Cryptocaryon irritans is an obligate parasitic ciliate of marine teleosts and is the causative agent of cryptocaryoniasis (marine white spot disease). ​The parasite infects the skin and gills of marine teleosts and often causes mass mortality in heavily infected fish. ​Therefore, this parasite is a major threat to marine aquaculture in tropical and subtropical waters, and effective control methods are required. ​Interestingly, it was reported that fish infected with the parasite acquired protective immunity against it, and it was suggested that a vaccine strategy could be realized. ​Since then, many researchers have attempted to develop vaccines against cryptocaryoniasis. ​This review presents the current research on vaccine development against cryptocaryoniasis to provide references for future experimental studies and propose future research directions.

Aquareovirus infection by hirame aquareovirus (HAqRV) is fatal in hatchery-reared Japanese flounder Paralichthys olivaceus. ​In the present study, we investigated the transmission route of HAqRV in order to develop outbreak prevention measures. ​HAqRV was isolated from six cases of Japanese founder juveniles from different locations in 2015 and 2016 and compared by molecular epidemiology. ​For three of these cases, apparently healthy broodstock that were used for Japanese flounder production were subjected to virus carrier tests using real-time PCR and ELISA targeting the viral genome and antibody against HAqRV, respectively. ​The viral genome was detected in the intestine and/or liver, with a detection rate of approximately 60% (n = 143). ​For the ELISA test, antibodies against HAqRV were detected in 100% of the examined broodstock (n = 109). ​Sequencing analysis of the RNA-dependent RNA polymerase gene indicated that the nucleotide sequences of the isolated virus, diseased juveniles, and apparently healthy adults were identical within the same case, but distinct among different cases. ​These results suggest that asymptomatically infected broodstock are a potential infection source of HAqRV outbreaks in Japanese flounder juveniles.

In 2019, a large number of microsporidian xenomas was confirmed in the gills of yamame trout (landlocked masu salmon), Oncorhynchus masou masou, cultured in Tokyo. ​The spore size was similar to that of the previously reported Loma salmonae, and the nucleotide sequence of the ribosomal RNA gene (1,839 bp) showed 99.9% similarity with that of L. salmonae from Oncorhynchus spp. in North America. ​Based on the morphological and molecular analyses, the microsporidian from yamame trout was identified as L. salmonae. ​During October 2019 to July 2021, four cases of L. salmonae infection in yamame trout were confirmed at three fish farms in Tokyo. ​Loma salmonae is the causative agent of microsporidial gill disease in salmonids, causing economic losses in salmonid aquaculture in North America and Europe. ​In Japan, Loma sp. was once reported from O. masou masou in Hokkaido, but it was not identified at the species level. ​This is the first confirmed report of L. salmonae infections in Japan, and also its new host record from O. masou masou.

The location Edwardsiella anguillarum causing edwardsiellosis in red sea bream Pagrus major enters the fish is hypothesized to be the nasal cavity because ulcers form on the head. ​To test this hypothesis, we conducted two infection experiments on red sea bream. ​Fish were inoculated with a bacterial suspension in each nasal cavity, the cumulative mortality plateaued at 60–70%. ​Fish had its nasal cavities obstructed were challenged by immersion infection, the cumulative mortality was significantly lower than control group. ​These results strongly suggest that the nasal cavity of fish is an important entry site for E. anguillarum.