Baseline Assessment of WOAH-Listed Protozoan Parasites in Wild Mediterranean Mussels Mytilus galloprovincialis and Pacific Oysters Crassostrea gigas from Port-Adjacent Coastal Waters of Korea in 2023

Systematic surveillance of World Organization for Animal Health (WOAH)-listed protozoan parasites is essential for maintaining the sanitary status of seafood exports and detecting the introduction of exotic pathogens into coastal ecosystems. In 2023, we examined wild Mediterranean mussels Mytilus galloprovincialis and Pacific oysters Crassostrea gigas collected from small harbors adjacent to ten major trading ports along the west and south coasts of Korea to assess the occurrence of WOAH-listed protozoan parasites and emerging Perkinsus species. A total of 1080 mussels and 1080 oysters from 18 sites were sampled in spring and autumn, and gill and digestive gland tissues were pooled from six individuals for DNA extraction. Species-specific PCR assays targeting Perkinsus marinus, P. olseni, P. beihaiensis, Bonamia ostreae, B. exitiosa, and Marteilia refringens were performed using previously validated primer sets and positive controls. All PCR assays were negative for the six protozoan parasite species in both host species across all sampling sites, indicating no detectable infections in port-adjacent wild mussel and oyster populations during the survey period. These negative results contrast with recent reports of P. marinus in wild C. gigas and B. ostreae in Ostrea denselamellosa on the west coast of Korea, suggesting that infections may currently be focal, transient, and host-specific rather than widespread in port-associated M. galloprovincialis and C. gigas populations. The present study provides baseline data on the distribution of protozoan parasites in bivalves inhabiting high-risk harbor environments and underscores the need for continued surveillance of transboundary shellfish diseases that is closely coupled with environmental monitoring.