Sparicotyle chrysophrii experimental infection of gilthead seabream (Sparus aurata): Establishment of an in vivo model reproducing the pathological outcomes of sparicotylosis

Sparicotyle chrysophrii (Microcotylidae) is considered the most threatening pathogen affecting the gilthead seabream (GSB; Sparus aurata) off-shore farming due to its economic impact. This study explores the best experimental conditions to set up an in vivo infection model capable of mimicking the sparicotylosis signs observed in farmed diseased fish. The experimental setup for parasite transmission consisted of a recipient (R) fish tank with naïve GSB receiving water from two S. chrysophrii-infected donor tanks in a recirculating aquaculture system (RAS). Egg collectors, consisting of a polyester mesh in a supporting plastic frame, were placed in the R tank in order to monitor the progression of the parasitosis. An additional tank with control unexposed naïve fish was maintained in parallel, with open water flow and disconnected from the RAS. After a preliminary trial, infective pressure in the R tank was increased by placing an additional egg collector already loaded with entangled parasite eggs, and by maintaining the fish number throughout the experiment. Adult S. chrysophrii parasite load correlated with most of the evaluated biotic and abiotic factors. Haemoglobin and haematocrit significantly dropped around 40 days after exposing GSB to S. chrysophrii. Furthermore, the abundance of eosinophilic granular cells and goblet cells in gill filaments, and splenic melanomacrophagic centres increased. In contrast, hepatic fat was depleted in S. chrysophrii-infected GSB. This study provides an advancement not only for studying S. chrysophrii's biology and its interaction with its host, but also for further studying the disease under experimental conditions in search of treatment alternatives and prophylactic measures.This work has been financed by the Spanish Ministry of Science and Innovation and Universities to the project SpariControl (RTI2018-098664-B-I00, AEI/FEDER, UE) and by the Spanish Ministry of Science and Innovation with funding from European Union NextGenerationEU project (ThinkInAzul, PRTR-C17·I1) and from Generalitat Valenciana to the project REMEDISA-PARASITE (GVA-THINKINAZUL/2021/022). ERF was supported by an FPI contract PRE2019-087409 (MCIN/AEI/10.13039/501100011033). MCP was funded by a Ramón y Cajal Postdoctoral Research Fellowship (RYC2018-024049-I & ACOND/2022 Generalitat Valenciana). RDP was contracted under the PTA Programme from the Spanish Ministry of Science, Innovation and Universities (PTA2018-015315-I). IE was funded by contracts from the Ministerio de Ciencia e Innovación and Generalitat Valenciana, Spain. ERF, MCP and RDP contracts were co-funded by the European Social Fund (ESF). We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI)