Evolution of virulence in heterogeneous host communities under multiple trade-offs

Many pathogens and parasites are transmitted through hosts that differ in species, sex, genotype, or immune status. In addition, virulence (here defined as disease-induced mortality) and transmission can vary during the infectious period within hosts of different state. Most models of virulence evolution assume that transmission and virulence are constant over the infectious period and that the host population is homogenous. Here, we examine a multi-species SIR model where transmission occurs within and between species, and transmission and virulence varied during the infectious period. This allows us to understand virulence evolution in a broader range of situations that characterize many emerging diseases. Because emerging pathogens are by definition new to their host populations, they should be expected to rapidly adapt after emergence. We illustrate these evolutionary effects using the framework of adaptive dynamics to examine how virulence evolves after emergence in response to the relative strength of selection on pathogen fitness and mutational variance for virulence. We illustrate the role of evolution by simulating adaptive walks to an evolutionarily stable virulence. We found that the magnitude of between-species transmission and the relative timing of transmission and mortality across species were of primary importance for determining the evolutionarily stable virulence.

多种病原体与寄生虫可通过物种、性别、基因型或免疫状态各异的宿主进行传播。此外，毒力（本文将其定义为疾病诱导的死亡率）与传播能力在不同状态宿主的感染期内可发生动态变化。绝大多数毒力进化模型均假设，在感染期内传播能力与毒力保持恒定，且宿主种群为均质种群。本研究针对多物种SIR模型（multi-species SIR model）展开分析，该模型中传播既可发生于物种内部，也可发生于物种之间，且传播能力与毒力在感染期内存在动态变化。这使得我们能够在更广泛的场景下解析毒力进化过程，而此类场景正是诸多新发传染病的典型特征。根据定义，新发病原体对其宿主种群而言属于全新引入的病原，因此在出现后理应快速发生适应性进化。本研究借助适应性动力学（adaptive dynamics）框架阐释此类进化效应，用以分析新发病原出现后，毒力如何响应病原体适合度选择的相对强度以及毒力相关突变方差而发生进化。我们通过模拟通往进化稳定毒力的适应性行走过程，直观展示了进化的作用。本研究发现，物种间传播的强度以及跨物种传播与死亡的相对时序，是决定进化稳定毒力的核心因素。