The common cold is probably common because a lot of different viruses cause similar symptoms. We usually treat it symptomatically or just endure it. We rarely expend much time, effort or money identifying which virus caused it. As as a result we undoubtedly haven’t identified all the viruses that can make us miserable in the inimitable way we identify as a “head cold.” When we entered the 21st century, some 8 years ago, there were a lot of stories about what the future might bring and I was interviewed by a well known medical TV reporter (Dr. Timothy Johnson) about what I thought would happen in the medical world. His first question: do you think we’ll ever find a cure for the common cold? I said I thought so. Truthfully, I’m not so sure, and a recent paper illustrates one of the reasons. We just don’t know that much about the viruses that cause head colds.
One of these viruses is something called human metapneumovirus (HMPV), first discovered in 2001 by Dutch scientists. HPMV has a lot in common with another respiratory virus we talk a lot about here, influenza. Both are negative sense RNA viruses (meaning that their genetic material is RNA instead of DNA and translates into protein via an intermediate copy); infections with both are prevalent in the population; both have forms that infect birds; both cause respiratory disease of varying severity, from asymptomatic or very mild to serious lower respiratory tract disease and fatal outcome. HPMV seems much less virulent than flu, on average. At least we think so. So that’s one difference. Another is that the genetic material of flu is packaged into eight segments, which can mix and match when a cell is co-infected with more than one kind of flu virus. The HPMV is unsegmented, although there is still plenty of genetic variation. Whether that variation is mainly driven by point mutations or recombinations we don’t know for sure, but most negative sense RNA viruses don’t engage in a lot of homologous recombination so it is probably random point mutation. And a lot of it. RNA viruses mutate rapidly, have short generation times and produce huge numbers when the replicate, and HPMV is no exception.
We’ve known that birds get infected with an avian version of this virus (Avian Metapneumovirus, AMPV) since 1978, van den Hoogen and his colleagues didn’t discover a human form of the virus until 2001. Nevertheless we now know it’s been around a much longer time. In a recent paper in Journal of General Virology de Graaf and colleagues examined banked serum samples from 1958 and found widespread infection then. Using other tissue specimens of both the avian and human variety, some of the former going back 25 years, the Dutch group working with E. C. Holmes’s group at Penn State, have started to reconstruct the evolutionary history of the metapneumoviruses, which exist in several avian subgroups and at least two human subgroups. It is the AMPV subgroup C that is most closely related to the human metapneumovirus.
There are always a number of assumptions that have to be made in doing these reconstructions, but using the latest sophisticated techniques the Holmes group estimates that the bird and human forms of the virus had a common ancestor about 200 years ago and had diverged into bird and human forms around 120 years ago (give or take a few years). The evidence suggests the virus started in birds and went to humans rather than the other way around. Despite this, humans don’t seem to get infected with the bird form. At least not that we know of. But of course we didn’t think humans got infected with bird influenza viruses, either, and they don’t usually. But now we know they can become infected and the results can be very, very nasty.
HPMV is a remarkably prevalent virus, circulating globally, like influenza. By age 5, most children have been infected. Unfortunately evidence suggests that the immune response is transient and that reinfection is common. So every cold that you get doesn’t have to be a different kind of virus. But it’s pretty sure that some of the colds you get will be from HMPV. Knowing that is nothing to sniff at.