Efficacy of praziquantel and a combination anthelmintic (Adecto®) in bath treatments against Tagia ecuadori and Neobenedenia melleni (Monogenea), parasites of bullseye puffer fish

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Diclidophoridae and Capsalidae are families of monogenean parasites that include some of the most pathogenic species for cultured finfish. In this study, the efficacy of praziquantel (PZQ) and a combination anthelmintic (PZQ, ivermectin, pyrantel pamoate and fenbendazole) commercialized under the name of Adecto® (Adler Pharma) were evaluated in vitro against adults and eggs of the diclidophorid Tagia ecuadori and the capsalid Neobenedenia melleni. Freshwater and formalin immersions were also evaluated against T. ecuadori for comparative purposes. In addition, the efficacy in vivo of Adecto® as a bath treatment to eliminate mixed infections as well as the median lethal concentration (LC50) of this drug in juvenile, healthy bullseye puffer fish (Sphoeroides annulatus) were determined. Triglycerides, hemoglobin, total protein and glucose levels were measured in three groups of fish: uninfected, infected and infected/treated with Adecto®. At the doses tested, PZQ did not have a concentration-dependent effect. Thus, 2.5 mg/L PZQ was 100% effective against adults of T. ecuadori after 20 h, and 3 mg/L killed 87% of N. melleni after 12 h. Adecto® had a concentration-dependent effect. The concentration required to kill all parasites in the minimum time was 20 mg/L Adecto® (12 h for T. ecuadori, and 16 h for N. melleni). Neither PZQ nor Adecto® were effective at inhibiting egg hatching. Adults of T. ecuadori were highly tolerant to freshwater; mortality was <40% after 24 h, whereas formalin was 100% effective against both adults and eggs in this species. In vivo, 20 mg/L Adecto® administered for 12 h was 100% effective against T. ecuadori; however, it was not 100% effective against N. melleni. Mixed infections provoked increases in hemoglobin and total protein levels in fish. Fish exposed to 20 mg/L Adecto® did not show signs of toxic effect after 24 h. For those fish, the 24-h LC50 of Adecto® was 30.8 mg/L. This study confirms that PZQ is effective against the parasitic phase of monogeneans but under prolonged exposure. Our results suggest that combination anthelmintics have the potential to kill all parasites in less time; however, alternative combinations should be investigated to find one that is effective under low concentrations to provide a greater safety margin.