Relative importance of chemical attractiveness to parasites for susceptibility to trematode infection

While the host immune system is often considered the most important physiological mechanism against parasites, pre-contact mechanisms determining exposure to parasites may also affect infection dynamics. For instance, chemical cues released by hosts can attract parasite transmission stages. We used the freshwater snail Lymnaea stagnalis and its trematode parasite Echinoparyphium aconiatum to examine the role of host chemical attractiveness, physiological condition and immune function in determining its susceptibility to infection. We assessed host attractiveness through parasite chemo-orientation behaviour, physiological condition through host body size, food consumption, and respiration rate, and immune function through two immune parameters (phenoloxidase-like and antibacterial activity of haemolymph) at an individual level. We found that although snails showed high variation in chemical attractiveness to E. aconiatum cercariae, this did not determine their overall susceptibility to i...