Immune status of Oreochromis niloticus experimentally infected with Aeromonas hydrophila following feeding with 1, 3 β-glucan and levamisole immunostimulants

Fish diseases cause high mortality in fish farms and lead to great economic losses. The immune status of Oreochromis niloticus was investigated after experimental infection with a 10% lethal dose 50 (LD50) of Aeromonas hydrophila. In this study, two different immunostimulants, 1,3 β-glucan and levamisole, were added to the fish diet for two weeks, and the fish were then challenged with A. hydrophila. Immunological activity was assessed after the cessation of mortality. Blood parameters including red blood cell (RBC) counts, haemoglobin (Hb), white blood cell (WBC) counts, serum total protein (TP), albumin (Alb), globulin (Glo), and α, β as well as γ Glo were measured three times, with a one-week interval. To assess the immune status of the fish, oxidative burst activity (OBA), phagocytic activity (PA), phagocytic index (PI) and serum antibacterial activity (SAA) were measured. Additionally, innate immunological responses, such as the pro-inflammatory cytokines tumour necrosis factor (TNF-α) as well as interleukin-1β (IL-1β) and the anti-inflammatory cytokine IL-10, were measured. The results revealed that although fish mortality had stopped, the fish were susceptible to bacterial infection, since immune activity was low in the challenged fish compared to that in the controls. Meanwhile, both of the examined immunostimulants, especially β-glucan significantly improved the fish immune status, and the relative level of protection (RLP). Therefore, new feeding practices in fish farming should include the addition of various additives to a fish feed formula to achieve better growth and to enhance fish capacity for disease resistance.