The efficacy of emamectin benzoate as an oral treatment of sea lice, Lepeophtheirus salmonis (Kroyer), infestations in Atlantic salmon, Salmo salar L.

The efficacy of emamectin benzoate as an oral treatment of sea lice, Lepeop htheirus salmonis (Kroyer), infestations in Atlantic salmon, Salmo salar L. , was evaluated in a dose titration study and two dose confirmation studies . Replicated groups of salmon with induced infestations of sea lice were gi ven emamectin benzoate on pelleted feed at doses of 0, 25, 50 and 100 mu g kg(-1) biomass day(-1) for seven consecutive days. Sea lice were counted at 7, 14 and 21 days from the start of treatment, and comparisons made with c ontrol fish given the same diet without emamectin benzoate. Total numbers o f sea lice were significantly reduced at all doses in all three studies whe n compared to control fish. There was no significant difference between dos es of 50 and 100 mu g kg(-1), but the 50 mu g kg(-1) dose resulted in signi ficantly fewer lice than the 25 mu g kg(-1) dose. Emamectin benzoate was hi ghly effective in reducing numbers of preadult and adult lice and prevented the maturation of chalimus to motile stages. The optimum therapeutic dose was selected as 50 mu g kg(-1) day(-1) for seven days. Treatment reduced th e incidence of epidermal damage by sea lice and, in one study, survival of treated fish was 48% higher than in control groups. No fish mortalities or adverse effects were attributed to treatment with emamectin benzoate at any of the doses tested.