Isolation and Characterisation of Serotype III in Miamiensis avidus (Ciliophora: Scuticociliatida) From Olive Flounder (Paralichthys olivaceus).

Miamiensis avidus (Ciliophora: Scuticociliatida, syn. Philasterides dicentrarchi) is a significant pathogen causing scuticociliatosis in olive flounder (Paralichthys olivaceus) in Korea and Japan. In this study, the BB19 strain, isolated from olive flounder, was identified as M. avidus through phylogenetic analysis of cytochrome c oxidase subunit 1 (cox1) sequences and classified as cox1 genotype IV. Serotype-specific PCR confirmed BB19 as serotype III. Agglutination assay and indirect ELISA also showed that BB19 exclusively reacted with anti-serotype III (Mie0301 and BB19) sera, with no cross-reactivity to other serotypes, making this the first report of this serotype in Korea. Western blot analyses revealed a unique 34 kDa protein in serotype III, distinguishing it from the 30 and 38 kDa proteins of other serotypes. These results suggest that serotype III has distinct antigenic properties, indicating that vaccines based on serotypes I and II may not offer cross-protection. Virulence tests showed that BB19 caused 65% cumulative mortality in olive flounder at a concentration of 1 × 105 cells/fish, a level similar to that of other serotypes. The high virulence of BB19 highlights the need for ongoing monitoring of emerging serotypes and development of effective preventive measures against M. avidus to support the aquaculture industry.