The evolution of virulence.--I have recently become very interested in understanding why parasites sometimes make their hosts so sick. A recent model suggests that parasite virulence may be associated with interactions among propagules (Lively 2001). The model also makes direct connections with genetic load theory and with the mutational theories for the maintenance of sex.

The basic result can be seen in the figure at the left. The thin lines give different relationship between host survivorship and the number of parasite propagules when they become transmissible. Note that adding additional propagules has an increasingly negative effect on host survivorship for the upper-most curves.

The thick line gives the equilibrium values for the number of parasite propagules produced by a single infection. The key result is that host survivorship increases at the parasite equilibrium as the interactions among propagules increases. This result is analogous to results from theoretical population genetics, which show that fitness at mutation-selection balance increases with increasing synergism among deleterious mutations. These results suggest that, all else equal, the most virulent parasites will be the ones for which additional propagules have diminishing effects on the host.

I am also interested in the effects of multiple infection on selection for within-host growth rates by parasites (Lively 2005).  My results differ from the classic result, which shows that multiple simultaneous infections in the same host leads to selection for more aggressive within-host growth by the parasite.  Instead I find that coinfection can lead to less aggressive within-host growth by each of the coinfecting strains.  I argue that the difference stems directly from different assumptions in the models. 

I am also now very interested in the ecology of virulence.  I have found using standard assumptions regarding the effect of density on host birth rates that parasites can be very virulent at carrying capacity, even if they have no effect on the intrinsic birth rate of infected individuals.  In fact, relatively small effects of infection on the sensitivity of infected individuals to competition can lead to situations in which the parasites effectively sterilize the host (Lively 2006).

Lively, C.M.  2001. Propagule interactions and the evolution of virulence. Journal of Evolutionary Biology 14:317-324. (preprint)

Lively, C.M.  2005.  Evolution of virulence: coinfection and propagule production in spore–producing parasites.  BMC Evolutionary Biology 2005, 5:64

Lively, C. M.  2006.  The ecology of virulence.  Ecology Letters 9: 1089-1095.

C. M. Lively, Dept. of Biology, Indiana University
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