Behavioural and physiological alteration in gammarids infected with a bird acanthocephalan, following the selective killing of parasite inside its intermediate host

Various parasites alter their intermediate host's phenotype in ways that increase parasite transmission to final hosts. To what extent infected intermediate hosts can recover from such "manipulation" is poorly documented, thus limiting our understanding of its proximate and ultimate causes. Here, we address the reversibility of several phenotypic alterations induced by the acanthocephalan Polymorphus minutus, a trophically-transmitted bird parasite, in its amphipod intermediate host. Using a recently developed laser-based technology, we selectively killed parasite larvae inside the body cavity of Gammarus fossarum, while preserving host viability. Following behavioural tests, parasite death was confirmed using DNA integrity assays. Alterations of geotaxis, locomotor activity and resting metabolic rate in infected gammarids remained unchanged one month after parasite's death. In contrast, elevated brain lactate concentration and hemolymph total phenoloxidase activity of treated gammarids hosting a dead cystacanth returned to control (uninfected) levels. Interestingly, melanotic encapsulation response to dead cystacanth was rare up to two months after treatment, with only 5.6% of cystacanths being fully or partially melanized, thus suggesting long-lasting protection from the acellular outer envelope. Irreversible behavioural but reversible physiological alterations appear to be a cost-effective strategy of host manipulation, and point to a putative role of epigenetic alterations in parasite manipulation.