Scuticociliates are recognized as the causative agents of mass mortalities in certain cultured marine fishes, resulting in enormous economic losses. This study aimed to investigate a fatal infection caused by scuticociliates in farmed large yellow croaker (Larimichthys crocea) in Fujian province, China. Microscopic examinations of focal organs, including the brain, eyes, gills, and skin, revealed the presence of parasites. Active masses of scuticociliates were observed in these organs, and the ciliates were subsequently isolated and maintained in vitro. An immersion challenge experiment revealed that L. crocea experienced cumulative mortalities reaching 73% within 7 d upon exposure to 1.0 × 10⁴ ciliates mL⁻¹. Based on the microscopic and PCR testing of infected fishes, the brain was comprehensively inferred as the main infection organ for the isolated strain. Microscopic and submicroscopic observations of the isolated scuticociliate, coupled with cortical ciliature patterns revealed through α-tubulin indirect immunofluorescence techniques, identified these scuticociliates as Miamiensis avidus. The sequencing of two genetic markers (small subunit ribosomal RNA, SSU rRNA and cytochrome c oxidase subunit I, COI) further confirmed that the isolated strains exhibited the highest sequence similarity to most M. avidus sequences in GenBank. However, significant differences in SSU sequences compared to the M. avidus strain Ma/2, and the lack of published COI and ITS (internal transcribed spacer) sequences for Ma/2, indicate the need for further molecular data to resolve whether there are potential cryptic species within the M. avidus complex.

Cryptocaryon irritans, an obligate ectoparasite, causes cryptocaryoniasis and heavy economic losses to marine aquaculture. Cathepsin B (CatB) is highly expressed in C. irritans theronts and hypothesized to participate in parasite host invasion, yet its biochemical properties and pathogenic roles remain poorly defined. In this study, we performed bioinformatic analysis of the CiCatB gene. Following codon optimization, CiCatB was heterologously expressed in Pichia pastoris, and the biologically active recombinant CiCatB protein was purified. Subcellular localization analysis revealed that CiCatB predominantly localized in the cytoplasm of theronts, with partial signals detected at the cell membrane and in the extracellular space. Enzymatic assays showed that yCiCatB hydrolyzed gelatin and type I collagen from large yellow croaker, with optimum activity at pH 5.0 and stable performance from 28 to 37 °C. Mg²⁺ increased enzyme activity, while Cu²⁺ almost completely abolished catalytic activity. Both E-64 and anti-CiCatB serum inhibited enzyme activity. Functional blocking using inhibitor or antiserum markedly reduced parasite recovery following challenge. Our results suggest that CiCatB may contribute to host invasion by degrading host structural proteins, advancing our understanding of C. irritans pathogenicity. CiCatB represents a potential target for exploring control strategies against cryptocaryoniasis.