Effect Of Oxytetracycline On The Biosafety, Gut Microbial Diversity, Immune Gene Expression And Withdrawal Period In Pacific Whiteleg Shrimp, Penaeus Vannamei

Aquaculture disease management has been one of the primary focuses of improved production and profitability. Oxytetracycline (OTC) is a broad-spectrum antibiotic approved for therapeutic applications in aquaculture. Though maximum residue levels, safety and withdrawal period is established for finfish and tiger shrimp (Penaeus monodon), similar information is scanty on Pacific whiteleg shrimp, Penaeus vannamei, farmed under the tropical climates of South Asia. In this study, P. vannamei was administered with OTC for 14 days at different doses, 2.25, 4.5, 9.0, 18.0 g kg- 1. OTC administration at 4.5 g kg- 1 feed dose or less caused no significant impact on the growth or survival of the shrimp during the study period. The residual concentration in shrimp muscle declined below 0.1 ppm in 96 h post-treatment and was undetected at 120 h post-treatment. There were no significant histological changes in hepatopancreas, antennal gland and hematopoietic tissue at the therapeutic dose (4.5 g kg- 1 feed), while dose-dependent toxic changes were observed in the lymphoid organ. Therapeutic dose of OTC administration did not influence the relative expression of crustin, penaeidin, lysozyme, pro-phenol oxidase and superoxide dismutase. Dose-dependent reduction in gut microbial diversity was observed as revealed by denaturing gradient gel electrophoresis (DGGE). In all the treatments, however, the microbial diversity was restored 14 days post-cessation of OTC administration. The observed 50% inhibition of phytoplankton Arthrospira platensis and Chlorella vulgaris (68.17 and 78.18 mg L-1 OTC, respectively) and 50% immobilisation of copepods and rotifers (173.89 and 257.16 mg L-1 OTC, respectively) was significantly higher than the therapeutic dose. Oxygen consumption, ammonia excretion, feeding rate and ingestion rate of copepod was significantly reduced at and above 187.5 mg L-1 OTC, while rotifers showed no impact of OTC up to 750 mg L-1, which is higher than the recommended therapeutic dose. The study has indicated that 96 h of withdrawal period may be required for safe use of OTC at 4.5 g kg- 1 feed for 14 days in Indian P. vannamei farming.