Heterogeneous host populations drive evolution of more virulent pathogens, modeling study shows

The evolution of pathogens has received attention in a wide range of scientific fields, such as epidemiology, demography, and evolutionary ecology. Understanding pathogen evolution is particularly urgent for rapidly evolving ...

Classical evolutionary theory states that virulence evolves to maximize a pathogen's basic reproduction ratio, i.e., the average number of secondary infections caused by one infected host. This approach provides insights into how pathogen virulence evolves under tradeoffs with other epidemiological parameters such as the rates of infection and recovery. Over time, the classical theory has been extended to a variety of ecological and epidemiological contexts.

Despite these advancements, most models continue to assume homogeneous host populations, thereby neglecting the impacts of all environmental heterogeneity. In reality, however, ecosystems often represent interconnected local populations that experience different local conditions, as described by ecological metapopulation theory. Moreover, the movement patterns of host individuals connecting these local populations also tend to be heterogeneous, and such imbalances in movement among local populations creates a "source-sink" structure, with some populations (sources) creating a net outflow and other populations (sinks) receiving a net inflow.

Contrast between idealized homogeneous and realistic heterogeneous metapopulations. Credit: SOKENDAI, AIST, OIST Read more

Evolved virulence is consistently higher in heterogeneous metapopulations compared to homogeneous metapopulations. Each point represents the evolved virulence in a metapopulation with randomly generated local environments that are heterogenous in terms of movement rates, birth rates, carrying capacities, or immunity-loss rates. The horizontal lines indicate the resultant average increases in the evolved virulence. Credit: SOKENDAI, AIST, OIST Read more

Heterogeneities in local environments create heterogeneity in the densities of local host populations. The left population is "resource-rich" for pathogens, as it offers a higher density of susceptible hosts, while the right population is "resource-scarce", as it offers a lower density of susceptible hosts. For higher susceptible-host densities, more virulent pathogens are favored, while for lower susceptible-densities, milder pathogens are favored. Crucially, the net effect of these selection pressures across the metapopulation is not balanced, as the resource-rich population makes a more significant evolutionary contribution than the resource-scarce population, causing evolution to drive up pathogen virulence and infectiousness. Credit: SOKENDAI, AIST, OIST Read more