The adaptive evolution of virulence: a review of theoretical predictions and empirical tests

Abstract

SUMMARYWhy is it that some parasites cause high levels of host damage (i.e. virulence) whereas others are relatively benign? There are now numerous reviews of virulence evolution in the literature but it is nevertheless still difficult to find a comprehensive treatment of the theory and data on the subject that is easily accessible to non-specialists. Here we attempt to do so by distilling the vast theoretical literature on the topic into a set of relatively few robust predictions. We then provide a comprehensive assessment of the available empirical literature that tests these predictions. Our results show that there have been some notable successes in integrating theory and data but also that theory and empiricism in this field do not ‘speak’ to each other very well. We offer a few suggestions for how the connection between the two might be improved.

Next step in the ongoing arms race between myxoma virus and wild rabbits in Australia is a novel disease phenotype

“…The ongoing evolution of MYXV in Australia has already contributed hugely to the understanding of pathogen adaptation and virulence evolution (1,(11)(12)(13)(14)(15)(16)(17)(18)(19). Critically, our data show that enhancements in host resistance have led to the evolution of strains of MYXV that are far more immunosuppressive than any of their ancestors.…”

“…Fenner and colleagues then went on to show that this attenuation was favored by natural selection because, by killing hosts so rapidly, highly virulent viruses had shorter infectious periods than more attenuated strains, which did not kill so rapidly (6)(7)(8)(9)(10). This work became the bedrock of the mathematical theory of virulence evolution developed in the 1980s (11)(12)(13)(14)(15)(16)(17)(18)(19), and it remains so because the combination of temporal field sampling and controlled experimentation demonstrating the relevant trade-offs is unique for a disease of vertebrates.…”

The industrialization of farming may be driving virulence evolution

“…With an increase in life span, the parasite's ESS will transition to become less virulent, a strategy better suited for a longer life span. Therefore, increasing the cohort duration, T , is similar to decreasing background host mortality, which has traditionally been shown to select for lower virulence (Cressler, McLeod, Rozins, Van Den Hoogen, & Day, 2016). …”

“…In traditional models of virulence evolution, this is a similar effect to a decrease in background host mortality (Cressler et al., 2016), and hence why we have referred to μ( v ) = ln ( k β( v ))/ T as an effective death rate. As background host mortality increases, the lifespan of the pathogen decreases, and thus, the cost of being very virulence decreases.…”

Short Rotations in Forest Plantations Accelerate Virulence Evolution in Root-Rot Pathogenic Fungi

“…Among biological traits assumed to be under strong evolutionary pressure, the virulence of pathogens, generally defined as the quantitative ability of a pathogen to induce host mortality [11,12], is the focus of considerable attention from evolutionary ecologists [12] and plant pathologists [13]. Despite still imprecise experimental characterizations, mainly based on proxies, and a complex theory including the transmission-virulence trade-off hypothesis [14], the evolution at this trait is thought to be tightly related to the spread of diseases [11,15,16].…”