Dietary Pelargonium Sidoides extract mitigates thermal stress in Oreochromis niloticus: physiological and immunological insights

Extreme water temperatures caused by climate change constitute a serious threat to aquaculture producers. Aquatic ectotherms, which are unable to regulate their body temperature, are highly affected by temperature changes and serve as excellent models for studying the effects of these factors. This study aimed to examine the impact of extreme temperature variation on the growth, biological indices, antioxidant capacity, histology, and disease resistance of Oreochromis niloticus. The fish were exposed to 17 °C, 25 °C (control), or 33 °C for 30 days, imitating extreme temperature events. Additionally, a mitigation trial was conducted via dietary Pelargonium sidoides root extract (PS). Compared with that at 25 °C, growth performance was significantly impaired by thermal stress. The exposure of O. niloticus to thermal stress, especially cold stress, elicited significant increases in stress biomarkers (cortisol and glucose), hepatorenal functions (AST, ALT, LDH, and creatinine), and lipid profiles (triglyceride, cholesterol, HDL, and LDL). Cold stress induced immunosuppression in terms of reduced total protein, phagocytosis, serum bacterial activity, total immunoglobulin, and white blood cells, all of which increased the susceptibility of the fish to Aeromonas hydrophila infection and increased mortality. In addition to severe pathological lesions in the gills, liver, and intestine, thermal stress causes an imbalance in antioxidant/oxidative stress. Nevertheless, the PS extract counteracted these detrimental impacts of thermal stress. In conclusion, PS may be recommended for enhancing immunity and antioxidative resistance to mitigate the effects of thermal stress on O. niloticus. These findings may be useful in developing hypotheses about the physiology and immunology of fish in extreme-temperature environments.