Frogs (by Haeckel)
“Batrachia”, by Ernst Haeckel
(Kunstformen der Natur, 1904)

There’s a constant viral assault on us humans, as there is on just about all other species. We as a species have to contend not only with the vast pool of human pathogens, those viruses that constantly circulate among humanity; but also with the continual probes on our defenses from other viruses, viruses that normally infect other species.  All of us are exposed to these on a regular basis: Dog and cat viruses, mouse viruses, crow and pigeon viruses, bat viruses, not to mention the ocean of insect and fungus and amoeba and plant viruses.

Almost all of these assaults don’t even scratch our defenses.  The viruses can’t even enter our bodies, and if they do then they can’t enter our cells, and if they do they can’t replicate in our cells, and if they do then they can’t  …

Most viruses, in other words, can’t effectively jump species.  Even when they do, they’re usually not well adapted to the new species, and they can’t establish a productive chain of infections. Even if they cause a disease, they burn themselves out, infecting fewer and fewer individuals each round of infection, until they disappear.

But every so often, in a tiny minority of cases, the virus does get a foothold.  This is one of the ways that “emerging infections” get started.  It covers things like HIV, SARS, parvovirus of dogs, Ebola, and of course the new H1N1 swine-origin influenza virus (SOIV), among many others.

Why did these guys take off, when so many other viruses failed? Why did SOIV infect people last year, while decades of exposure to pigs and swine H1N1 influenza viruses didn’t lead to earlier pandemics?  Basically, we don’t know, and we’d really, really like to know, so we have a chance of predicting the next SOIV or HIV before it’s a pandemic.

OK, so that explains why I’ve written a fair number of posts here on species-jumping in viruses (here, here, here, here, and here), and partly explains why I want to mention a new paper from Bertram Jacobs‘ lab1.  (The rest of the reason is, as always, that I just think it’s  cool.)  I’m not sure why Jacobs has done this particular project, because he’s more of an interferon guy, but he’s looked at the origins of ranaviruses and finds evidence for lots of species shifts in their history.

Dekay - Salamanders & turtle
“The Smooth Terrapin (Emys terrapin)”, by James Dekay
(Zoology of New York; or, The New York fauna, 1843)

Ranaviruses are probably best known as frog viruses, but they infect a bunch of cold-blooded animals — fish, frogs, salamanders, turtles, and so on — and several of them are causes of emerging infectious disease (as I discussed last time I talked about ranaviruses, here).  Jacobs’ group looked at about a dozen of them whose genomes are completely sequenced2, and tried to put together their evolutionary history, which turns out to involve all kinds of cross-species jumps:

…we hypothesize that the most recent common ancestor of the ALRVs was an ancestral fish virus …  Both of these hypotheses suggest that for the majority of evolutionary time vertebrate iridoviruses were confined to fish, and much more recently, there appear to have been at least three species jumps, from fish to frogs, from fish to salamanders, and from frogs to reptiles, and perhaps as many as four species jumps, including a jump from tetrapod amphibians back to fish. It is tempting to speculate that activities associated with human harvesting of aquatic organisms during the past 40,000 years led to the more common recent jumping of ranaviruses among aquatic organisms.1

(My emphasis) They don’t offer any specific reasons why the ranaviruses should be able to leap from species to species like the chamois of the Alps, but they do make the general point that these viruses tend to be rather promiscuous to start with.  Not only are closely-related viruses able to infect different hosts, but even the same viruses often are able to infect a wide range of species; the fish virus they sequenced in this paper, epizootic hematopoietic necrosis virus, can infect a half-dozen different species of fish.  They raise an interesting comparison:

In addition, the ability of this group of viruses to infect such a wide variety of host species suggests that more host shifts are likely. Therefore, it is important that we understand more of the evolutionary traits of this unique group of viruses, as there is no other closely related group of viruses that infect such a broad group of hosts, with the possible exception of the orthomyxoviruses.1

Orthomyxoviruses, of course, include influenza viruses, which notoriously infect humans, pigs, ducks, chickens, wild waterfowl, horses, and dogs; and you’ll recall all the reports during the epidemic phase of SOIV of the virus infecting all kinds of other pets and domestic animals.  Influenza viruses are apparently evolving at an even faster pace than the ranaviruses, and experimenting with even more species; but there may be lessons for us (as influenza hosts) in the ranaviruses.

  1. Jancovich, J., Bremont, M., Touchman, J., & Jacobs, B. (2009). Evidence for Multiple Recent Host Species Shifts among the Ranaviruses (Family Iridoviridae) Journal of Virology, 84 (6), 2636-2647 DOI: 10.1128/JVI.01991-09[][][]
  2. Including epizootic hematopoietic necrosis virus, whose genome they sequenced themselves[]