Effects of Dietary Emamectin Benzoate at Recommended and Overdose Levels on the Pathophysiological Responses and Tolerability of Oreochromis niloticus at Fry Stage

Emamectin Benzoate (EB) has been recommended as a feed additive for the control of fish ectoparasites. This investigation examined the impact of dietary EB on physiological responses, antioxidant defence mechanisms, and histopathological changes in key organs, and on the kinetics of residue deposition and elimination in the muscle of Oreochromis niloticus fry. Fry were administered EB feeds at 50 µg (recommended dose) and 500 µg/kg biomass/day (overdose) for 7 days and observed for 42 days post-EB-dosing. The reductions in superoxide dismutase, calcium, chloride, and acetylcholinesterase and the elevations in glucose and alkaline phosphatase, indicated failure in the antioxidant capacity, ionic equilibrium, brain and liver functions, and physiological responses. The dose-dependent mortalities and biomass of fry were recorded. Histopathological alterations in the livers, kidneys, intestines, and brains indicated the toxicity of EB. The brain tissue exhibited a reduction in neuron density, with remaining cells appearing scattered. Upon cessation of medication, the physiological responses of the fry were normalised. The EB residues peaked on day 7 (4.18 ng/g) and decreased significantly to trace levels on day 42 post-dosing (0.12 ng/g) in the recommended dose group. The muscle EB residue accumulation was within the permissible maximum residue limits (100 ng/g) set by international agencies. The findings on pharmacological behaviour and multiple biomarkers suggested that EB at the recommended dose is convincingly safe for O. niloticus fry.