In many places around the world where tilapia aquaculture is practiced, abrupt changes in water temperature are recorded. For instance, in Egypt, water temperatures during the day can reach 25°C, dropping to 15°C at night.
Even though Nile tilapia can survive within a temperature range of 16 to 38°C, they stop consuming food outside this range and experience a high mortality rate. For Nile tilapia, temperatures below 16°C can impair various metabolic functions, including immunity and food assimilation, and temperatures below 10°C are lethal.
Temperature fluctuations can act as a ‘stress factor’ for fish. The immune response of fish can be suppressed by deviation from their typical physiological temperature range, reducing their resistance to certain infections.
Scientists from the Agriculture Research Center (ARC) conducted a study to gain a deeper understanding of the immunosuppression status of Nile tilapia during repeated changes in water temperature and to evaluate the vulnerability of Nile tilapia to aeromoniasis.
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Balanced Oxidative State
During a balanced oxidative state, where an organism’s antioxidant defenses properly regulate the production of reactive oxygen species (ROS), oxidative stress is described as excessive ROS generation in relation to the cell’s, organ’s, or animal’s antioxidant defense.
Several studies have reported that high temperatures and hypoxia present in aquaculture systems are stressful conditions known to interfere with fish homeostasis and significantly increase ROS generation.
To maintain organism homeostasis and protect it from pathogens, including bacteria, parasites, fungi, and viruses, the defense mechanism must respond.
The pathogenesis of a disease is significantly influenced by water temperature, such that increased temperature can promote bacterial growth and resistance to host immune elimination.
To determine the impact of temperature fluctuations on Nile tilapia’s ability to resist Aeromonas hydrophila infection, 20 fish were intraperitoneally injected with 0.1 mL of a bacterial solution.
The fish were challenged using the cohabitation method, with two infected fish added to a control aquarium.
“In this study, proinflammatory cytokines TNF-α and IL-1β significantly decreased under heat fluctuation stress and then increased in week 4, but at a lower level than in the control group,” report the researchers.
According to the study results, during the first three weeks of heat stress, IL-10 and HSP-70 were significantly elevated, followed by a significant decrease.
“Water temperature fluctuations reduced immune responses of Nile tilapia in skin mucus, such as lysozyme, peroxidase, and ABA, as well as cytokine gene expression in the head kidney. The mortality rate was between 35% and 40% in fish exposed to A. hydrophila infection, and bacteria were isolated at a rate of 61.54% to 75% based on surviving fish’s time, compared to 41.67% in the control group of this study,” highlight the researchers.
Finally, the researchers indicate that heat stress can reduce humoral immunity, explaining the increased vulnerability of Nile tilapia to bacterial infection.
“The immunological and oxidative state of Nile tilapia was negatively influenced when water temperature changed from 25 to 15°C every 12 hours for 4 weeks,” the researchers conclude.
They further note that temperature fluctuations diminish the immunity of Nile tilapia and induce oxidative stress, making them more susceptible to bacterial infections.
The study was funded by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).
Ahmed H. Sherif
Fish Diseases Department
Animal Health Research Institute AHRI
Agriculture Research Center ARC
Kafrelsheikh, 12619, Egypt
Reference (open access)
Sherif, A.H., Farag, E.A.H. & Mahmoud, A.E. Temperature fluctuation alters immuno-antioxidant response and enhances the susceptibility of Oreochromis niloticus to Aeromonas hydrophila challenge. Aquacult Int (2023). https://doi.org/10.1007/s10499-023-01263-9