Introduction

Nile tilapia (Oreochromis niloticus) is considered the most important freshwater-cultured fish and is widely produced in many tropical and subtropical countries due to its unique properties, such as a quick growth rate, cheap maintenance cost, and great economic value. In 2020, the total tilapia output exceeded 4.5 million tons (FAO 2020). Nile tilapia, which is native to Africa and is a member of the Cichlidae family, can survive in a temperature range of 16–38 °C but stops consuming food outside this range and experiences a high mortality rate (Zhou et al. 2019). Since fish are ectothermic animals, they depend on their surroundings to control their body temperature and metabolic activity. For Nile tilapia, temperatures below 16 °C can damage several metabolic functions, including immunity and food intake, while temperature below 10 °C is lethal (Kumar et al. 2023).

Aquatic animals’ immune systems can be impacted by environmental factors such as salinity, temperature, pH, and dissolved oxygen (Velmurugan et al. 2019; Sood et al. 2021; Zhang et al. 2018). In open systems, environmental change can cause physiological changes in both the host and pathogen sharing the same habitat. There are numerous physiological and biochemical mechanisms involved in fish growth and development that might influence the prevalence of streptococcosis (Hazel and Prosser 1974; Sherif et al. 2022a). A disease epidemic cannot be caused by a pathogen alone since environmental factors must also be considered (Amal and Zamri-Saad 2011; Abdelsalam et al. 2022; Elgendy et al. 2022). In addition, temperature fluctuations may serve as a “stress factor” in poikilotherms. A fish’s immune response can be suppressed by a temperature deviation from its physiologically typical range, which lowers its resistance to certain infections (Karvonen et al. 2010). Nile tilapia suffers from a retarded growth rate and elevated mortality rate upon exposure to extremities of water temperatures below 12 °C and above 35 °C (Xie et al. 2011; Qiang et al. 2013).

In contrast to a balanced oxidative state, wherein an organism’s antioxidant defenses properly regulate the production of reactive oxygen species (ROS), oxidative stress is described as an excessive generation of ROS in relation to the antioxidant defense of the cell, organ, or animal system (Sies 1985). High temperatures and hypoxia are frequent environmental stresses in aquaculture systems that are known to interfere with fish homeostasis and significantly increase ROS generation (Cadenas 1989). ROS can form excessively in response to environmental stress conditions, thereby leading to oxidative damage (Kammer et al. 2011). To maintain an organism’s homeostasis and protect it from pathogens including bacteria, parasites, fungi, and viruses, its defense mechanism must be responsive (Velmurugan et al. 2019). To maintain anaerobic homeostasis in organisms, oxidative stress must be reduced by upregulating the activities of antioxidant enzymes such as total antioxidant capacity (T-AOC), superoxide dismutase (SOD), and catalase (CAT) (Li et al. 2016). Healthy tilapia is extremely vulnerable to Streptococcus agalactiae infection due to its long-term survival at high temperatures, which also causes extremely high levels of inflammation in infected tilapia (Amal and Zamri-Saad 2011; Ndong et al. 2007). The pathogenesis of a disease is significantly influenced by water temperature (Kayansamruaj et al. 2014), such that an increase in temperature can aid bacterial growth and resistance to host immunological clearance (Hu et al. 2017). Temperature is regarded as the most crucial aspect of tilapia and barramundi farms and has been linked to the prevalence of diseases (Mian et al. 2009).

Accordingly, this study was conducted to gain a deeper insight into the immunosuppression status of Nile tilapia during repeated changes in water temperature and to assess the vulnerability of Nile tilapia to aeromoniasis.

Materials and methods

Fish accommodation and experimental design

A total of 365 healthy Nile tilapia weighing 24 ± 2.5 g was purchased from a local fish farm located in Kafrelsheikh governorate, Egypt. Fish were tranquilized during transportation with 40 mg/L MS-222 and disinfected with 20 ppm/L iodine upon arrival at the wet laboratory of the Animal Health Research Institute (AHRI) (Sherif et al. 2022b; Eldessouki et al. 2023). Fish were acclimatized in a fiberglass water tank (2 × 1 × 1 m) for 14 days at 25 ± 1.5 °C, pH 7.6, dissolved oxygen 5.8 mg/l, total ammonia TAN less than 0.01 mg/l. The feeding rate was 4% of their body weight divided into two meals per day.

At the end of the acclimatization period, fish were investigated for any behavioral abnormality (swimming, food approach, and operculum movements), five fish were randomly investigated for parasites, the other five were bacteriology examined, and no growths were recorded on general media. Fish were divided into 18 glass aquaria (0.5 × 0.4 × 0.4 m) with 20 fish per aquarium, 240 Nile tilapia were randomly and evenly stocked in 12 aquaria and subjected to water temperature fluctuation, every week one aquarium exposed to bacterial infection, 120 Nile tilapia served as control 60 fish for immunological studies and the other 60 fish (20/aquarium) for bacterial infection, control positive, and control negative (water temperature was adjusted at 27 ± 1.5 °C).

In the experimental aquaria, the water temperature was adjusted at 27 ± 1.5 °C for 12 h from 08:00 a.m. to 08:00 p.m. and at 18 ± 1.5 °C for the of the day for 4 weeks. To simulate the fish farm climate, the fluctuation of water temperature was controlled using electric heaters with a thermometer. Fish feed was offered once a day at a rate of 1.5% at 01:00 p.m. as the feed approach was decreased and to avoid accumulating of uneaten food pellets that require large volumes of water to be changed. Water parameters were kept consistent by removing fish waste and replacing one-third of the aquarium water with dechlorinated water with a temperature of 27 ± 1.5 °C on alternate days at 11:00 p.m.

The cumulative mortality rate (CMR)% was calculated as follows:

$${CMR}\ \left(\%\right)=\left({number}\ {of}\ {deaths}\ {in}\ {the}\ {experiment}{al}\ {period}/{total}\ {population}\ {at}\ {the}\ {beginning}\ {of}\ {the}\ {experiment}\right)\times {100}$$

Gene expression

The expression of immune-related genes; tumor necrosis factor-α, interleukin (IL)-1β, IL-10, IL-8, and heat shock protein (HSP)-70 were determined, also antioxidant enzymes (SOD), catalase (CAT), and glutathione peroxidase (GPx) were assessed in head kidney tissues, the collected samples were preserved at − 80 till the analyses was performed. National Center for Biotechnology Information (NCBI) database; all primers were provided by Sigma-Aldrich (Sigma-Aldrich Chemie GmbH, Steinheim, Germany) and are shown in Table 1. Total RNA was extracted from the head kidney tissues that were collected from three fish in each group on the last day of the feeding trial using the Trizol reagent (iNtRON Biotechnology Inc., Korea) following the manufacturer’s procedure. The quantity and quality of extracted RNA were assessed by Nanodrop D-1000 spectrophotometer (NanoDrop Technologies Inc., USA). The complementary DNA (cDNA) was synthesized by the reverse transcription polymerase chain reaction using SensiFAST cDNA synthesis kit (Bioline, USA) following the manufacturer’s protocol. For studying the gene expression, the resulting cDNA was used as a template in the quantitative real-time PCR using Nile tilapia-specific primers, while the gene encoding β-actin was used as the housekeeping gene, owing to its constitutive expression. After the efficiency of PCR was confirmed to be around 100%, the data of gene expression were calculated using the Eq. 2−ΔΔCT method according to the procedure of Livak and Schmittgen (Livak and Schmittgen 2001).

Table 1 Primers for cytokines and antioxidants
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Bacterial infection

To determine the impact of temperature fluctuations on Nile tilapia’s capacity to resist Aeromonas hydrophila (A. hydrophila) infection. Accordingly, 20 fish were intraperitoneally (I/P) injected with 0.1 mL of a bacterial solution of an A. hydrophila AHRAS2 strain containing (LD50) 2.4 × 105 CFU/mL. The bacteria strain was grown in tryptic soy agar (TSA) at 28 °C for 18 h; the bacteria suspension was adjusted to LD50 in phosphate buffer saline using the McFarland scale. This strain was previously isolated and identified by Sherif and Abuleila (Sherif and AbuLeila 2022) and deposited to GenBank under the accession number MW092007; the bacterial strain was isolated from moribund Nile tilapia during mass mortality in fish cages; the strain harbors the virulence genes cytotoxic gene (act) and enterotoxin gene (alt). Pure saline solution (0.65%) was parallel injected similarly in twenty fish and stocked in control aquaria as a negative control (Boijink et al. 2001).

Fish were challenged using the cohabitation method (Sherif et al. 2022b), wherein two clinically diseased fish (experimentally infected with A. hydrophila AHRAS2) were added to one control aquarium (positive control) and one of a stressed aquarium every week, and each dead fish was counted only if A. hydrophila was re-isolated for 14 days, the mortality rate (MR) was estimated as follows:

$$MR\ \left(\%\right)=\left( number\ of\ deaths\ in\ a\ specific\ period/ total\ population\ during\ that\ period\right)\times 100$$

Following the method outlined by Austin and Austin (Austin and Austin 2012), swabs were taken from skin lesions, kidneys, gills, liver, spleen, and heart, and were inoculated onto brain heart infusion agar and incubated at 28 °C for 24 h. Pure colonies were streaked on Rimler–Shotts agar and Aeromonas selective agar base with ampicillin supplement were incubated at 28 °C for 24 h. The phenotypic profiles of the isolated bacteria were determined according to previous recommendations (Bergey 1994). All isolates were identified biochemically using API 20E strips (bioMérieux 1984).

Mucus analyses

Four Nile tilapia were randomly collected and used every week for skin mucus collection (Sherif and AbuLeila 2022). Fish were tranquilized with 40 mg/L MS-222; then, they were placed into a polyethylene bag with 10 mL of 50 mM NaCl and then gently massaged inside the bag for 30 s. The solutions were centrifuged at 1500 rpm for 10 min at 4 °C, and only the sediment was collected. The recovered mucus was then stored at − 80 °C for future use.

Lysozyme activity in the experimental Nile tilapia was assessed using the method of Parry et al. (Parry Jr et al. 1965). Briefly, 100 μL of skin mucus of fish was put into 96-well plates, in triplicate, and then, 175 μL of Micrococcus luteus suspension (0.3 mg M. luteus (Sigma-Aldrich, USA), in 1 mL of 0.1 M citrate phosphate buffer, pH 5.8) was loaded. The activity was presented in microgram per milliliter using a microplate reader compared to standard at 540 nm every 30 s and for 10 min at 25 °C. The peroxidase activity in skin mucus was determined following the protocol developed by Quade and Roth (Quade and Roth 1997), using the ELIZA at 405 nm, and the results were presented in mU. Ml−1.

Skin mucus antibacterial activity was determined according to Kumari et al. (Kumari et al. 2019). Briefly, equal amounts (100 μL) of the experimental Nile tilapia mucus and sterile normal saline were vortex-mixed and incubated at 25 °C for 1 h in nutrient agar broth inoculated with A. hydrophila suspension containing 106 CFU. The mucus–bacteria combination (100 μL) was cultivated on nutrient agar and incubated at 27 °C for 24 h. Counting the colonies developed on nutrient agar plates yielded the number of live bacteria.

Statistical analyses

To determine the impact of water temperature fluctuation on the immune and oxidative status of Nile tilapia, as well as their susceptibility to bacterial infection, the results were analyzed using SPSS software for windows (SPSS Statistical and package for social science, 2004). Analysis of variance was employed. All values were expressed as the mean ± SE (standard error) at a significance level of 0.05.

Biosafety

This study applied biosafety measures following the instruction provided by pathogen safety data sheets: infectious substances—A. hydrophila, Pathogen Regulation Directorate (Public Health Agency of Canada 2010).

Results

Clinical signs and cumulative mortality

Nile tilapia exposed to temperature fluctuation had a higher cumulative mortality rate (CMR) in the first week compared to weeks 2–3 and the control. Lethargy and decreased food intake were the most predominant clinical signs in the first week and then began to subside gradually (Table 2).

Table 2 Mortality rate of the experimental fish
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Immune responses of Nile tilapia exposed to temperature fluctuation

As shown in Figs. 1 and 2, heat stress led to changes in cytokine gene expression. Proinflammatory cytokines TNF-α and IL-1β had the similar trend, significantly declining over the course of 3 weeks before somewhat increasing over the course of week 4; however, it was still less than the control group. Heat stress had no effect on the gene expression of IL-8; however, after 3 weeks of exposure to heat stress, heat shock protein (HSP)-70 showed a significant increase. After 1 week of heat stress, the anti-inflammatory IL-10 was highly upregulated (3.25-fold change), but in the subsequent 3 weeks, it was significantly downregulated compared to the control.

Fig. 1
figure 1

Gene expression of tumor necrosis factor-α, interleukin (IL)-1β, and IL-10 of the experimental fish. The same letter in the same indicates the nonsignificant difference at different time points, at P ≤ 0.05

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Fig. 2
figure 2

Gene expression of interleukin (IL)-8 and heat shock protein (HSP)-70 of the experimental fish. The same letter in the same indicates the nonsignificant difference at different time points, at P ≤ 0.05

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Fluctuations in the water temperature had no effect on the gene expression of the antioxidant enzyme SOD. However, CAT and GPx were considerably elevated in the first week and then reduced linearly until they no longer differed from the control group (Fig. 3).

Fig. 3
figure 3

Gene expression of antioxidant enzymes of the experimental fish. The same letter in the same indicates the nonsignificant difference at different time points, at P ≤ 0.05

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Mucus analyses of the experimental fish

The first line of protection against external stressors in fish is the skin mucus. Fluctuations in water temperature influenced the mucus’s lysozyme, peroxidase, and ABA activities. Compared to the control group, mucus lysozyme significantly decreased in weeks 1 and 2 and then began to increase in weeks 3 and 4 (Figs. 4 and 5).

Fig. 4
figure 4

Lysozyme and peroxidase activity in the mucus of the experimental Nile tilapia. The same letter in the same indicates the nonsignificant difference at different time points, at P ≤ 0.05

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Fig. 5
figure 5

ABA activity in the mucus of the experimental Nile tilapia. The same letter in the same indicates the nonsignificant difference at different time points, at P ≤ 0.05

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Challenge fish with virulent A. hydrophila

As indicated in Table 3, a twenty Nile tilapia were challenged against A. hydrophila infection using the cohabitation method every week. In these fish, the MR increased, ranging between 35 and 40%, as compared to the control fish. The emergence of clinical symptoms took 2 days longer in this group compared to the control. The isolation rates of A. hydrophila also increased over time. Bacteria were isolated at a rate of 61.54 to 75% in the survived fish, compared to 41.67% in the control group.

Table 3 Mortality rate of the experimental fish challenged against A. hydrophila
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Discussion

Inflammation is one of the initial and most significant reactions to pathogen invasion, and cytokines are pivotal to the signaling of inflammation (Zhang et al. 2018; Dinarello 2009). In this study, the proinflammatory cytokines TNF-α and IL-1β decreased significantly under the stress of heat fluctuation and then increased in week 4 but at a lower level than in the control group. Inconsistency, a modulation of IL-1β and TNF-α expression, which are closely related to each other (Dinarello 2009), could be attributed to environmental factors (Van Doan et al. 2023). Similar exposures to fluctuating temperatures in rainbow trout (Oncorhynchus mykiss) led to a delayed expression of IL-1β at lower temperatures compared to higher temperatures (Raida 2007). In contrast, Dellagostin et al. (Dellagostin et al. 2022) observed an upregulation of the IL-1β gene expression in the spleen, kidney, and liver of fish exposed to cold water temperatures while no significant difference in the TNF-α gene expression was observed in the liver. These differences may be due to the exposure period to cold water and not a fluctuation in temperature, as is the case in this study.

In our study, during the first 3 weeks of heat stress, IL-10 and HSP-70 were significantly elevated and followed by a significant decrease. Similarly, to prevent damage to tissues during the inflammatory process, stress induces an anti-inflammatory mechanism by activating the production of IL-10 (Sabat et al. 2010); also, the HSP-70 gene activates in the liver tissue of fish to facilitate protection and thermotolerance to maintain homeostasis under adverse environmental conditions, such as heat stress and extreme water temperatures (Mladineo and Block 2009; Zhou et al. 2010; Liu et al. 2019). In contrast to chronic exposure, fish can acquire resistance through the fast production of the anti-inflammatory cytokines IL-10 and HSP-70 and the downregulation of proinflammatory cytokines when the water temperature fluctuates from 15 to 25 °C (Dellagostin et al. 2022).

Since antioxidant enzymes (SOD, CAT, and GPx) prevent ROS formation by removing their precursors, they have been widely employed as early-warning bioindicators of heat stress (El-Deep et al. 2019). In this study, the gene expression of SOD was unaffected by changes in water temperature; however, CAT and GPx significantly increased in the first week and subsequently linearly decreased, indicating that the fish had acclimated to the temperature fluctuations. In agreement with these findings, Vicente et al. (Vicente et al. 2019) discovered that Nile tilapia held under heat/dissolved oxygen-induced stress (32 °C/2.3 mg/L dissolved oxygen) had upregulated levels of CAT and GPx after 72 h but not after 48 h. In contrast, the SOD of rainbow trout exposed to how high heat (25 °C) exhibited significant decreases and abrupt increases in serum glucose when exposed to high-temperature settings in grass carp (Ctenopharyngodon idellus) (Cui et al. 2014). Additionally, stressful conditions downregulated antioxidative enzymes, including T-AOC, SOD, and CAT (Cheng et al. 2017). However, in our investigation, Nile tilapia could outperform the stress condition. According to several authors (Lushchak 2011; Sherif et al. 2019, 2021a, b, c, 2022c, d, e), the physiological reaction induced to preserve or restore homeostasis that causes the association between exposure to stressors and systemic or cellular oxidative stress.

Water temperature fluctuations reduced the immunological responses of Nile tilapia in skin mucus such as lysozyme, peroxidase, and ABA, as well as cytokine gene expression in head kidney. The MR was between 35 and 40% in fish challenged with A. hydrophila infection and bacteria were isolated at a rate of 61.54 to 75% time-depended from survived fish, compared to 41.67% in the control group in this study. Accordingly, Ndong et al. (2007) conducted a similar experiment transferring of Oreochromis mossambicus after 12 h from 27 °C to low temperatures (19 and 23 °C) and the transfer of fish from 27 °C to high temperatures (31 °C and 35 °C) resulted in lowering their immunological competence and resistance to Streptococcus iniae, concurrently their phagocytic and lysozyme activity was lowered. In agreement with our findings, Ndong et al. (2007) confirmed that an abrupt temperature reduction reduces a fish’s capacity to make antibodies essential to an initial immune response, and a delay in the immune response may allow bacteria to colonize, multiply, and establish infection (Harper and Wolf 2009). Furthermore, Kurata et al. (1995) discovered that cold temperatures inhibit the defensive abilities of nonspecific leukocytes known as natural killer (NK) cells in common carp (Cyprinus carpio), suggesting that NK cells may be able to adapt to temperature fluctuations over time. In this study, heat stress may reduce humoral immunity, explaining the increased vulnerability of O. niloticus to bacterial infection. Moreover, A. hydrophila infection is reported to be exacerbated by sudden temperature changes, handling, crowding, insufficient nutrition, and a lack of oxygen (LeungI et al. 1994; Roberts 2001; Sherif and Kassab 2023; Dominguez et al. 2004).

Conclusion

The immunological and oxidative state of Nile tilapia was negatively influenced as the water temperature changed from 25 to 15 °C every 12 h for 4 weeks. TNF-α and IL-1β cytokine gene expression reduced throughout the 4 weeks, while anti-inflammatory IL-10 gene expression significantly increased within the first week. In weeks 1 and 2, there was a considerable drop in skin mucus lysozyme, and the gene expressions of the antioxidant enzymes CAT and GPx were significantly increased. The MR increased and ranged between 35 and 45%, while bacteria were isolated from fish that survived at a rate of 61.54–75%. Temperature fluctuations decrease the Nile tilapia’s immunity and induce oxidative stress, making them more susceptible to bacterial infection.