We investigated the efficacy of praziquantel (PZQ) and fenbendazole (FBZ), each administered by bath and orally, against the monogenean Lepidotrema bidyana Murray, a gill parasite of the freshwater fish silver perch, Bidyanus bidyanus (Mitchell). PZQ and FBZ were each administered by bath at 10 mg L⁻¹ for 48 h and on surface-coated feed pellets at 75 mg kg⁻¹ per body weight (BW) per day for 6 days. Bath treatments of PZQ and FBZ had an efficacy of 99% and 91%, respectively, against adult L. bidyana. Oral treatments of PZQ and FBZ had an efficacy of 79% and 95%, respectively, against adult L. bidyana. Fish rejected feed pellets surface-coated with PZQ, suggesting that palatability of surface-coated PZQ-medicated feed is poor, which undermined efficacy. In all trials, some juvenile parasites were present on fish after treatment during efficacy assessment, indicating that efficacy may be lower against juvenile parasites or that recruitment occurred post-treatment, demonstrating that repeat treatments are necessary to effectively control L. bidyana in aquaculture.

Praziquantel (PZQ) is registered for oral use against Benedenia seriolae infecting Seriola spp. in Japan, but its bitter taste causes poor palatability. Incorporating PZQ in a moist pellet may help mask the flavor to improve intake. Altering delivery, however, may influence efficacy. We assessed the minimum effective concentrations of PZQ in moist pellets delivered by intubation for the monogeneans Zeuxapta seriolae and B. seriolae infecting yellowtail kingfish Seriola lalandi in flow-through tanks. The optimised dose was then evaluated in a sea-cage of S. lalandi to assess feed acceptance and efficacy. During intubation trials, efficacy was assessed as a percent reduction compared to control groups; in the field trial, efficacy was assessed by a percent reduction after treatment. PZQ delivered by intubation at 70 mg kg −1 body weight (BW) for 3 d was 99.7 and 81.7% effective against Z. seriolae and B. seriolae, respectively. Increasing the dose to 120 mg kg −1 BW for 3 d had a similar efficacy against Z. seriolae (98.4%) and increased efficacy against B. seriolae to 89.2%, but partial emesis of the medicated feed was sometimes noted. S. lalandi in a sea-cage at 17°C readily consumed PZQ administered daily in moist pellets at 70 mg kg −1 BW for 3 d (inclusion rate: 5.15 g kg −1), which significantly reduced Z. seriolae and B. seriolae abundance with 99.4 and 81.6% efficacy, respectively. Juvenile B. seriolae were common on the eyes of fish post-treatment which indicates a strategically timed repeat treatment is necessary.

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Ichthyophthirius multifiliis Fouquet, 1876, a ciliate protozoan, is a common cosmopolitan parasite of freshwater teleosts and is a recurring problem during the summer months on Australian rainbow trout Oncorhynchus mykiss (Walbaum, 1792) farms. Preventative strategies include increasing water flow and filtration, but when an infection is established, chemical intervention is often required. Formalin (FOR) has been traditionally used on Australian trout farms as a treatment for I. multifiliis. Treatment using sodium percarbonate (SPC) that releases hydrogen peroxide when dissolved is being implemented on a number of farms. To assess anecdotal reports of low efficacy we evaluated 1 h exposures of FOR and SPC at 12 °C and 17 °C in both hard and soft water against free-living stages of I. multifiliis. Each free-living stage were exposed to FOR and SPC in vitro; theronts were exposed to 8, 16, 32 or 64 mg/l SPC or FOR every 15 min, for a maximum of 6 h, and the number of live theronts at each time point was recorded. Prototomonts and tomocysts were exposed to 64, 128, 256 and 512 mg/l SPC and 16, 32, 64 and 128 mg/l FOR for 1 h, incubated, with the percentage viability and the number of theronts produced recorded. Theronts were more sensitive to treatment than tomonts, and prototomonts were more sensitive to treatment than tomocysts. FOR and SPC killed all theronts within 15 min at 64 mg/l at both temperatures. FOR was effective against all prototomonts at ≥64 mg/l at both temperatures and was effective against all tomocysts at 128 mg/l at 17 °C but did not achieve complete mortality in any doses tested at 12 °C. SPC was effective against prototomonts and tomocysts at 64 m/l at 17 °C but required ≥256 mg/l at 12 °C. These results can be used to aid development of specific treatment strategies for the management of I. multifiliis on Australian rainbow trout farms.

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