Detection and characterization of the causative pathogen of Scuticociliatosis in olive flounder (Paralichthys olivaceus) in Jeju Island

Abstract

The disease known as "Scuticociliatosis" is one of the major problems in farmed olive flounder (Paralichthys olivaceus) and is considered as a parasitic disease with significant economic implications in aquaculture due to its high mortality rate and inadequate treatment options. In particular, when the juvenile olive flounder is infected with Scuticociliates during their early stages, the parasites, including Miamiensis avidus and Uronema marinum, penetrate various organs, such as the gills, skin, eyes, brain, and internal organs, causing substantial challenges in fish farming. Therefore, the prevention of infections and multifaceted research and approaches are crucial to minimize losses. In this study, we conducted a six-month disease monitoring from July to December 2022, targeting 21 aquaculture farms in Jeju, South Korea. The focus was on two species of Scuticociliates, namely M. avidus and U. marinum, which are commonly associated with infections in domestic farmed olive flounder. We measured the weight and length of all olive flounder used in the analysis and observed external and internal symptoms suspected of Scuticociliatosis to analyze disease occurrence patterns. For the diagnosis of infections caused by these two species, we conducted polymerase chain reaction (PCR) on DNA extracted from the muscle tissues of the fish. Out of the total 3,352 farmed olive flounders used in the experiment, M. avidus was detected in 234 individuals, while U. marinum showed negative results in all 3,352 fish. The 234 M. avidus-positive individuals were confirmed to have 100% sequence identity with 10 reference M. avidus 18S ribosomal RNA gene sequences. Additionally, we attempted classification using the cytochrome c oxidase 1 (cox 1) gene and found a correlation between cox 1 gene types and serotypes through related research. This study investigated the trends in Scuticociliatosis outbreaks in the region of Jeju, which is geographically isolated and accounts for half of South Korea's farmed olive flounder production. The analysis of disease characteristics presented in this research can serve as fundamental data for the prevention of Scuticociliate infections in the future.