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.

2019年に東京都内で養殖ヤマメの魚病診断を行ったところ，鰓に微胞子虫のキセノマが多数確認された。胞子の形態観察と計測を行ったところ，既報のLoma salmonaeのものと概ね一致した。また，リボソームDNAの遺伝子解析では 1,839 bpの塩基配列が得られ，北米のOncorhynchus spp.から得られたL. salmonaeの登録配列と99.9％一致した。以上の結果から，本研究では本微胞子虫をL. salmonaeと同定した。また，2019年10月～2021年7月までに東京都内の3養魚場でL. salmonaeの寄生が4例確認された。Loma salmonaeは北米およびヨーロッパのサケ科魚類養殖で被害をもたらしている。国内においては北海道で，Loma sp.の寄生報告が1例あるが，種同定されたのは初である。また，陸封型のヤマメを含めたサクラマスで寄生が確認されたのは世界初であり，ここに報告を行う。

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.