One of the reasons for epidemics and pandemics, is a virus that jumps from one species to a new one. Among the original population (let’s call it the “natural host”), there’s a certain level of immunity . Individuals have been infected and survived, and walk away with some resistance to the virus. That limits the virus’s ability to spread among the natural host. If the virus can jump into a new host species, then none of the population will have ever seen that virus before, and there is the potential to burn through the population in a sudden, explosive pandemic.
Some obvious examples should jump to your mind: HIV jumping from non-human primates into humans; West Nile virus moving into North America. The best-documented example may be canine parvovirus entering the world-wide dog population in 1978, which I talked about here.
A more common scenario is for a virus to dip its toe into the new population, but not to establish a permanent, ongoing infection in that population. Ebola periodically jumps into the human population from bats (probably) and causes serious problems for a while, but hasn’t moved into the larger human population yet. SARS virus, ditto. Sin Nombre virus, Avian Influenza (so far!), canine distemper moving into seals and lions — these cause sudden but limited epidemics that burn out and don’t set up a long-term relationship in the new host.
How can we tell which route will be followed? Did SARS in humans fizzle out because it wasn’t well adapted (even though it was visibly evolving to be human-adapted at a furious rate) or did the containment policies that were slapped on travel and so forth catch it before it has time? I have no idea how to generalize, and I don’t think anyone does.
Which means it’s probably a good idea to watch closely for any examples of species jumping, and to monitor it closely.
Right now, there’s apparently an example of a coronavirus (same virus type as causes SARS, by the way) that has moved from pigs into dogs. 1 In fact, these viruses seem to be hydrids — recombinants that are part dog coronavirus, part swine coronavirus. This is very reminiscent of the major changes in influenzaviruses that happens a couple of times per century, and that is the concern with avian flu.
So far, these new viruses don’t seem to be very virulent, at least on their own; they didn’t cause disease experimentally, though they were isolated from natural cases of sick dogs. Something like this has apparently happened at least once before, about 10 years ago, with a previous canine/pig coronavirus recombinant,2 and that one didn’t take off in the canine population.
I don’t know if it’s possible to draw general conclusions about new viruses and pandemics, but I am sure it’s worth trying.
- N. Decaro, V. Mari, M. Campolo, A. Lorusso, M. Camero, G. Elia, V. Martella, P. Cordioli, L. Enjuanes, C. Buonavoglia (2008). Recombinant Canine Coronaviruses Related to Transmissible Gastroenteritis Virus of Swine Are Circulating in Dogs Journal of Virology, 83 (3), 1532-1537 DOI: 10.1128/JVI.01937-08[↩]
- Wesley, R. D. 1999. The S gene of canine coronavirus, strain UCD-1, is more closely related to the S gene of transmissible gastroenteritis virus than to that of feline infectious peritonitis virus. Virus Res. 61:145-152[↩]