Lactococcus garvieae serotype III, a newly emerging pathogen that causes lactococcal infection, has been detected in fish farms since 2021. This pathogen has caused severe damage to the striped jack Pseudocaranx dentex and greater amberjack Seriola dumerili. This study collected strains from various fish species on farms between 2021 and 2023 and conducted an epidemiological and characterization study of L. garvieae serotype III using biochemical characterization tests, agglutinating titers against diagnostic antisera, biased sinusoidal field gel electrophoresis (BSFGE), and drug susceptibility tests. Capsule-like structures were found on the cell surfaces of L. garvieae serotype III strains, and strains showed similar characteristics except for the reaction of acidification ribose in biochemical characterization test using the API 20 STREP system. All strains were agglutinated with antiserum raised against L. garvieae serotype III and lower or non-agglutinated with antiserum raised against L. garvieae serotype I. BSFGE analysis identified three types of electrophoretic pattern: A, B, and C. BSFGE types A and B spread to fish farms in 2021, while type C found in 2022. All strains analyzed in this study were thought to be resistant to lincomycin but not to erythromycin.

We investigated the antibody dynamics at different growth stages of Japanese amberjack Seriola quinqueradiata to determine the timing of first IgM production. Vaccination trials of juveniles were then conducted using a formalin-inactivated Vibrio anguillarum serotype J-O-3 vaccine to characterize the changes in vaccine-specific IgM and efficacy at each growth stage. In whole-body analyses, although total IgM was slightly detected at 1 day post-hatching (dph), it was undetectable until 15 dph and then increased from 23 to 57 dph. Levels of IgM specific to V. anguillarum J-O-3 were significantly elevated in fish vaccinated at and after 60 dph. Relative percentage survival of vaccinated fish was 52.6% and 100% at 51 dph and ≥61 dph, respectively. These results suggest that the IgM response to the vaccine developed between 51 and 60 dph (body weight: 0.45 ± 0.04 to 1.15 ± 0.06 g; standard length: 2.68 ± 0.09 to 3.90 ± 0.08 cm), coinciding with the increase in vaccine efficacy. These data also suggest that development of the humoral immunity was delayed from the initiation of IgM production.

Isolates of Flavobacterium psychrophilum that produce brown pigments were obtained from ayu Plecoglossus altivelis at a farm in Hiroshima Prefecture, Japan in 2014 and 2015. In 2016, similar bacteria were isolated from ayu caught in a river in a different area. Eleven isolates producing brown pigments were obtained from both farmed and riverine ayu, and PCR-RFLP revealed the same A/S/QS/C genotype in all isolates. Infection experiments conducted with two isolates from farmed and riverine ayu revealed that both isolates had a similar LD₅₀ and relatively weak pathogenicity. ​However, the property of this brown pigment remains unknown and requires further investigation.

Host specificity of the parasitic copepod Salmincola markewitschi was examined in the Sapporo Salmon Museum in Hokkaido, Japan. Of the ten salmonid species we examined, seven were infected with S. markewitschi. Particularly, three species（white-spotted charr, Salvelinus leucomaenis leucomaenis; Nikko charr, Salvelinus leucomaenis pluvius; brook trout, Salvelinus fontinalis）had high infection levels with prevalence of 30.3–56.7%, suggesting that this copepod species utilizes Salvelinus spp. as primary hosts.