Prior infections or defence priming: what determines the risk of trematode infections in amphipod hosts?

Inducible defences against parasites that are only activated when needed can mitigate the cost of immune or behavioural evasion of parasites. Priming of the immune system and activation of behavioural defences can follow exposure to cues associated with imminent infection risk. In contrast, prior infection can cause immune depression or leave the host with less energy to defend itself against further infections. We investigate the priming of anti-parasite defences and the effect of prior infections in the amphipod Paracalliope fluviatilis, the second intermediate host of the trematode Coitocaecum parvum. During experimental infections, amphipods that had been primed by exposure to chemical cues (from first intermediate snail hosts infected by C. parvum) of infection risk were not better at avoiding further infection than control amphipods. All amphipods showed the same swimming behaviour, whether or not they had been primed by chemical cues from infected snails, or whether or not they were in the presence of live infective stages. In contrast, regardless of whether or not they had been exposed to control water or chemical cues from infected snails, amphipods harbouring prior infections acquired in nature were significantly more likely to acquire new parasites under controlled conditions. These results suggest that the induction of defences via external cues associated with the threat of infection do not play a role in the amphipod's anti-parasite strategy. However, prior infections may pre-dispose a host to acquire further parasites, with consequences for the distribution of parasites among host individuals and the regulation of the host population.