Shoaling with infected conspecifics does not improve resistance to trematode infection

Group‐living animals can gain protection against parasitic infections through social contacts with previously infected conspecifics (social immunization). Recent research suggests that such protective effects can be induced through visual or chemical cues released by infected individuals, resulting in anticipatory immune upregulation among group members. Here, we study cue‐induced social resistance in rainbow trout Oncorhynchus mykiss exposed to a trematode parasite, the eye‐fluke Diplostomum pseudospathaceum. We established groups of naïve individuals (receivers) that were paired with previously infected individuals (donors) at different ratios of donors to receivers and at different time points since donor exposure to capture varying concentrations of the anticipated cues. While the pre‐infection elevated resistance among the donors, there was no evidence of social transfer of resistance, regardless of the ratio of donors and receivers in a group or the time since the pre‐infection. The results suggest that resistance through social signalling may be system‐specific and requires further study into the generality of the phenomenon as well as the nature of the cues involved.