (H2) and polymerase (PB1) genes of the H2N2 virus with two new avian genes, H3 and a new PB1. Today, the descendants of this virus continue to cause the majority of influenza infections in humans (see diagram). Five of the genes of today's H3N2 influenza virus have their origin in the 1918 pandemic.
http://content.nejm.org/cgi/content/full/353/21/2209
"Swine flu showed signs of posing a human pandemic threat more than a decade ago - but the danger was largely ignored, it has been claimed.
In 1976 there was an outbreak of swine flu in humans at a military camp in New Jersey with one death. The virus was not able to spread and soon "fizzled out".
But in 1998 the H1N1 strain combined with both human and bird viruses to produce "triple reassortants" that surfaced in Minnesota, Iowa and Texas.
This was a much more aggressive form of flu that should have set alarm bells ringing, according to New Scientist.
The bird element - two genes for the RNA polymerase enzyme - allowed the viruses to replicate rapidly and become more virulent. By 1999 they had become the dominant flu strain in North American pigs and were actively evolving, the report said.
Many different versions evolved, making the creation of a strain that could infect humans more likely. And by switching surface proteins, the viruses adapted to evade the pig immune system.
Today there are so many kinds of swine flu that outbreaks at pig farms are no longer seasonal, New Scientist said. One in five US pig producers were said to make their own vaccines because the vaccine industry could not keep up with the changes.
One US expert, Richard Webby from St Jude's Children's Research Hospital in Memphis, Tennessee, warned in 2004 that pigs in the US were "an increasingly important reservoir of viruses with human pandemic potential", the report said.
A similar warning was issued last year by Amy Vincent, of the US Department of Agriculture, it was claimed. She said the rapid evolution of swine flu posed the "potential for pandemic influenza emergence in North America".
The Mexican virus strain causing panic today has pig surface proteins the human immune system cannot recognise.
It also harbours bird flu genes similar to those that have made other strains lethal to humans. The flu virus that killed around 50 million people in 1918 is thought to have originated in birds.
Despite the warnings from veterinary scientists, medical researchers focused on human flu viruses failed to see the threat, said New Scientist.
New Scientist said: "All the evidence suggests that swine flu was a disaster waiting to happen. But it got little research attention, perhaps because it caused mild infections in people which didn't spread. Now one swine flu virus has stopped being so well behaved."
