Efficacy of Bacillus probiotic mixture on the immunological responses and histopathological changes of Nile tilapia (Oreochromis niloticus, L) challenged with Streptococcus iniae

Isolates of Streptococcus iniae were recovered from the diseased fish during summer season, identified and characterized using different laboratory techniques. Three hundred and sixty Nile tilapia (100 ± 5.0 g average weight) were divided into two subgroups in triplicates with 20 fish per replicate and received either sterile PBS (B0, B1 and B2, respectively) or S. iniae (B0+ S. iniae, B1+ S. iniae and B2 S. iniae, respectively). At 15 days post‐challenge, results showed low mortality rates in probiotic‐fed groups. The mortalities ranged from 6.67% in B2 to 81.67% in B0 following S. iniae challenge. The haematological parameters showed a significant increase (p < 0.05) in haemoglobin, red blood cells and white blood cells in B1, B1+S. iniae, B2 and B2+ S. iniae groups compared to B0+ S. iniae group. The alanine aminotransferase (ALT) level was elevated (p < 0.05) in tilapia challenged with S. iniae compared to the control groups. Glucose and creatinine levels were elevated (p < 0.05) in all groups compared with B0. Both albumin and globulin levels were significantly low (p < 0.05) in B0+ S. iniae compared to other groups. The total protein level, phagocytosis and phagocytic index were significantly lower (p < 0.05) in B0+ S. iniae than other groups. Histological analysis showed that the hepatopancreas of B0, B1 and B2 groups represents normal hepatocyte architecture, while the infected tilapia showed severe diffused necrosis, mononuclear cell infiltration and loss of the normal architecture. Spleen of PBS control groups did not show any pathological changes, while that of infected tilapia revealed several necrotic areas in addition to marked depletion of lymphoid cells. A loss of skin layers was observed in infected tilapia with marked interstitial oedema and leucocytic cell infiltration which was not observed in the probiotic‐fed fish. The current study highlights the immunomodulatory effect of Bacillus probiotic mixture against S. iniae infection in tilapia that can be further applied to control the disease in farms.