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Monday, November 07, 2005 

Back 'n' forth about evolution of virulence and avian flu

There's been some interesting discussion over on SciAm Observations about the evolution of infectious diseases, and notably, influenza. It all started with editor John Rennie's post, discussing an article on H5N1 written by Wendy Orent, here:

Part I

Wendy Orent and evolutionary biologist Paul Ewald both replied in Part II and Part III, respectively:

Part II

Part III

Both discussed Rennie's mention of a critique of Orent's article on the public health blog, Effect Measure. The pseudomynous Revere from that blog then replied in Part IV:

Part IV

I've said before that I have a lot of interest in Ewald's work (and we touched on some of Ewald's claims about influenza virulence in the discussion in this thread). I also take issue with Ewald's claim that:

Being an expert on the molecular biology or epidemiology of influenza viruses does not translate into expert assessments of the future evolution of influenza viruses. One need only peruse the writings of such influenza experts to recognize that they generally fail to incorporate the most essential component of the evolutionary process into their arguments--they talk as though mutation and reassortment were the only processes relevant to the evolution of influenza viruses. To discuss evolutionary processes with any degree of expertise one must focus on natural selection. Mutations and reassortments generate the variation on which natural selection acts. But natural selection molds viral evolution.

I can't imagine that there are really influenza experts out there who don't realize the importance of natural selection. I agree that perhaps discussion of that factor of the equation gets overlooked and a disproportionate amount of time is given to discussion of what mutations might occur, but that's hardly the same as "failing to incorporate" selection into the models and discussions.

And this brings us to Ewald's main beef--that not all scientists agree with him that virulence can be predicted. His theory states that high virulence should only occur in an immobilized host--such as was the condition on the western front during WWI, which he claims enabled the Spanish flu to become so virulent. I've read many of Ewald's works on this (for those of you unfamiliar with his theory, the most comprehensive discussion is in 1994's Evolution of Infectious Diseases; other writings are listed here, and a freely available Emerging Infectious Diseases article on the topic is here). I think he has some excellent ideas, but I agree with Revere (though I'd say it in a much nicer way :) ) that Ewald's claims are "too sweeping." They're thought-provoking and he's done wonders advancing study into the evolution of infectious disease (and specifically, the evolution of virulence), but he seems to allow for no exceptions to his rule. This, IMO, is a mistake. We all know that biology is messy, and that it's tough (impossible?) to predict anything in this field with 100% certainty. Sure, we can say there are trends one way or another, and one outcome may be significantly more likely than another, but I certainly wouldn't want to bet the farm on the idea that we'll never have another monster influenza pandemic without something like WWI conditions to select for a highly virulent virus.

(Edited to add: Revere has some additional remarks on the exchange and the topic here).


About me

  • I'm Tara C. Smith
  • From Iowa, United States
  • I'm a mom and a scientist, your basic stressed-out, wanna-have-it-all-and-do-it-all Gen Xer. Recently transplanted from Ohio to Iowa, I've spent most of my life in the midwest (with 4 years of college spent out east in "soda" territory). My main interest, and the subject of my research, is infectious disease: how does the microbe cause illness? What makes one strain nasty, and another "avirulent?" Are the latter really not causing any disease, or could some of those be possible for the development of chronic disease years down the road? Additionally, I've spent a lot of time discussing the value of teaching evolution, and educating others about "intelligent design" and other forms of creationism. My interest in history of science and medicine is also useful as a way to tie all of the above interests together. [Disclaimer: the views here are solely my own, and do not represent my employer, my spouse, that guy who's always sitting by the fountain when I come into work, or anyone else with whom I may be remotely affiliated.]
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