In this study, we assessed the influence of different water temperatures on the swimming ability of the oncomiracidium of Neobenedenia girellae. ​In addition, we investigated the temporal change in their infectivity to Seriola dumerili after hatching at 25°C. ​Most oncomiracidia in the cell culture flask filled with seawater deciliated and predominantly crept on the bottom by the end of 6 h experiment. ​Approximately 9% of the oncomiracidia in seawater-containing wells of a 12-well cell culture plate were non-swimming and became trapped in the air-water surface interface. ​However, these effects were mitigated when a polyethylene sheet was placed on the seawater surface in the wells. ​When conducting the experiment using this method, the oncomiracidia remained active and were still swimming at 60, 72, and 108 h after hatching at 30°C, 25°C, and 20°C, respectively. ​The retention time of the oncomiracidia's swimming ability decreased as the water temperature increased. ​The oncomiracidial infectivity was reduced after 24–25.5 h or more after hatching. ​At 5 days after the exposure to S. dumerili, the number of attached N. girellae was significantly lower, and their body length was significantly shorter when using oncomiracidia that were 36–37.5 h after hatching compared to freshly hatched ones.

Flavobacterium psychrophilum can infect rainbow trout Oncorhynchus mykiss and ayu Plecoglossus altivelis, detrimentally affecting global aquaculture. ​This study aimed to understand the infectivity and host specificity of two F. psychrophilum strains, SG950607 and PH0424, which were isolated from the kidneys of rainbow trout and ayu, respectively. ​Rainbow trout (fry) and ayu (adult and fry) were infected via two methods: bath immersion and intraperitoneal injection. ​We investigated the ability of each F. psychrophilum strain to survive in the serum of non-original host fish species. ​Rainbow trout died upon being infected by PH0424. ​In addition, the complement of the rainbow trout did not show a bactericidal effect against PH0424. ​On the other hand, ayu infected with SG950607 survived, and their complement eliminated this strain. ​The difference in mortality rates is likely associated with the susceptibility of each strain to the complement activity possessed by the host fish.

To reduce losses induced by infectious hematopoietic necrosis (IHN), selective breeding of rainbow trout Oncorhynchus mykiss strains was conducted using artificial infection with the IHN virus (IHNV) TK8901, which was isolated in 1989. ​The cumulative mortality rate of the artificially infected rainbow trout strain decreased after successive rounds of selective breeding, reaching less than 5% from the 8th generation of selection. ​The cumulative mortality rate of the 9th generation of this strain infected with IHNV isolated in Tokyo in 2016 was significantly lower than that of the unselected strain. ​These results indicate that, after the 8th generation, rainbow trout strains that have survived from artificial infection have become resistant to IHNV infection.

Susceptibilities of three hybrids between crucian carp including Carassius cuvieri (Ccu), Carassius buergeri grandoculis (Cbg) and Carassius buergeri subsp.1 (Cbs) and common carp Cyprinus carpio (Cc) to KHV were evaluated in an infection experiment. ​Infection rates of KHV in each hybrid were 30% and 20% in Ccu × Cc, 40% and 30% in Cbg × Cc, and 10% and 0% in Cbs × Cc, at 14 and 44 dpe, respectively. ​This indicates that Cbs × Cc shows lower sensitivity to KHV than the other two hybrids. ​More importantly, Ccu × Cc and Cbg × Cc transmitted the virus to co-habited naïve koi.