Data from: Host age modulates parasite infectivity, virulence and reproduction

1. Host age is one of the most striking differences among hosts within most populations, but there is very little data on how age-dependent effects impact ecological and evolutionary dynamics of both the host and the parasite. 2. Here, we examined the influence of host age (juveniles, young and old adults) at parasite exposure on host susceptibility, fecundity and survival as well as parasite transmission, using two clones of the water flea Daphnia magna and two clones of its bacterial parasite Pasteuria ramosa. 3. Younger D. magna were more susceptible to infection than older ones, regardless of host or parasite clone. Also, younger-infected D. magna became castrated faster than older hosts, but host and parasite clone effects contributed to this trait as well. Furthermore, the early-infected D. magna produced considerably more parasite transmission stages than late-infected ones, while host age at exposure did not affect virulence as it is defined in models (host mortality). 4. When virulence is defined more broadly as the negative effects of infection on host fitness, by integrating the parasitic effects on host fecundity and mortality, then host age at exposure seems to slide along a negative relationship between host and parasite fitness. Thus, the virulence–transmission trade-off differs strongly among age classes, which in turn affects predictions of optimal virulence. 5. Age-dependent effects on host susceptibility, virulence and parasite transmission could pose an important challenge for experimental and theoretical studies of infectious disease dynamics and disease ecology. Our results present a call for a more explicit stage-structured theory for disease, which will incorporate age-dependent epidemiological parameters.

1. 宿主年龄是多数种群内宿主间最显著的差异特征之一，但目前关于年龄依赖效应如何影响宿主与寄生虫的生态及进化动态的相关数据却极为匮乏。
2. 本研究以两种克隆株的大型溞（Daphnia magna）及其细菌性寄生虫拉氏巴斯德菌（Pasteuria ramosa）为实验对象，探究了寄生虫感染暴露时宿主的年龄（幼体、成年早期个体与成年晚期个体）对宿主易感性、繁殖力、存活率以及寄生虫传播能力的影响。
3. 无论宿主与寄生虫的克隆株类型如何，年幼的大型溞均比年长个体更易被感染。此外，被感染的年幼大型溞出现生殖功能被抑制的速度快于成年宿主，而宿主与寄生虫的克隆株也会对该性状产生影响。进一步研究发现，早期感染的大型溞所产生的寄生虫传播体数量远多于晚期感染的个体；不过若按照模型中的定义（以宿主死亡率衡量），感染时的宿主年龄并不会影响毒力。
4. 若将毒力更宽泛地定义为感染对宿主适合度的负面影响，并综合寄生虫对宿主繁殖力与死亡率的作用，则感染时的宿主年龄似乎会沿着宿主与寄生虫适合度间的负相关关系发生变化。由此可见，不同年龄类群间的毒力-传播权衡差异显著，而这又会反过来影响最优毒力的预测结果。
5. 年龄依赖效应对宿主易感性、毒力以及寄生虫传播能力的影响，可能会给传染病动态与疾病生态学的实验及理论研究带来重大挑战。本研究结果呼吁构建更明确的疾病阶段结构理论，该理论需纳入年龄依赖的流行病学参数。