In several of my influenza posts, I’ve discussed ways that the viruses can evolve. These are termed “antigenic drift,” where the virus accumulates small mutations in the RNA genome; and antigenic shift, where large sections of the genome are swapped, generally in their entirety. While it was long thought that the latter was the most likely type of mutation to cause a pandemic, we now know that even the right kind of antigenic drift may be enough to allow a novel influenza virus to enter the human population, which seems to have happened in 1918 (and is, of course, the current concern regarding H5N1). A new paper in the journal Emerging Infectious Diseases, however, highlights the need for broader influenza surveillance and pandemic response plans (such as those here) by showing how a novel influenza virus can appear right under our noses.
I mentioned recombination and the pandemic threat. Pigs in particular have been cited as a contributor to this threat, as the cells of their lungs can bind both human and avian-type influenza viruses; they’ve thus been dubbed a potential “mixing vessel” for the recombination of the two virus lineages. The worst possible outcome from this would be, of course, a human/avian hybrid virus that could easily spread among humans and cause a high mortality rate.
Currently, 3 serotypes of influenza viruses are circulating in swine: H1N1, H3N2, and H1N2. H1N1 has been in the population since the original 1918 outbreak; this is known as “classical” swine flu, and was the dominant serotype in swine until just a decade ago. In 1998, H3N2 emerged and quickly spread throughout the pig population. It was a “triple reassortant,” containing genes from human, avian, and swine influenza viruses. Shortly thereafter, another reassortant emerged: serotype H1N2 (which was the result of a recombination between the H3N2 virus and the old H1N1 strain). In the intervening years, wholly avian viruses (serotypes H4N6, H3N3, and H1N1) were also found in swine populations, but there is no evidence they’ve spread. The new paper linked above describes another triple reassortant: serotype H3N1, “that may have arisen from reassortment of an H3N2 turkey isolate, a human H1N1 isolate, and currently circulating swine influenza viruses.” To make matters even a bit messier, many of the turkey viruses seem to have come from swine originally, suggesting frequent transfer of these viruses between the two species. This is worrisome, as the authors note:
However, our results underline the scenario in which swine can be a mixing vessel for human, swine, and avian influenza viruses to create new reassortants that may be dangerous to human health. Turkeys are more susceptible to influenza viruses from waterfowl than are other domestic poultry, and a high degree of genetic reassortment most likely occurs in domestic turkeys. This finding may indicate that influenza A viruses could sequentially acquire new genes during transmission from waterfowl via turkey to swine and humans.
Keep in mind that there are roughly 250 million turkeys in the U.S. alone, along with approximately 60 million hogs. That’s a lot of potential there for recombination, especially since in many areas, folks who raise turkeys and pigs may not be all that far apart. And that’s not even factoring in any effect from wild turkeys (such as those pictured to the left), which are common here in Iowa as well.
What’s the bottom line? There’s no doubt that these novel influenza viruses will continue to pop up. Some will spread, such as the reassortants have in swine (though I should emphasize that it’s not yet known whether the new H3N1 reassortant will spread widely and become endemic in swine, if it will have marginal spread, or if it will simply burn out and not be seen again). The epidemiology of human influenza viruses over the past 20-odd years has been similar: we’ve mostly seen H1N1 and H3N2, but H2N2, H9N2, H7N7, and of course, H5N1 have all popped up sporadically and infected humans. Will one of these become the next pandemic? Will it be another recombinant, perhaps from swine, or will it be a wholly avian virus that mutates slightly to become efficiently transmitted among humans? Obviously we don’t know yet, but good surveillance coupled with better vaccine capability would go a long way toward protecting us from whatever nature throws at us.
Lekcharoensuk P, Lager KM, Vemulapalli R, Woodruff M, Vincent AL, Richt JA. Novel swine influenza virus subtype H3N1, United States. Emerg Infect Dis. 2006. Available from http://www.cdc.gov/ncidod/EID/vol12no05/05-1060.htm
Image from http://www.cynthiasrandallcattle.com/image/obj176geo138pg6p5.jpg