Histopathology and Wound Healing in Oxytetracycline Treated Oreochromis niloticus (L.) Against Aeromonas hydrophila Intramuscular Challenge

Antibiotics are very important tools for the control of fish bacterial diseases. Yet, there are strict regulations controlling the use of antibiotics in aquaculture. This study assessed the efficacy oxytetracycline dihydrate (OTC) at a dose of 2 g, 4 g, 6 g and 8 g/100 pounds fish/day against Aeromonas hydrophila challenge in Nile tilapia Oreochromis niloticus as well as the histopathological alterations in kidney and muscle, and wound healing. The commercial pellet feed was top dressed with OTC using 5 ml vegetable oil as a binder. The fish were injected intramuscularly with A. hydrophila at ≈1 × 108 cells/fish and then fed OTC feeds at 2% of their body weight for 10 days. The fish fed with 8g OTC/100 pounds fish/day recorded the lowest mortality (3.33%). The untreated fish recorded 8.33% mortality. Histologically, the kidney tissues of O. niloticus exhibited nephropathy and glomerulopathy. The kidney of OTC fed groups had improved organization of nephritic tubules and glomerulus. The muscle tissues exhibited haemocyte infiltration with mild necrosis initially, followed by melanization and disrupted muscle bundles. The results demonstrated that the OTC treatment for 10 days could bring out improved functioning of fish kidneys that carry infectious agents. Within 3 days of OTC therapy, tissue reddening and inflammation subsided with the formation of black scar. Full recovery of normal skin architecture was reached within 26-31 days post-injection. Based on the results, prudent use of 4-8 g OTC/100 pounds fish/day is recommended for the control of A. hydrophila infection in O. niloticus.