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.

EHP causes hepatopancreatic microsporidiosis in shrimp, leading to growth retardation without notable mortality. To examine potential environmental carriers, farmed bivalves were collected near shrimp ponds in Korea and screened for EHP using nested PCR targeting the spore-wall-protein gene. DNA of EHP was mainly detected in gill and digestive tissues of oysters, scallops, mussels, and clams. Histopathology after 24 h immersion exposure confirmed spores within the intestinal epithelium of clams, indicating short-term invasion potential. These results suggest that bivalves can retain and mechanically transfer EHP spores through shared seawater between shrimp farms and surrounding areas. Considering the environmental persistence of EHP, continuous surveillance of local invertebrates is recommended to reduce the risk of EHP introduction and further spread in aquaculture systems.

Systematic surveillance of World Organization for Animal Health (WOAH)-listed protozoan parasites is essential for maintaining sanitary status for seafood export 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 1,080 mussels and 1,080 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 disease that is closely coupled with environmental monitoring.

In pond-based shrimp culture, seawater exchange may introduce white spot syndrome virus (WSSV) from coastal waters, but infection risk at a given seawater viral concentration may vary with temperature, stocking density, and exposure duration. This study evaluated waterborne WSSV susceptibility in whiteleg shrimp (Penaeus vannamei) under these conditions. Shrimp were reared at 25 or 30 °C and at low (70), medium (140), or high (210 shrimp/m²) stocking densities; physiological responses were assessed, followed by short-term and continuous immersion challenges and a cohabitation challenge. Higher stocking-density conditions were associated with stress-related physiological changes and greater WSSV susceptibility. Under short-term exposure, 10⁷ WSSV genome copies/L caused infection only in the high-density group at 25 °C. Under continuous exposure, the minimum infective dose (MID) values were 10⁵ and 10⁴ WSSV genome copies/L at low and high stocking densities at 25 °C, and 10⁶ and 10⁵ WSSV genome copies/L, respectively, at 30 °C. Cohabitation transmission occurred at seawater viral loads below those associated with short-term immersion infection. By linking seawater viral concentrations with experimentally defined MID values and cohabitation-transmission outcomes, these results indicate that waterborne WSSV transmission risk should be assessed in relation to viral concentration, exposure duration, water temperature, and stocking-density conditions.