In Vivo Challenge Trials with Dietary Antiparasitics Reveal New Insights into the Practical Use of Fenbendazole against Sparicotyle chrysophrii (Monogenea) Infection in Sparus aurata (Teleostei)

Abstract

The microcotylid Sparicotyle chrysophrii is one of the most harmful parasites for the aquaculture of the gilthead seabream Sparus aurata, for which effective treatments are needed. The efficacy of dietary praziquantel (PZQ, 30–50 mg kg body weight⁻¹ day⁻¹), emamectin benzoate (EMB, 0.015–0.050 mg kg BW⁻¹ day⁻¹), and fenbendazole (FBZ, 40–75 mg kg BW⁻¹ day⁻¹) against S. chrysophrii on gilthead seabream was tested in four different in vivo challenge trials. Specimens of gilthead seabream naturally infected with S. chrysophrii were maintained in three separate recirculating aquaculture systems of 142 L (1 control vs. 2 treated). Fish were fed (ration 1.2% BW day⁻¹) 5 days a week for 4 weeks, alternating weeks with medicated and control feed. On days 0, 14, and 28, samples of 7–9 fish per group were collected and the gills were examined for S. chrysophrii. The gills of hosts treated with EMB and FBZ were evaluated by histopathology. The trials showed that in-feed PZQ has low palatability, causing rejection which did not allow its efficacy to be adequately investigated. While dietary EMB was associated with adverse effects, such as gill de-epithelialisation and necrosis, with no demonstrated efficacy against S. chrysophrii. On the other hand, all the fish treated with FBZ lost all parasites, showing 100% efficacy and also suggesting a preventive/repellent action against the monogenean. In conclusion, the present study provides indications for the use of dietary FBZ as a medical device for the treatment/prophylaxis of sparicotylosis in aquaculture.