Assessment of therapeutic effect and residue depletion kinetics of oxytetracycline dihydrate in rainbow trout, Oncorhynchus mykiss (Walbaum, 1792)

Oxytetracycline (OTC) dihydrate, an approved broad-spectrum antibiotic, is widely used in aquaculture to combat bacterial infections like Aeromonas hydrophila in rainbow trout ( Oncorhynchus mykiss ). However, its optimal dosing, residue depletion kinetics, and withdrawal times remain underexplored in exotic species under Indian conditions, raising food safety concerns due to maximum residue limits (MRL) of 100 µg kg − 1 in muscle. This study evaluates the therapeutic efficacy, tissue residue profiles, and withdrawal times of OTC dihydrate in O. mykiss experimentally infected with A. hydrophila RTS 02, identifying safe doses for aquaculture use. Effective OTC doses (44.1, 88.2, 132.3, and 176.4 mg kg − 1 day − 1 ) were administered orally for 10 days to infected rainbow trout. Survivability and histopathology (liver, kidney, muscle) were assessed post-treatment. Residues in muscle, liver, kidney, intestine, and plasma were quantified via LC-MS/MS, with withdrawal times calculated using WT 1.4 software for muscle and all tissues. Doses of 88.2–176.4 mg kg − 1 day − 1 boosted survivability to 64–74%, with marked histopathological improvements in liver, kidney, and muscle. Peak residues (C max ) reached 2120 µg kg − 1 in plasma and 6740 µg kg − 1 in intestine after day one. At the therapeutic dose of 80 mg kg − 1 day − 1 , muscle residues fell below 100 µg kg − 1 MRL after 35.89 days; notable levels included 78.0 µg kg − 1 (intestine, day 8), 80.1 µg kg − 1 (plasma, day 12), and 46.6 µg kg − 1 (kidney, day 20). The graded OTC concentrations tested (88.2, 132.3, and 176.4 mg kg − 1 day − 1 ) demonstrated a balance between therapeutic efficacy and safety. Among these, 88.2 mg kg − 1 day − 1 is suggested as the minimum effective therapeutic dose for rainbow trout under the present experimental conditions. Residue depletion analysis further indicated a muscle withdrawal period of 35.89 days following administration at 80 mg kg − 1 day − 1 . To our knowledge, this study provides the first data describing OTC dihydrate residue depletion in rainbow trout under Indian aquaculture conditions.