Tolerance vs. resistance to disease

I just finished reading this paper from a recent issue of Science.. The Title: Disentangling Genetic Variation for Resistance and Tolerance to Infectious Diseases in Animals, which can be found (by subscribers) here. It's a very nice paper, and brings up an interesting issue, one that I have not previously considered. Here it is...

We traditionally think of host-parasite interactions as a relationship characterized by host resistance (immunity, etc.) and pathogen evasion. They pull from the plant literature to report on host tolerance. The idea is that there are some situations where how a host repsponds to infection is much more important that it's ability to evade infection.

Here is the abstract:

Hosts can in principle employ two different strategies to defend themselves against parasites: resistance and tolerance. Animals typically exhibit considerable genetic variation for resistance (the ability to limit parasite burden). However, little is known about whether animals can evolve tolerance (the ability to limit the damage caused by a given parasite burden). Using rodent malaria in laboratory mice as a model system and the statistical framework developed by plant-pathogen biologists, we demonstrated genetic variation for tolerance, as measured by the extent to which anemia and weight loss increased with increasing parasite burden. Moreover, resistance and tolerance were negatively genetically correlated. These results mean that animals, like plants, can evolve two conceptually different types of defense, a finding that has important implications for the understanding of the epidemiology and evolution of infectious diseases.

They use a mouse model of human malaria. Specifically- Mus and Plasmodium chabaudi. It's pretty cool that they do show that there is heritable variation for tolerance.. That some mouse strains are more tolerant to microparasitic infection than others.. Although they suggest that tolerance is likely subject to selection, and show that there is heritable variation in the trait- they do not show that there are actual fitness differences- although that seems probable.

The remaining question pertains to candidate loci- and this is what would really piqu my interest. If we can say that tolerance is under the influence natural selction- can we determine which genes are responsible.. The answer is surely yes, but not by me.

I hope these guys follow up this work and identify particular genes under selection.