Bacterial cold-water disease (BCWD), caused by Flavobacterium psychrophilum, is a major infectious disease of ayu Plecoglossus altivelis in Japan. In this study, serological and multiplex PCR (mPCR) analyses were conducted on 11 isolates from BCWD-affected ayu. The results revealed the coexistence of two distinct types, Type-1 and Type-3 in ayu. Type-1 strains, previously reported only in salmonids, were identified in ayu for the first time. The PCR-RFLP typing pattern of the Type-1 strain isolated from ayu corresponded to the A/S/QS/C types commonly detected among ayu isolates in previous studies, suggesting that infection originating from other fish species is unlikely. Vaccine efficacy was evaluated using formalin-killed cell (FKC) preparations. Monovalent vaccines provided strong protection against homologous strains but showed limited protection against heterologous strains, indicating serotype-specific immunity and limited cross-protection. In contrast, a bivalent vaccine containing both Type-1 and Type-3 strains conferred high protection against both types. These findings demonstrate that multiple mPCR types of F. psychrophilum are involved in BCWD in ayu and highlight the importance of multivalent vaccines for effective disease control.

A strain of Azumanishiki variety of goldfish Carassius auratus resistant to herpesvirus hematopoietic necrosis, caused by cyprinid herpesvirus 2 (CyHV-2), has been established. In this study, we investigated virus kinetics and histopathological changes in resistant Azumanishiki and susceptible goldfish after experimental infection via intraperitoneal (IP) injection or immersion. Following experimental challenge, the resistant strain exhibited low mortality even after IP injection with a virus dose 10,000-fold higher than the lethal dose for susceptible fish. In the immersion-challenged susceptible fish, the virus DNA loads reached over 10⁷ copies/mg of hematopoietic tissue at 5 days post-infection (dpi), whereas in the resistant fish, the virus DNA level peaked at approximately 5 dpi at less than 10⁶ DNA copies/mg. Histopathological analysis revealed a fundamental divergence in cellular responses: the susceptible fish exhibited characteristic infected cells with enlarged nuclei and chromatin marginalization, followed by diffuse necrosis in hematopoietic tissues. In contrast, the resistant strain showed only small infection foci with less pronounced nuclear enlargement, followed by marked leukocyte infiltration at the foci and subsequent tissue recovery, with no diffuse necrotic lesions observed. These findings suggest that both the suppression of virus replication and the subsequent immune response play a role in the mechanisms underlying CyHV-2 resistance.

Edwardsiellosis in red seabream caused by Edwardsiella anguillarum results in economic losses. Fish were experimentally infected under natural temperature conditions (21.4–27.3°C), reaching a cumulative mortality of 26.5% at 134 days. Survivors retained the bacterium for >100 days, with re-isolation rates of 60–90%, suggesting long-term persistence and their potential role as reservoirs in aquaculture systems. Four reinfection trials were conducted at 28°C between days 29 and 134 post initial experimental infection. In all trials except one, survival was significantly higher in persistently infected fish than in naïve fish, suggesting resistance to reinfection.

We developed a protocol for epidemiological analysis of infectious hematopoietic necrosis virus (IHNV) that includes virus isolation from dead fish using limiting dilution titration on EPC cells, followed by RT-PCR and sequencing targeting partial regions of the nucleocapsid protein and glycoprotein genes. Application of this protocol to a spontaneous IHN-associated outbreak in rainbow trout fry in 2019 revealed that multiple IHNV strains belonging to two lineages (Nagano and North Kanto) were involved and appeared to originate from two upstream farms. These findings suggest that IHN outbreaks in farms supplied with river water can involve the simultaneous circulation of multiple virus strains rather than simple transmission of a single strain within a fish population.

In 2016, Lactococcus formosensis M-T2 was isolated from a farmed greater amberjack Seriola dumerili. Electron microscopy revealed the absence of capsule-like structures in M-T2. Infection and vaccination trials in greater amberjack showed that M-T2 was avirulent and that a formalin-killed vaccine prepared from M-T2 failed to provide protection against either L. garvieae or L. formosensis. Comparative genomic analysis between the virulent L. formosensis strain 122061 and M-T2 revealed differences at 23 loci, including a nonsense mutation in a glycerophosphotransferase gene potentially involved in capsular polysaccharide synthesis. Differences in plasmid composition were also observed among strains. These findings are expected to contribute to the identification of virulence-associated factors and vaccine antigens in L. formosensis.