The impact of heat stress on oxidative stress and histopathological dynamics in red hybrid tilapia experimentally infected with Streptococcus agalactiae

Abstract

The most common predisposing factor to Streptococcosis in the aquaculture industry is heat stress. The impact of heat stress on oxidative stress and histopathological changes in red tilapia with Streptococcosis is not well understood. This study aimed at evaluating the impact of heat stress on oxidative stress and histopathological dynamics in red hybrid tilapia experimentally infected with Streptococcus agalactiae. Apparently healthy 105 red hybrid tilapia (Oreochromis nolyticus) were used. The fish weighing 120 to 130 g were acclimatized for 2 days prior to onset of the experiment. The 105 tilapias were randomly separated to group 1, group 2 and group 3 of 35 fishes each in 200 litre tanks. Group 1 was inoculated with 0.1 mL of S. agalactiae at 109 CFU/mL intraperitoneally and subjected to heat stress (33°C). Group 2 was injected intraperitoneally with the same quantity of S. agalactiae in similar manner to group 1 without heat stress as positive control while group 3 served as negative control. Samples of blood was collected from five fish in every group before and at 6, 12, 24, 48, 72 h and 96 hours post-inoculation (pi) for lipid peroxidation [malondialdehyde (MDA)] and erythrocyte superoxide dismutase (SOD) analysis. The fish were necropsied and samples were taken from spleen, liver and kidney for microbiology and histopathology. Statistical analysis from the value of MDA and SOD revealed that there were significant differences (p < 0.05) between all the groups. Predominant lesions in the spleen were hemosiderin deposition, lymphoid depletion and haemorrhages. In the liver, main lesions were hepatocellular necrosis, mononuclear cell infiltration and haemorrhages while in the kidney, predominant histopathological lesions were mononuclear cell infiltration, necrosis, degeneration of tubular epithelial cells and haemorrhages. The lesions were significantly higher in spleen, liver and kidney of fish in group 1, followed by group 2 as compared to 3 which had no significant histological damage. Based on the findings of this study, heat stress alters significantly, the oxidative stress status and histopathology of Streptococcus infection in red hybrid tilapia.