The Effect of Acute Heat Stress on the Chinook Salmon Immune System and on Ability to Combat 𝘝𝘪𝘣𝘳𝘪𝘰 𝘢𝘯𝘨𝘶𝘪𝘭𝘭𝘢𝘳𝘶𝘮 Infection

Abstract

The frequency, duration, and intensity of heatwaves in western Canada is expected to rise in the coming years. As a result, shallow rivers and streams have also experienced drastic changes, and water temperature may fluctuate by up to 13 °C throughout the duration of a single day. This poses a problem for salmon species, as they travel from marine to freshwater environments to spawn. Additionally, salmon are poikilothermic, meaning their physiological functions are influenced by their surrounding water temperature. Encountering water that is outside of their optimal growth temperature of 11-17 °C may result in behavioural and metabolic changes and, in extreme cases, rapid death. This may also result in increased susceptibility to infectious disease, as warming waters may render them temporarily immunocompromised and increase the virulence factors of some pathogens. Salmon have several cellular mechanisms to survive these stressful events, including heat shock proteins, immune responses, and secretion of glucocorticoids. Interestingly, the concept of thermal preconditioning is emerging as a method to aid salmonids’ ability to handle increasing water temperatures – exposure to a controlled, short term stimulus triggers a physiological response that prepares them for future, more extreme stress exposure. However, thermal preconditioning has never been tested before exposure to a pathogen. Further, the immune effects of heat waves have scarcely been investigated in salmon. In this study, we performed an environmentally applicable mock heat shock on juvenile Chinook salmon (Oncorhynchus tshawytscha) and measured their immunological responses up to 14 days afterwards. Chinook salmon also received an injection of live Vibrio anguillarum after heat shock to determine if heat deterred their ability to fight a systemic bacterial infection. The transcripts of IL-1β, IL-8, TNF-α, IL-10, TGF-β, IFN-γ, IL-2, cathelicidin, hepcidin, MHC1α, tapasin, MHC2α, MHC2β, HSP47, HSP70, and HSP90 were quantified by qPCR in the spleen, gills, and hindgut, as well as assessment of the stress response by measuring glucose, lactate, cortisol, and HSP47 levels in the plasma. Heat shock did not affect mortality rates due to vibriosis compared to salmon that received V. anguillarum alone. Additionally, heat shock mitigated the pro-inflammatory and corresponding anti-inflammatory responses needed to combat infection by initially upregulating il1b, tnfa, il8, and il10, then returning to normal levels by three days post-infection. The antimicrobial peptides cathelicidin and hepcidin played a significant role in combatting infection in both V. anguillarum treatment groups. Transcript levels of hsp47 and hsp90 were upregulated in response to both heat shock and bacterial infection; however, hsp70 expression was surprisingly low through the duration of the trial. HSP47 proteins measured by ELISA in the serum did not differ between any treatment groups. Heat shock caused a significant increase in plasma and cortisol lactate concentrations that both returned to basal levels 6-hours post-heat shock. Altogether, these data indicate that the experimental heat shock had a positive preconditioning effect on Chinook salmon and provides new insights on the interactions between the host, environment, and pathogen over a 14-day period. This research also provides a first step in understand how increasing river temperatures affect the salmon immune and stress responses and can help inform decision making in policy and sustainability initiatives to protect Pacific salmon populations in the coming years.