Can host ecology and kin selection predict parasite virulence?
The July article of the month from Parasitology is Can host ecology and kin selection predict parasite virulence? , Parasitology, Volume 141, issue 8, by Alyssa M. Gleichsner and Dennis J. Minchella. Alyssa Gleichsner, from Purdue University, discusses the research below.
“Parasite virulence, or the damage a parasite does to its host, is of wide interest to the parasitology community because of its impact on disease severity and transmission. Studies investigating the population genetics and transmission of parasites in natural populations exist for a wide variety of systems (both microparasites and macroparasites), but studies of virulence in natural parasite populations are rare and have not considered ecological variables that could influence virulence. Laboratory parasites and hosts often lack the genetic diversity found in field populations and that may limit their ability to infect or resist infection. Because of this, results from studies using laboratory host-parasite systems may not reflect virulence patterns found in nature.
The genetic relatedness of co-infected parasites within a host has been shown to influence virulence. For instance, in parasites that require a living host for transmission, infections of related parasites tend to have lower virulence than infections of unrelated parasites. According to kin selection theory, this is because unrelated parasites must directly compete for finite host resources while related parasites can benefit from the inclusive fitness of their counterparts. While more evidence accumulates in favor of the influence of kin selection on virulence, little remains known about how ecological factors such as host resistance and movement alter the genetics and relatedness of parasite populations, and consequently their virulence.
Our review article in Parasitology provides a broad overview of virulence studies and associated trends and draws connections to existing knowledge about natural parasite populations. It discusses how parasite and host parameters can interact over time to select for or against virulence. For instance, we predict that high host movement will create parasite population admixture, decreasing relatedness in affected parasite populations and selecting for higher virulence. We also predict that high host resistance should select for lower virulence and examine how different host parameters, such as host movement and resistance, could interact and create cyclical patterns of high and low virulence over time. We consider virulence selection in the context of parasites with complex life cycles and include parasites with high movement hosts at one or more life cycle stages. In addition, habitat characteristics, such as the degree of fragmentation, can alter host behavior and parasite virulence. We examine how the size of habitat fragments could be used to predict parasite virulence and discuss the influence of fragmentation on the fate of host and parasite populations.
With the increasing cost effectiveness of molecular techniques it is increasingly possible to study the genetics of parasite populations and investigate the impact of ecological parameters on the virulence in natural parasite populations. The paper outlines possible experimental designs using both trematodes and Plasmodium and encourages the inclusion of natural or semi-natural experiments into future virulence research. Incorporating these variables into experiments will increase our understanding of virulence evolution and disease dynamics.”