Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, Michigan, 48109, USA.
Department of Biological Sciences, Florida State University, Tallahassee, Florida, 32306, USA.
Ecology. 2021 Aug;102(8):e03422. doi: 10.1002/ecy.3422. Epub 2021 Jul 12.
The likelihood an individual becomes infected depends on the community in which it is embedded. For environmentally transmitted parasites, host community composition can alter host density, the density of parasites that hosts encounter in the environment, and the dose to which hosts are subsequently exposed. While some multi-host theory incorporates some of these factors (e.g., competition among hosts), it does not currently consider the nonlinear relationships between parasite exposure dose and per-propagule infectivity (dose-infectivity relationships), between exposure dose and infected host mortality (dose-mortality relationships), and between exposure dose and parasite propagule excretion (dose-excretion relationships). This makes it difficult to predict the impact of host species on one another's likelihood of infection. To understand the implications of these nonlinear dose relationships for multi-host communities, we first performed a meta-analysis on published dose-infectivity experiments to quantify the proportion of accelerating, linear, or decelerating dose-infectivity relationships; we found that most experiments demonstrated decelerating dose-infectivity relationships. We then explored how dose-infectivity, dose-mortality, and dose-excretion relationships might alter the impact of heterospecific host density on infectious propagule density, infection prevalence, and density of a focal host using two-host, one-parasite models. We found that dose relationships either decreased the magnitude of the impact of heterospecific host density on propagule density and infection prevalence via negative feedback loops (decelerating dose-infectivity relationships, positive dose-mortality relationships, and negative dose-excretion relationships), or increased the magnitude of the impact of heterospecific host density on infection prevalence via positive feedback loops (accelerating dose-infectivity relationships and positive dose-excretion relationships). Further, positive dose-mortality relationships resulted in hosts that traditionally decrease disease (e.g., low competence, strong competitors) increasing infection prevalence, and vice versa. Finally, we found that dose relationships can create positive feedback loops that facilitate friendly competition (i.e., increased heterospecific density has a positive effect on focal host density because the reduction in disease outweighs the negative effects of interspecific competition). This suggests that without taking dose relationships into account, we may incorrectly predict the effect of heterospecific host interactions, and thus host community composition, on environmentally transmitted parasites.
个体感染的可能性取决于其所处的社区。对于环境传播的寄生虫，宿主群落组成可以改变宿主密度、宿主在环境中遇到的寄生虫密度以及宿主随后暴露的剂量。虽然一些多宿主理论包含了其中的一些因素（例如，宿主之间的竞争），但它目前不考虑寄生虫暴露剂量与每一个传播体感染力之间的非线性关系（剂量感染力关系）、暴露剂量与受感染宿主死亡率之间的非线性关系（剂量死亡率关系），以及暴露剂量与寄生虫传播体排泄之间的非线性关系（剂量排泄关系）。这使得预测宿主物种对彼此感染可能性的影响变得困难。为了理解这些非线性剂量关系对多宿主群落的影响，我们首先对已发表的剂量感染力实验进行了荟萃分析，以量化加速、线性或减速剂量感染力关系的比例；我们发现大多数实验都显示出减速剂量感染力关系。然后，我们使用两宿主一寄生虫模型探索了剂量感染力、剂量死亡率和剂量排泄关系如何改变异宿主密度对传染性传播体密度、感染流行率和焦点宿主密度的影响。我们发现，剂量关系要么通过负反馈环降低异宿主密度对传播体密度和感染流行率的影响幅度（减速剂量感染力关系、正剂量死亡率关系和负剂量排泄关系），要么通过正反馈环增加异宿主密度对感染流行率的影响幅度（加速剂量感染力关系和正剂量排泄关系）。此外，正剂量死亡率关系导致传统上降低疾病的宿主（例如，低能力、强竞争者）增加感染流行率，反之亦然。最后，我们发现剂量关系可以产生正反馈环，从而促进友好竞争（即，增加异宿主密度对焦点宿主密度有积极影响，因为疾病减少的影响超过了种间竞争的负面影响）。这表明，如果不考虑剂量关系，我们可能会错误地预测异宿主相互作用以及因此宿主群落组成对环境传播寄生虫的影响。
