Looking for Greener Pastures: In Vitro Screening of Phytogenics for the Control of Sparicotyle chrysophrii in Gilthead Sea Bream

The gill parasite Sparicotyle chrysophrii poses a significant threat to gilthead sea bream (Sparus aurata) aquaculture in the Mediterranean, causing considerable mortality and economic losses. As traditional chemotherapeutic treatments, like formalin, face growing regulatory restrictions, there is an urgent need for alternative control strategies. This study screened the in vitro antiparasitic activity of 16 phytogenic active ingredients (AIs), including essential oils (EOs) and commercial feed additives, against adult S. chrysophrii. Worms were exposed to a range of concentrations of each compound, and mortality was assessed over 24 h. Lethal dose 50% (LD 50) values were calculated and categorised by short-, mid-, and long-term efficacy. Several AIs, such as Cinnamomum zeylanicum, Origanum vulgare, Thymus vulgaris, and the commercial formulations Arotec-G and OA + HE+EO, demonstrated rapid and strong toxicity, with low LD 50 values observed within 2 h. In addition to lethality, characteristic morphological damage was detected in exposed parasites, indicating a direct and severe parasiticidal effect. These results highlight the high efficacy of various AIs at concentrations significantly lower than those used in standard formalin baths. Overall, the study identifies several promising candidates for the development of alternative treatments against S. chrysophrii, providing a solid scientific basis for future in vivo validation and integration into sustainable parasite control programs in marine aquaculture.