Biosafety, withdrawal and efficacy of anti -parasitic drug emamectin benzoate in Asian Seabass (Lates calcarifer)

Parasitic infestations are frequently reported in brackishwater fish. Hence, prevention and control strategy is paramount for successful brackishwater aquaculture. Emamectin benzoate (EMB) is found to be an effective in-feed treatment for controlling several ecto-parasites in fish. The objective of this study was to understand the biosafety, withdrawal and efficacy of EMB for use in brackishwater reared Asian Seabass (Lates calcarifer) under tropical climate. Biosafety was evaluated at doses of 0, 50, 250 and 500 mu g kg(-1) of fish body weight (BW) day(-1) for 21 days. EMB was administered per os through coated pellet feed. Gross, haematological and histopathological observations were made and found that there were no biologically significant adverse changes between control and treated groups. Withdrawal period was estimated with similar experiment with same doses but for seven consecutive days. Samples were collected on 0th, 7th, 14th, 21st and 28th day of post-medication for QTRAP 4000 LC-MS/MS analysis. The data showed that the level of EMB in whole fish treated with 50, 250 and 500 mu g kg(-1) of fish BW day(-1) were 21.0 +/- 0.0, 142.4 +/- 0.1 and 209.7 +/- 0.1 parts per billion (ppb) on 0th day of withdrawal, respectively. These values were found drastically decreased on 7th day of withdrawal and reached undetectable level by 28th day. For assessing the environmental safety, acute immobilization test was conducted using 24 h old daphnid. The EC50 and LC50 values in daphnids were 107 and 160 times higher than the the treatment dose, respectively. Efficacy was tested with the therapeutic dose on Asian Seabass fingerlings infested with naturally sea louse (Caligus minimus). Medicated pellet feed was fed at 4% BW day(-1) divided equally between three feedings. Efficacy trial showed that there was significant improvement in survival (92.0 +/- 2.3%; P < .01), reduction in parasitic load (P < .05), percent efficacy (76.2%) and haematological parameters (P < .01; P < .05) on and after fifth day of treatment. Based on the results, it was concluded that there was an adequate margin of safety and 100% efficacy associated with administering EMB-medicated feed to Asian Seabass fingerlings at the proposed therapeutic treatment regimen of 50 mu g of EMB kg(-1) fish BW day(-1) for seven consecutive days.