Early developmental exposure to heat shock altered the response to handling stress but had limited impacts on the susceptibility to the bacterial pathogen Yersinia ruckeri in juvenile rainbow trout (Oncorhynchus mykiss)

Disease is a major limiting factor for aquaculture production, having significant implications for production as well as fish welfare, hence, novel approaches for preventing and alleviating these impacts are required. Exposure of fish to heat shock during early embryo development has the potential to alter responses to stress in later life. Due to crossover between thermal stress and immune responses, and documented ability for elevated temperatures to influence immune function, we hypothesise that heat shock during early developmental life stages could influence responses and survival during infection. To test this hypothesis, rainbow trout embryos were heat shocked during early development, or maintained as naive controls. Once hatched and fully developed, fish from both groups were exposed to bacterial pathogen Yersinia ruckeri, or mock challenged (exposed to handling stress associated with the disease challenge but in the absence of the bacterial pathogen) in parallel. No significant difference in survival following infection was observed between naive and heat shocked fish (log-rank test, p = 0.7). Interrogation of hepatic transcription prior to infection revealed minimal differences between naive and heat shocked fish. However, comparing the response of naive and heat shocked fish to the stress associated with mock challenge revealed substantial differences in hepatic transcription. Enriched pathways including unfolded protein response, apoptosis, metabolism, oxidative stress and DNA damage, showed that the embryonic exposure to heat shock modified the response to subsequent handing stress. In contrast, naive and heat shocked fish exposed to Y. ruckeri displayed minor differences in transcriptomic responses, and no alterations in pathogen-associated mortalities. The lack of observed differences in susceptibility and transcriptional responses may have been due to the high exposure dose applied during the challenge.Overall, our data demonstrate the potential for developmental heat shock exposure to condition the response of fish to subsequent stressors, particularly to handling stress. Further work is required to build on these findings and determine the potential for developmental heat shock as a management tool to alleviate the adverse effects of exposure to stressors in aquaculture.