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Outbreaks of oyster diseases caused by the World Organization for Animal Health (OIE)-listed protozoan pathogens have been a global concern for several decades. With the wide application of species-specific and sensitive polymerase chain reaction (PCR) techniques, it is now well known that even the Pacific oyster, insusceptible to Perkinsus marinus, Bonamia ostreae, Bonamia exitiosa could act as a carrier host. Furthermore, one of the transmission pathways of oyster pathogens includes anthropogenic activities, such as the discharge of ballast water or hull attachment during international maritime shipping. In this survey, OIE-listed pathogens, including B. ostreae, B. exitiosa, and Marteilia refringens were screened in the Pacific oysters growing in small bays off the south coast of Korea using PCR with OIE-recommended species- or genus-specific (Bonamia) primers. The survey sampled 1980 oysters from 35 sites in small bays in the south, where more than 95% of the national productions are yielded. The PCR screening indicated that none of the oysters screened in this study were positive for B. ostreae, B. exitiosa, P. marinus, and M. refringens. This study first investigated the OIE-listed protozoan pathogens in the Pacific oysters culturing in Korea, and the results indicated that the Pacific oysters raised on the south coast of Korea are free from those OIE-listed pathogens. For secure and sustainable oyster farming on the south coast, however, regular monitoring of such invasive oyster pathogens is recommended, as the international maritime trade activities are increasing in Korea. KEY WORDS: Crassostrea gigas, OIE-listed protozoan parasite, PCR assay, Korea, INTRODUCTION Oysters have been one of the essential global aquaculture resources, contributing 10.9% of the world production in 2020 (FAO 2022). Despite its rapid growth in the world aquaculture, there [...]

Simple Summary: Acute hepatopancreatic necrosis disease (AHPND) is a plasmid-encoded PirA/B toxin mediated bacterial disease that causes significant economic losses to the global shrimp industry. Since the first AHPND outbreak of Penaeus vannamei cultures in Korea in 2016, AHPND-associated Vibrio parahaemolyticus (VpAHPND) is still considered a major threat to the Korean shrimp industry. Although several diagnostic methods have been developed against AHPND, several concerns have been raised about the misdiagnosis of the pathogen due to the transferability and mutation of the plasmid. In this study, we report the emergence of a natural mutant VpAHPND (pirA−, pirB+), which was isolated from cultured P. vannamei in Korea, and also provide its detailed genomic and pathological characteristics. Acute hepatopancreatic necrosis disease (AHPND) is one of the most important diseases in the global shrimp industry. The emergence of mutant AHPND-associated V. parahaemolyticus (VpAHPND) strains has raised concerns regarding potential misdiagnosis and unforeseen pathogenicity. In this study, we report the first emergence of a type II (pirA−, pirB+) natural mutant, VpAHPND (strain 20-082A3), isolated from cultured Penaeus vannamei in Korea. Phenotypic and genetic analyses revealed a close relationship between the mutant strain 20-082A3 and the virulent Korean VpAHPND strain 19-021-D1, which caused an outbreak in 2019. Detailed sequence analysis of AHPND-associated plasmids showed that plasmid pVp_20-082A3B in strain 20-082A3 was almost identical (>99.9%) to that of strain 19-021-D1. Moreover, strains 20-082A3 and 19-021-D1 exhibited the same multilocus sequence type (ST 413) and serotype (O1:Un-typeable K-serogroup), suggesting that the mutant strain is closely related to and may have originated from the virulent strain 19-021-D1. Similar to previous reports on the natural mutant VpAHPND, strain 20-082A3 did not induce AHPND-related symptoms or cause mortality in the shrimp bioassay. The emergence of a mutant strain which is almost identical to the virulent VpAHPND highlights the need for surveillance of the pathogen prevalent in Korea. Further investigations to elucidate the potential relationship between ST 413 and recent Korean VpAHPND isolates are needed. [ABSTRACT FROM AUTHOR]

Background: Salinomycin, an antibiotic, have potential as a veterinary drug for fish due to its anti-parasitic activity against several fish parasites. Thus the residual levels of salinomycin in muscles of two significant aquaculture species in Korea, olive flounder and black rockfish, were analyzed using HPLC-MS-MS. Results: The proper method to analyze the residual salinomycin in fish muscles using LC-MS-MS was settled and the method was validated according to CODEX guidelines. The residues in three distinct groups for two fish species were analyzed using the matrix match calibration curves at points of five different times following oral administration. After oral administration, salinomycin rapidly breaks down in both olive flounder and black rockfish. After 7th days, the average residue in all groups of two fish spp. decreased below limit of quantitation (LOQ). Conclusion: Due to low residue levels in fish muscles, salinomycin may therefore be a treatment that is safe for both fish and humans. This result could contribute to establishment of MRL (minimal residual limit) for approval of salinomycin for use in aquaculture. [ABSTRACT FROM AUTHOR]

White tail disease (WTD) is caused by the Macrobrachium rosenbergii nodavirus (MrNV) and an extra-small virus (XSV). MrNV belongs to the Nodaviridae family. While the role of XSV in the pathogenicity of WTD remains unclear, MrNV is considered to be a significant factor in the disease. To study WTD infection in giant freshwater prawns (Macrobrachium rosenbergii), adult and post-larval (PL) prawns were collected from three giant freshwater prawn farms in Gyeongsangnam-do, Korea in 2021. Although the adult and PL prawns did not display any gross signs of WTD, MrNV was detected in both adult and PL in this study. However, XSV was not detected in both prawns. Phylogenetic analysis revealed that the capsid protein gene sequences of MrNV obtained in this study were robustly clustered with the MrNV group, and were clearly distinguished from Alphanodavirus and Betanodavirus groups of the family Nodaviridae. Although Zenker's necrosis and myolysis were observed histopathologically in the abdominal striated muscle of adult and PL prawns, no gross signs associated with white tail were observed because of local lesions. [ABSTRACT FROM AUTHOR]