Data from: Quality attracts parasites: host condition-dependent chemo-orientation of trematode larvae

1. Environmental factors impairing physiological condition of organisms are assumed to predispose them to parasite infections. This is because host immune function is typically condition-dependent. However, poor physiological condition has been reported to reduce host susceptibility to parasites in various systems. 2. We examined whether such an effect can be due to altered exposure of hosts to active parasite transmission stages by investigating chemo-orientation of free-swimming cercariae larvae of a parasite Echinoparyphium aconiatum towards its snail host Lymnaea stagnalis. 3. We used both long-term and short-term feeding treatments to manipulate the body condition and physiological traits related to food processing in experimental (i.e. target) snails and measured the preference of cercariae towards snail-conditioned water (SCW) over clean water. 4. We found that chemo-orientation of cercariae depended on the nutritional status of target snails. High physiological condition (long-term feeding) attracted parasites, but cercariae did not show preference towards SCW from individuals in poor physiological condition (long-term starvation). Food processing (short-term feeding treatments) did not affect chemo-orientation. 5. Our results suggest that host condition-dependent chemo-orientation of parasite larvae is a likely mechanism explaining the reduced susceptibility of snails to infection due to food limitation in our study system. In general, the use of condition-dependent host cues can be highly beneficial for parasites as it increases their transmission to high-quality hosts. Furthermore, evolving counter adaptations to such a transmission strategy can be very difficult for hosts.

1. 损害生物体生理状态的环境因子被认为会使宿主更易感染寄生虫。这是因为宿主免疫功能通常具有状态依赖（condition-dependent）性。然而，已有研究在多个研究系统中报道，较差的生理状态反而会降低宿主对寄生虫的易感性。
2. 本研究通过探究寄生虫刺缘吸虫（Echinoparyphium aconiatum）的自由游动尾蚴（cercariae）对其椎实螺宿主静水椎实螺（Lymnaea stagnalis）的化学趋向性，检验了该效应是否源于宿主与寄生虫活性传播阶段的接触暴露发生改变。
3. 本研究通过长期和短期投喂处理，对实验用（即目标）螺类的身体状态及与食物加工相关的生理性状进行调控，并测定了尾蚴对螺源条件水（snail-conditioned water，SCW）相较于清水的偏好性。
4. 研究发现，尾蚴的化学趋向性取决于目标螺类的营养状态。生理状态良好（长期投喂）的螺类会吸引寄生虫，而尾蚴并未表现出对生理状态较差（长期饥饿）螺类的螺源条件水的偏好。食物加工相关处理（短期投喂）并未对化学趋向性产生影响。
5. 本研究结果表明，寄生虫幼虫的状态依赖（condition-dependent）宿主化学趋向性，或是解释本研究系统中因食物限制导致螺类感染易感性降低的潜在机制。总体而言，利用状态依赖的宿主信号对寄生虫而言极具益处，因为这能提升其向高质量宿主的传播效率。此外，宿主演化出针对该传播策略的反向适应机制通常极为困难。