Abalone asfa-like virus (AbALV) has been strongly implicated as the causative agent of amyotrophia in Haliotis spp. However, investigations targeting AbALV in abalone hatcheries have been very limited. The present study continuously monitored this virus in a hatchery rearing black abalone H. discus discus. In surveys of 0-year-old juvenile abalone, amyotrophia with high mortality was confirmed from April to June or July, and copy number of AbALV in the foot muscle remained high during this period. From late July to August, AbALV copy number decreased along with a decline in mortality, supporting the conclusion that AbALV is the causative agent of amyotrophia. Additionally, the present study first detected AbALV in early February, followed by mortality in March, suggesting occurrence of amyotrophia in early spring around March. In the survey of 1-year-old abalone, AbALV copy number increased from September to December. Additionally, mortality and AbALV copy number were much lower than those in 0-year-old juveniles from April to July. During seed production in the hatchery, AbALV was frequently detected in wild broodstock. However, AbALV was not detected in fertilized eggs after washing, nor in swimming larvae that developed from those washed eggs.

In recent years, there have been many publications reporting mass mortality events and genotyping of carp edema virus (CEV) around the world, but molecular phylogenetic knowledge of CEV in Japan is scarce. Therefore, in this study, we performed a comprehensive survey to understand the prevalence and genotypes of CEV in koi fish farms in Japan. PCR was performed to amplify a portion of the p4a gene of CEV from 585 samples collected in 2021 across 96 aquaculture farms in 16 prefectures. The results showed that 62.5% of prefectures and 21.9% of fish farms were found to be positive for CEV. Overall, 42 samples were positive for CEV, and a total of 12 genotypes were detected by sequencing a portion of the p4a gene. Phylogenetic analysis revealed that all of the genotypes obtained in this study belonged to genogroup II. Of the 12 CEV genotypes found in Japan, 10 were identical to CEVs reported in other parts of the world. We speculate that the international koi trade has contributed to the spread of genogroup II. This study represents the first large-scale genetic survey of CEV in koi in Japan.

The mass mortality episodes of the Pacific oyster Crassostrea gigas larvae, which occurred in a Japanese hatchery between 2013 and 2014, were investigated bacteriologically. A total of 77 bacterial isolates were obtained from dead larvae, seawater from rearing tanks, and other sources. Immersion infection challenge experiments revealed that 34 of the 77 isolates were highly virulent towards C. gigas larvae, with the LD₅₀ of less than 10⁵ CFU/mL. These virulent isolates were identified as Vibrio coralliiyticus (n = 28), V. tubiashii (n = 3), and unknown Vibrio species (n = 3), based on PCR and partial sequence analyses of three genes (16S rRNA, recA, rpoA). This indicates that both V. coralliilyticus and V. tubiashii, which have long been known worldwide as important pathogens of hatchery-reared bivalve mollusk larvae, are involved in mass mortality of C. gigas larvae cultured in the hatchery. Furthermore, some virulent isolates from diseased oyster larvae in the same hatchery in 1997, which were previously identified as V. splendidus biovar II using conventional phenotypic characterization tests, were re-identified as V. coralliilyticus. These results imply that V. coralliilyticus is a major pathogen of C. gigas larvae in the hatchery.

Infectious hematopoietic necrosis virus (IHNV) was detected in female masu salmon Oncorhynchus masou broodstock in Hokkaido, Japan, in the fall of 2021 and 2022. In 2021, an outbreak of the disease occurred in yearling masu salmon at another nearby hatchery. We investigated the origin of the IHNV strains detected in 2021 and 2022 by genetic analysis and assessed the pathogenicity of the 2021 strain in fish ranging from fingerlings to juveniles weighing up to 50 g. The results showed that the strains detected in both years shared nearly identical nucleotide sequences and belonged to the JN lineage, which is prevalent in Honshu. Furthermore, the 2021 strain was found to be highly pathogenic even to juveniles over 10 g in weight. These findings revealed that a highly pathogenic strain, similar to those found in Honshu, has spread into rivers in Hokkaido. Until this virus is eradicated from Hokkaido, it is essential to implement strict measures such as treating wastewater during egg collection, disinfecting eggs and rearing equipment, and continuing surveillance for IHNV in rivers.

Piscine intestinal coccidia, including Cryptosporidium spp., are significant pathogens in marine aquaculture, particularly affecting juvenile fish and leading to high morbidity and mortality. However, the pathological effects of coccidiosis in pearl gentian grouper (Epinephelus lanceolatus ♂ × E. fuscoguttatus ♀) have not been thoroughly studied. This research aimed to explore the histopathological changes and inflammatory responses linked to oxidative stress-induced tissue damage in juvenile pearl gentian groupers reared in marine cage farming systems. A total of 44 intestinal samples from juvenile groupers were classified as infected and non-infected based on the presence of Cryptosporidium. Nucleotide sequencing and phylogenetic analysis of the partial 18S rRNA gene identified the Cryptosporidium species detected in this study as C. bollandi. Histopathological analysis and immunofluorescence techniques were used to assess reactive oxygen species (ROS) production, DNA damage, and cellular apoptosis. The findings showed a significant increase in inflammatory cell infiltration, ROS production, DNA damage, and cellular apoptosis in infected fish compared to uninfected ones. These results indicate notable pathological alterations associated with C. bollandi infection in juvenile pearl gentian groupers. This research emphasizes the importance of understanding the effects of intestinal cryptosporidiosis on juvenile fish, as these pathological changes contribute to the health challenges that marine aquaculture currently faces.

Lactococcus garvieae serotype III infections have been found in striped jack Pseudocaranx dentex since 2021. In the absence of a vaccine against serotype III, erythromycin (EM) is used to treat this infection. In 2024, five EM-resistant L.garvieae serotype III isolates were obtained from striped jack specimens. These isolates carried erm(B), an EM-resistance gene. The filter mating experiment confirmed that the EM-resistance trait was successfully transferred to L.formosensis as the recipient. All biased sinusoidal field gel electrophoresis (BSFGE) patterns of these strains were similar to the BSFGE type A of the previous study and coincided with each other.