Vindas et al. Brain-infecting parasites leave lasting effects on behaviour even in resistant hosts

Parasites can have profound effects on intra- and inter-specific
interactions at the population and community levels through their
influence on host behaviour, physiology, and fitness. While host
phenotypic changes are typically thought of in terms of established
infections, parasite encounters may be sufficient to induce behavioural
changes, even when no viable infections are established. Here, we use the
Japanese rice fish medaka Oryzias latipes and the brain-infecting
microsporidan parasite Pseudoloma neurophilia to understand how parasite
resistance influences behaviour. Although a previous study suggested that
medaka are a suitable host for P. neurophilia, an eight-week parasite
exposure regime resulted in no detectable infection in our study. Both
parasite-exposed and control (no parasite exposure) medaka were tested in
behavioural assays that assessed boldness, activity, and sociality. We
detected considerable changes in medaka behaviour following parasite
exposure, with parasite-exposed fish being more active, less bold, and
more social when compared to control fish. These data indicate that
parasite encounters may induce behavioural alterations even in
non-susceptible hosts. In addition to established infection, individual
differences in parasite exposure must also be considered in studies of
host responses across ecological scales.