Amberjacks of the genus Seriola (Japanese amberjack S. quinqueradiata, greater amberjack S. dumerili and yellowtail amberjack S. aureovittata) are most important fish resources, caught and cultured in Japan. ​Eight species of parasites are known to infect the skeletal muscle of Japanese Seriola spp.: the microsporidian Microsporidium seriolae, the myxozoans Kudoa amamiensis, Kudoa megacapsula and Unicapsula seriolae, the digenean Hirudinella ventricosa, the cestode Protogrillotia zerbiae and the nematodes Philometroides seriolae and Anisakis pegreffii. ​They induce detrimental effects on the quality of the muscle tissue and lower marketable value due to anesthetic appearances. ​Additionally, U. seriolae and A. pegreffii may cause food poisoning and zoonosis, respectively. ​In this paper, literature of these parasites from wild and cultured Seriola spp. is reviewed, and current status of their studies and issues are discussed.

Lactococcus garvieaeⅠ型およびⅡ型を含む市販の混合ワクチンを投与した宮崎県の養殖カンパチで，2021年8月からL. garvieae感染症に類似した症状を呈する疾病が確認されるようになった。分離された菌株は，L. garvieae診断用抗Ⅰ型およびⅡ型血清に対して非凝集性を示し，多遺伝子座配列解析（MLSA）においてL. garvieaeⅠ型に近く，Ⅱ型とは遺伝学的相違性が認められた。同菌株を用いたカンパチに対する腹腔内接種による感染試験では，半数致死濃度は 1.6×10³ CFU/FISH未満であった。また，Ⅰ型およびⅡ型の混合ワクチンを接種したカンパチにおいて，今回分離した菌株に対する感染防御効果は認められなかった。一方，今回分離した菌株のホルマリン不活化ワクチンは，同菌株の感染に対して高い防御効果が確認された。以上の結果から，今回分離された従来のⅠ型およびⅡ型診断用抗血清に凝集せず，Ⅰ型およびⅡ型ワクチンの効果がない菌株をL. garvieaeⅢ型と呼称することを提案する。

Flavobacterium psychrophilum, the causative agent of bacterial cold-water disease, was first reported in spawning chum salmon Oncorhynchus keta in Hokkaido in 2005. ​In the present study, Hokkaido was divided into five regions by chum salmon stock, and the distribution of this bacteria in spawning chum salmon and the other three species of salmonid fish that return to rivers in each region​—masu salmon O. masou, pink salmon O. gorbuscha, and sockeye salmon O. nerka—was surveyed over a three-year period from 2006 to 2008. ​Samples were collected from the ovarian fluid and kidneys of female salmon, and in some rivers, milt from males was also sampled. ​F. psychrophilum was detected in all rivers surveyed in all regions, and in males and females of all four spawning salmon species. ​The detection rates were high in many rivers (> 70%). ​These results confirm that the pathogen is widespread in Hokkaido regardless of stock, species, or sex of salmons. ​Furthermore, PCR-RFLP genotyping of pathogen isolates from chum salmon revealed multiple genotypes in some rivers. ​In these rivers, the pathogen was thought to have invaded early and differentiated, or to have invaded as multiple strains with different genotypes.

In Japan, Bacterial cold-water disease (BCWD) is an economically important fish disease, occurring in ayu Plecoglossus altivelis altivelis not only at aquaculture farms but also in rivers. ​In this study, an epizootiological study of BCWD was conducted to understand the route of infection in ayu and the source of the causative agent, Flavobacterium psychrophilum in rivers. ​We used a genotyping system with a combination of four PCR-restriction fragment length polymorphisms (PCR-RFLP). ​For five years from 2014 to 2018, various fish, mainly ayu, were caught and sampled in the Kanna River, Gunma Prefecture, Japan. ​A total of 852 isolates of F. psychrophilum from these fish species were used as PCR-RFLP genotyping samples. ​Nine PCR-RFLP genotypes were identified among these isolates, some of which were the typical epizootic types for ayu in this river, while species-specific genotypes exist in other fish species. ​In addition, fish species other than ayu, decoy ayu, and land-locked ayu were suspected as reservoirs of epizootic F. psychrophilum genotypes for ayu in the Kanna River. ​These results provide the basis for an epizootiological assessment of the transmission dynamics of ayu BCWD not only in the Kanna River but also in rivers in general.

An outbreak of ectoparasitic copepods occurred on the Japanese rice fish Oryzias latipes stocked in an outdoor tank for the extraterritorial conservation program. ​Only the adult female copepods were found on the skin and fins of the host fish. ​The parasite was identified as Neoergasilus japonicus from the morphological characters including an inflated projection on the second segment of the exopod of the first swimming leg, and the single-segmented exopod and endopod of the fourth leg. ​Partial sequence of 28S rDNA and cox1 genes also supported the morphological identification. ​This is a new host record for N. japonicus. ​A bath treatment with 1% salt concentration was effective in detaching the parasite within a day.

Edwardsiella bacteria infect Japanese medaka Oryzias latipes; however, the difference in susceptibility between E. piscicida and E. anguillarum is unknown. ​In this study, we performed challenge tests with medaka using four Edwardsiella strains, which were difficult to be identified by specific primers. ​The four strains were reclassified as three E. piscicida strains and one E. anguillarum strain using phylogenetic analysis. ​The immersion challenge demonstrated that medaka were more susceptible to E. piscicida than to E. anguillarum. ​Increases in the bacterial copy number in the infected medaka kidneys were correlated with the worsening of the external symptoms. ​Medaka is considered a valuable model organism for understanding the etiology of edwardsiellosis.

Wild, one-year yellowtail amberjack Seriola aureovittata (n = 13) were examined for parasites after about one month culture in Nagasaki and Kagoshima Prefectures, Japan. ​All fish were infected with the microsporidian Microsporidium seriolae (all qPCR positive; 11 with cysts in the skeletal muscle, four with cysts in the heart as well). ​Considering fish size, culture period, and time required for cyst formation, it is assumed that the fish had been infected with the microsporidian in the wild. ​Additionally, two fish were infected with a nematode in the skeletal muscle, tentatively identified as Philometroides seriolae, which had been recorded only from S. quinqueradiata.

魚病問題の解決のためには，国内外の防疫体制の継続的な見直しと改善が不可欠です。しかしながら，我が国の防疫体制は約8年間にわたって見直しが行われておりません。そこで，平成26–27年に開催された水産防疫専門家会議メンバーを加えた魚病専門家の有志で検討し，農林水産大臣あての「水産防疫体制の改善と見直しに関する提言」を取りまとめました。提言とその説明資料は，2023年10月12日に農林水産省において高橋光男農林水産政務官に手交いたしました。以下にその全文をお知らせいたします。