Pacific oysters Crassostrea gigas infected with the ovarian protozoan parasite Marteilioides chungmuensis often develop ovary enlargement disease characterized by nodular-like ovaries, resulting in a loss of marketability due to their unattractive appearance. ​This disease was previously limited to oyster farms in western Japan, but recently a new occurrence was recognized in an open water oyster farm in eastern Hokkaido, the northernmost region of Japan. ​In this study, we developed a quantitative real-time PCR to detect the causative parasite and conducted an epidemiological survey in the oyster farm from April 2021 to March 2022. ​Prior to introduction to the farm, parasite DNA was detected in oysters originating from Miyagi Prefecture (Miyagi oysters), then infection of the parasite was histologically confirmed, suggesting that Miyagi oysters were already infected with the parasite before introduction to the oyster farm in Hokkaido, leading to the development of the disease. ​Conversely, in oysters produced in a local hatchery in Hokkaido (local oysters), parasite DNA was not detected prior to introduction to the farm, but detected in August and September after introduction. ​Although infection was not confirmed histologically in local oysters during the survey period, transmission of the parasite in the survey area needs to be further examined.

The susceptibility of larvae and juvenile koi carp (0–56 days post hatch; dph) against carp edema virus (CEV) was investigated by experimental infection with the immersion method using homogenized suspension from gill tissues of the diseased fish. ​Although no CEV mortality was occurred during the larval stage (0–7 dph), CEV-positive fish were found at all dph (positive rate 12–48%). ​In the juvenile stage (14–56 dph), the positive rate and mortality increased with dph. ​The positive rates and mortality were positively correlated with the total length of fish. ​The positive rates of the fish that were greater than 20 mm in total length exceeded 50% and the dead fish caused by CEV-infection were observed at 30 mm in total length or greater. ​These results indicate that the CEV susceptibility of larvae and juvenile koi carp increase with fish size rather than dph.

Viral endothelial cell necrosis of eel (VECNE) is a disease caused by the Japanese eel endothelial cells-infecting virus (JEECV) and significantly impacts the eel aquaculture in Japan. ​Although Japanese eel Anguilla japonica are often exposed to virus-contaminated water during aquaculture operations, the risk of infection via rearing water has not been well-studied. ​Using a waterborne challenge test, we showed JEECV could be transmitted via rearing water. ​Additionally, we examined the effects of water temperature on JEECV infection. ​The challenged eels started to die on day 18, and the cumulative mortality during the experiment was 32%, 15%, and 0% at 30°C, 32.5°C, and 35°C, respectively. ​Eels that died on day 18–20 showed high viral loads and typical symptoms of VECNE. ​Eels that survived the challenge test at 30°C and 32.5°C were also positive for JEECV, whereas no viral DNA was detected in eels at 35°C. ​The serum antibody titers against JEECV were high and moderate in the surviving eels at 30°C and 32.5°C, respectively. ​The titers in those at 35°C were low and not significantly different from those in the uninfected control. ​These imply that JEECV remains infectious outside of a host and that high water temperatures of approximately 35°C prevent its transmission.

Mycobacteriosis caused by several mycobacterial species is often found in indoor pools where yearling koi carp are reared during the winter season in Niigata, Japan. ​In this study, we isolated several mycobacterial species from juvenile fish cultured in earthen ponds in August and October 2019. ​Further, antibody titers against two major causative species (Mycobacterium paragordonae and Mycolicibacterium sp. WS-type) rose not only in infected fish but in some individuals from which no bacterium was isolated. ​These data suggest that juvenile carp are infected in earthen ponds by autumn, the disease progresses, and subsequently the fish mortality occurs from winter to the next spring.

In this study, we investigated the influence of body-size and temperature on the susceptibility of carp Cyprinus carpio to the new ulcer disease caused by atypical Aeromonas salmonicida. ​We observed that 4–7 cm long carp, weighing 1.6–8.6 g, exhibited similar susceptibility to the disease. ​Furthermore, A. salmonicida cultured at 25°C demonstrated significantly reduced virulence compared to the bacterium cultured at 20°C. ​Additionally, a higher disease occurrence rate was noted at a water temperature of 25°C during infection, with faster symptom recovery.