First report of Bonamia ostreae in wild flat oysters (Ostrea denselamellosa) from the west coast of Korea: Molecular and histopathological confirmation.

Bonamia ostreae is a protozoan parasite of significant concern in oyster aquaculture due to its impact on flat oyster species. In this study, we screened 30 wild flat oysters (Ostrea denselamellosa) collected from the western coastal region of South Korea for B. ostreae infection using two molecular assays. Species-specific BOSTRE-F/R primers targeting the ITS region detected 14/30 (46.67 %) positive samples, while Bonamia genus-specific Bo/Boas primers targeting the SSU rRNA region yielded 19/30 (63.33 %) positives. Although the higher detection rate with Bo/Boas primers raised the possibility of other Bonamia species, further PCR using B. exitiosa-specific primers confirmed the absence of B. exitiosa. To enhance species identification and phylogenetic analysis, extended SSU rDNA sequences (1,094 bp) were amplified using two primer sets, one newly developed for this study. Phylogenetic analysis demonstrated that the South Korean isolate clustered tightly within the B. ostreae clade alongside strains from Europe and North America, suggesting low genetic diversity and a possible recent introduction via human-mediated oyster movements. Histological examination and in situ hybridization (ISH) confirmed the presence of B. ostreae microcells localized within hemocytes and surrounding connective tissues. Despite successful detection, the infection intensity was low, and histopathological changes were mild, suggesting that O. denselamellosa may be relatively less susceptible to B. ostreae, though further investigation is needed to assess its role in the parasite's transmission dynamics. These findings highlight the need for enhanced molecular surveillance and international biosecurity measures, as this study represents the first confirmed report of B. ostreae infection in O. denselamellosa and the first record of this parasite in East Asia.