1. Juveniles of diverse host taxa, including vertebrates, invertebrates and plants, are more susceptible to parasitic infections than adults, a phenomenon still poorly understood.
2. To shed light on the mechanisms underlying this host age effect, we investigated its expression during different steps of the infection process of a bacterial parasite of the planktonic crustacean Daphnia. To do so, we infected juvenile and adult Daphnia magna with the bacterium Pasteuria ramosa and monitored parasite development in experiments specific to different steps of the infection process.
3. We did not find an effect of host age on parasite attachment, but found a strong increase in the penetration time of the parasite into hosts with increasing age. Host age at exposure also affected within‐host parasite development, which was delayed in old adult D. magna in comparison to young adults and juveniles. Furthermore, parasite clearance was observed only in old adults, suggesting that old hosts have a better immune response than young adults and juveniles.
4. Our results reveal a step‐specific effect of host age on disease progression, giving support for different hypotheses explaining high juvenile disease susceptibility. While these mechanisms are likely to work independently from each other, earlier steps of the infection process influence later steps, and thus may respond more readily to selection than later steps.
5. Our results also highlight that clearance can occur both during early parasite establishment and surprisingly at progressed infection phases, an undocumented phenomenon in invertebrates, where late infections are often considered chronic, lasting until host death.

中文翻译：

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解开导致青少年疾病易感性的感染过程步骤

1. 包括脊椎动物，无脊椎动物和植物在内的多种寄主分类单元的幼虫比成年人更容易受到寄生虫感染，这一现象仍知之甚少。
2. 为了阐明这种宿主年龄效应的潜在机制，我们调查了其在浮游甲壳类水蚤的细菌寄生虫感染过程的不同步骤中的表达。为此，我们用细菌巴斯德氏菌感染了幼年和成年的水蚤，并在特定于感染过程不同步骤的实验中监测了寄生虫的发育。
3. 我们没有发现寄主年龄对寄生虫附着的影响，但是发现随着年龄的增长，寄生虫渗透到寄主的时间会大大增加。暴露时的寄主年龄也影响寄主内寄生虫的发育，与年轻人和少年相比，成年D. magna的寄主年龄有所延迟。此外，仅在老年人中观察到了寄生虫清除，这表明老年人宿主比年轻人和青少年具有更好的免疫反应。
4. 我们的研究结果揭示了宿主年龄对疾病进展的特定步骤影响，为解释高青少年疾病易感性的不同假设提供了支持。尽管这些机制可能彼此独立起作用，但是感染过程的早期步骤会影响后续步骤，因此与后期步骤相比，对选择的响应可能更容易。
5. 我们的研究结果还强调，清除可能会在寄生虫早期建立期间以及出人意料的感染阶段发生，这在无脊椎动物中是无证的现象，在无脊椎动物中，后期感染通常被认为是慢性的，一直持续到宿主死亡。