Wednesday, April 29, 2009
The Mexican swine flu virus is a swine influenza A/H1N1 virus hybridized (mixed) with human and bird viruses. We have some immunity to human flu and to some strains of swine influenza A/H1N1; We don't have immunity to bird flu, which is why that virus is so virulent - with a kill ratio of almost 50% -- and why so much pandemic planning and preparedness focused on bird flu.
But in 1998, says Richard Webby of St Jude's Children's Research Hospital in Memphis, Tennessee, swine H1N1 hybridised with human and bird viruses, resulting in "triple reassortants" that surfaced in Minnesota, Iowa and Texas. The viruses initially had human surface proteins and swine internal proteins, with the exception of three genes that make RNA polymerase, the crucial enzyme the virus uses to replicate in its host. Two were from bird flu and one from human flu. Researchers believe that the bird polymerase allows the virus to replicate faster than those with the human or swine versions, making it more virulent.
By 1999, these viruses comprised the dominant flu strain in North American pigs and, unlike the swine virus they replaced, they were actively evolving. There are many versions with different pig or human surface proteins, including one, like the Mexican flu spreading now, with H1 and N1 from the original swine virus. All these viruses still contained the same "cassette" of internal genes, including the avian and human polymerase genes, reports Amy Vincent of the US Department of Agriculture (USDA) in Ames, Iowa (Advances in Virus Research, vol 72, p 127). "They are why the swine versions of this virus easily outcompete those that don't have them," says Webby.
But the viruses have been actively switching surface proteins to evade the pigs' immunity. There are now so many kinds of pig flu that it is no longer seasonal. One in five US pig producers actually makes their own vaccines, says Vincent, as the vaccine industry cannot keep up with the changes. This rapid evolution posed the "potential for pandemic influenza emergence in North America", Vincent said last year. Webby, too, warned in 2004 that pigs in the US are "an increasingly important reservoir of viruses with human pandemic potential". One in five US pig workers has been found to have antibodies to swine flu, showing they have been infected, but most people have no immunity to these viruses.
While researchers focused on livestock problems could see the threat developing, it is not one that medical researchers focused on human flu viruses seemed to have been aware of. "It was confusing when we looked up the gene sequences in the database," says Wendy Barclay of Imperial College London, who has been studying swine flu from the recent US cases. "The polymerase gene sequences are bird and human, yet they were reported in viruses from pigs."
So where did the Mexican virus originate? The Veratect Corporation based in Kirkland, Washington, monitors world press and government reports to provide early disease warnings for clients, including the CDC. Their first inkling of the disease was a 2 April report of a surge in respiratory disease in a town called La Gloria, east of Mexico City, which resulted in the deaths of three young children. Only on 16 April - after Easter week, when millions of Mexicans travel to visit relatives - reports surfaced elsewhere in the country.
Local reports in La Gloria blamed pig farms in nearby Perote owned by Granjas Carroll, a subsidiary of US hog giant Smithfield Foods. The farms produce nearly a million pigs a year. Smithfield Foods, in a statement, insists there are "no clinical signs or symptoms" of swine flu in its pigs or workers in Mexico. That is unsurprising, as the company says it "routinely administers influenza virus vaccination to swine herds and conducts monthly tests for the presence of swine influenza." The company would not tell New Scientist any more about recent tests. USDA researchers say that while vaccination keeps pigs from getting sick, it does not block infection or shedding of the virus.
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.