Some infectious diseases, such as Salmonella in ornamental fish, are important because of their potential for human transmission and antibiotic resistance. In this study, 250 Koi fish with an average length of 10±3 cm and a weight of 20±1 gr were randomly divided into four groups with two repetitions. The test was performed for 24 days with diet and 72 hours' exposure to Salmonella Typhimurium. Fish are classified into four groups T1; receiving lactobacillus casei (1.5 × 108 CFU / ml) probiotic

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... and not exposed to the pathogen, T2; getting probiotics and exposed to the pathogen, T3; no probiotics received but exposed to the pathogen, and control group (C). In the present investigation, to evaluate the damage caused by Salmonella typhimurium, Alkaline phosphatase (ALP), Alanine aminotransferase (ALT), and Aspartate transaminase (AST) were tested on days 0, 24 and 27 of the experiment. Based on the results, the effect of Lactobacillus casei probiotic in comparison with the control group on the improvement of liver function in this species of fish was investigated in such a way that the presence of probiotics alone may cause liver function in fish under normal or optimized conditions. However, the group that was not exposed to any probiotic agents showed a clear increase in the level of ALP, AST and ALT in the liver, which indicates the destruction of liver cells (p<0.05). However, in the group exposed to the pathogen along with the probiotic agent, the pathogen first increased the amount of some enzymes, but finally, with the action of probiotic factors, a decrease in enzyme levels in fish can be seen (p<0.05). In the third group, which did not use any probiotics, to increase the level of liver function enzymes in fish is observed significantly (p<0.05). In the study of three main indicators of liver diagnosis, it was found that one of the proposed ways to have healthy fish both from the point of view of aquaculture and as an animal model; using probiotics such as Lactobacillus casei can significantly improve liver and therefore metabolic function.