Monday, December 15, 2014

Epidemiology meets evolutionary ecology

I read an article titled “Epidemiology meets evolutionary ecology” a few weeks ago, but have not had a chance to fully digest it. I think it’s potentially useful to think about viruses this way so I’m going to write down a few points that I find interesting.  Probably the most important point that this paper makes is the definition of virulence, or “the extent of parasite-induced reduction in host fitness.” (Although it still seems odd to me that virulence is defined based on the host rather than the parasite itself....)

1. Optimal Virulence
-Viruses face a tradeoff between host survival and fecundity. Hosts face a tradeoff between the cost of resistance and the risk of infection.

-Estimates of R0 generally do not take into account vertical transmission or multiple infections.

2. Transmission Patterns
-Vertical transmission tends to reduce virulence compared to horizontal transmission because vertical transmission depends on host survival and reproduction. But vertical transmission and horizontal transmission are usually related so it’s difficult to consider these transmission patterns individually. 

3. Spatial structuring of disease transmission and dispersal mechanism
-Recent models that include spatial structuring tend to predict lower virulence than those that don’t.

4. Host heterogeneity
-A host parasite that is optimally adapted to get around one host’s immune system and not another, host heterogeneity selects for lower virulence.
-The relationship between parasite virulence and host resistance is more complex than was originally assumed.

5. Competition strains and within-host dynamics
-Viral strains in the same host are competing to a limited amount of resources, which leads to selection for greater virulence.
-Kin selection between related strains within one host is predicted to reduce virulence.

6. Phylogenetic analysis
-Phylogenetic analysis is useful in providing information about emerging strains

7. Public Health Policy
-Evolutionary ecology should be taken into account when implementing public health interventions.

8. Sex and Virulence
-It is thought that sex in parasites increases virulence and sex in hosts reduces virulence.

Phew! That just made my brain hurt a little bit more. And I just discovered that there’s a great table in this article, which distills down the major points better than I just did. (There’s a screen shot below, but you should check out the original!) Also, the author makes the point that “the misconception that parasites will ultimately evolve towards avirulence has only been dispelled gradually from the medical literature.” (Proof of these “misconceptions” prevalence can be found in the abstract of the paper that Emi sent out on HIV.)

By Olivia

References:
Galvani, A. “Epidemiology meets evolutionary ecology.” Trends in Ecology and Evolution Mar. 2003. Vol. 18(3): 132-139.

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