Efficacy of current and alternative bath treatments for Lepidotrema bidyana infecting silver perch, Bidyanus bidyanus

• We evaluated the current treatments for Lepidotrema bidyana.
• We screened five potential alternate treatments in vitro; and where effective, in vivo.
• The current treatments were ineffective.
• Praziquantel was the only effective alternative short term bath treatment.
• Effective management of this parasite requires a repeat praziquantel bath.

Lepidotrema bidyana, a gill monogenean, infects silver perch (Bidyanus bidyanus) in aquaculture. L. bidyana is managed using short duration baths during grading, prior to the fish being returned to grow-out ponds. In response to anecdotal reports of poor efficacy, we evaluated the effectiveness of treatments used in the silver perch aquaculture industry in Australia: formalin, trichlorfon and sodium chloride. We also assessed the efficacy of peracetic acid, chloramine-T, praziquantel, hydrogen peroxide and sodium percarbonate using a panel of in vitro and in vivo trials. During in vitro trials parasite mortality was calculated as a percent reduction and treatment doses were considered effective when greater than 90% parasite mortality was observed. The minimum effective dose for each treatment after 100 min in vitro was 100 mg/L for formalin; 10 g/L for sodium chloride; 20 mg/L praziquantel; 175 mg/L hydrogen peroxide; and 50 mg/L sodium percarbonate. Trichlorfon, peracetic acid and chloramine-T were ineffective in vitro. During in vivo trials, praziquantel administered at 40, 15 and 5 mg/L for 60 min had an efficacy of 77, 68 and 47%, respectively, surviving worms were predominantly juveniles located at the base of the secondary lamellae. Silver perch treated with hydrogen peroxide has a significantly lower intensity of L. bidyana, however, fish were highly sensitive to hydrogen peroxide, resulting in mortalities. Treatment with formalin, sodium chloride and sodium percarbonate was ineffective at all doses trialled in vivo. These results indicate that the current treatment recommendations are ineffective for managing L. bidyana and that praziquantel is a potential alternative short duration bath treatment.