Carl Zimmer has a good article in today’s New York Times describing the swine flu virus (“H1N1”) as well as other pathogenic viruses, where they come from, and how they evolve. It turns out that the swine flu virus actually derived from humans — from the strain that caused the terrible influenza epidemic of 1918 (which killed my paternal grandmother when my father was only six months old). This virus infected swine, and then combined back with genes from humans and birds to create a more virulent strain:
. . . From time to time, a new kind of flu emerges that causes far more suffering than the typical swarm of seasonal flu viruses. In 1918, for example, the so-called Spanish flu caused an estimated 50 million deaths. In later years, some of the descendants of that strain picked up genes from bird flu viruses.
Sometimes reassortments led to new pandemics. It is possible that reassortment enables flu viruses to escape the immune system so well that they can make people sicker and spread faster to new hosts.
Reassortment also played a big role in the emergence of the current swine flu. Its genes come from several ancestors, which mainly infected pigs.
Scientists first isolated flu viruses from pigs in 1930, and their genetic sequence suggests that they descend from the Spanish flu of 1918. Once pigs picked up the flu from humans, that so-called classic strain was the only one found in pigs for decades. But in the 1970s a swine flu strain emerged in Europe that had some genes from a bird flu strain. A different pig-bird mix arose in the United States.
In the late 1990s, American scientists discovered a triple reassortant that mixed genes from classic swine flu with genes from bird viruses and human viruses. All three viruses — the triple reassortant, and the American and European pig-bird blends — contributed genes to the latest strain.
It is possible that the special biology of pigs helped foster all this mixing. Bird flu and human flu viruses can slip into pig cells, each using different receptors to gain access. “We call the pig a mixing vessel because it can replicate both avian and mammalian influenza virus at the same time,” said Juergen Richt of Kansas State University. “The mixing of these genes can happen much easier in the pig than in any other species.”
H1N1 (swine flu) virus. From today’s NYT article
Fortunately, experts don’t think this virus has the stuff to cause a serious pandemic. But they’re still worried about mutation and evolution that could increase its virulence. As Bill Maher said in yesterday’s clip, creationists who are worried about swine flu are worried about evolution.
The devastation of the 1918 flu makes me wonder what role influenza may have had in evolution. Could it have led to the extinction of the Neanderthals? At least there, with the availability of their genome, maybe we’ll sometime have an idea whether they might have had both receptors, like pigs. Or, presumably wild boars have both receptors and could have done the mixing. The same might apply to any of our other extinct cousins. A casual online search suggests variable susceptibility of other primates (chimpanzees apparently less so). A recent Science paper (312: 394-97, 2006) doesn’t directly address any of this, but heads a bit in that direction.