Dietary arginine modulates nonspecific immune responses in Indian Major Carp, Cirrhinus mrigala exposed to hypoxia

Hypoxia in aquatic environment is often a recurrent stressor which can affect growth and health of fishes. However, nutritional intervention can help aquatic animals to cope up with environmental stressors so as to alleviate the adverse effects. Here, we examined the impact of L-Arginine (Arg) supplementation on hypoxia-induced changes in immune responses and resistance to Aeromonas hydrophila infection in Indian Major Carp, Cirrhinus mrigala. The fish were fed with graded levels of Arg for 60 days and exposed to 72 h sublethal level of hypoxia (0.50 +/- 0.16 mg/L dissolved oxygen [DO]). The six treatment groups with three replicates having different levels of Arginine such as N0.0 (0% Arg + Normoxia), H0.0 (0% Arg + Hypoxia), N0.7 (0.70% Arg + Normoxia), H0.7 (0.70% Arg + Hypoxia), N1.4 (1.40% Arg + Normoxia), and H1.4 (1.40% Arg + Hypoxia) were used in the study. Subsequently, the fish of different treatment groups were challenged with A. hydrophila. The hypoxia exposed fish exhibited significantly higher (P < .05) cumulative mortality compared to the unexposed fish, while Arg-fed groups showed reduced mortality irrespective of the level of dissolved oxygen. The albumin-globulin ratio, complement-3 response, respiratory burst, myeloperoxidase, serum antiprotease and lysozyme activities were significantly lower (P < .05) in hypoxia-exposed carp, revealing the increased susceptibility of fish to A. hydrophila is due to the immunosuppressive effect of hypoxia, which was improved in Arg-supplemented groups. We conclude that there is a possible role of hypoxia in sporadic outbreaks of bacterial diseases in carps as acute hypoxia causes immunosuppression which can be reversed by 0.7% Arg supplementation.