Edwardsiellosis in the red seabream Pagrus major is one of the most significant bacterial infections in Japanese marine aquaculture. In this study, we investigated whether fish carcasses harboring Edwardsiella anguillarum (the causative agent of edwardsiellosis) serve as a source of infection using three experimental infection trials. In Experiment 1, healthy red seabream exhibited scavenging behavior on infected carcasses, resulting in increased E. anguillarum concentrations (viable counts and environmental DNA) in the rearing water and the bacterial detection in the intestines. This bacterium was subsequently isolated from the trunk kidney of fish in the same tank, and the fish developed edwardsiellosis. In Experiment 2, fish were infected with E. anguillarum through cohabitation with infected carcasses. Higher infection ratios were observed in the group with scavenging access to the carcasses than in the group without. In Experiment 3, oral administration of either an E. anguillarum suspension or a kidney homogenate from diseased fish successfully induced infection. In conclusion, infected fish carcasses may serve as a reservoir of E. anguillarum, with scavenging behavior playing a critical role in edwardsiellosis transmission in red seabream farms by contributing to both the release of the bacterium into surrounding water and the oral consumption of infected tissues.

The innate antiviral response through the RIG-1-like receptor pathway is oriented towards a key regulatory molecule, mitochondrial antiviral signaling protein (MAVS). ​Localized to the mitochondria, MAVS coordinates the interferons (IFN) production through regulated signaling cascades. ​Protein-protein interaction, post-translational modifications, and mitochondrial dynamics considerably regulate MAVS signaling on the mitochondrial membrane. ​This review gives insight into the inevitable role of MAVS in the innate antiviral immune response of fish.

日本の海産魚類養殖におけるLactococcus感染症の原因細菌は血清型および遺伝学的解析によりLactococcus garvieae Ⅰ型，L. formosensis (従来のL. garvieae Ⅱ型) およびL. garvieae Ⅲ型に分類される。各型の判別法として用いられる抗血清によるスライド凝集試験では，複数の抗血清に凝集を示す株の型判別が困難であり，従来のPCRでは，Ⅰ型レンサ球菌とⅢ型レンサ球菌を判別できなかった。本研究では，各型に特異的な遺伝子配列からプライマーを設計し，マルチプレックスPCRによる判別法の開発を試みた。マルチプレックスPCRを実施した結果，各型から予想サイズの増副産物が確認され，海産魚類以外を由来とするLactococcus属細菌からは増副産物は得られなかった。本マルチプレックスPCRは，日本で発生しているレンサ球菌の各型の判別法として有効であることが示された。

This study aimed to elucidate the genetic characteristics of lincomycin (LCM) resistance in the absence of erythromycin (EM) resistance in fish pathogenic Lactococcus garvieae serotype I. ​Two novel lsa(D) variants were found in clinically LCM-sensitive and resistant strains. ​The amino acid sequences of the two Lsa(D) variants shared 94.77% identity with that of Lsa(D) in L. garvieae serotype II. ​One of the lsa(D) variants, designated as lsa(D)36A, conferred cross-resistance to lincosamides, streptogramins A, and pleuromutilin, which was defined as the LSAP-resistant phenotype. ​This phenotype was confirmed by the minimum inhibitory concentrations of the lsa(D)36A-disruption mutant Δlsa(D)36A, and its complementation strain. ​However, a single-point mutation, which led to an amino acid substitution, was found in lsa(D)36D in clinically LSAP-sensitive strains. ​In addition, single-nucleotide replacement of lsa(D)36D alleles was identified in laboratory-induced LCM-resistant mutants. ​Meanwhile, the lsa(D)36D or lsa(D)36A was also detected in EM-resistant strains carrying erm(B). ​These strains carried lsa(D)36D with erm(B) or lsa(D)36A with erm(B), which conferred the EM- and LCM-resistant (ML-resistant) or MLSAP-resistant phenotype, respectively. ​In conclusion, lsa(D)36D was an intrinsic in the LSAP-sensitive L. garvieae serotype I, and its mutated allelic variant was named as lsa(D)36A, which explains the characteristic resistance of this strain to the LSAP-resistance phenotype.

A myxozoan parasite Kudoa septempunctata causing the food poisoning of humans which consume raw olive flounder Paralichthys olivaceus has become a new threat to public health. ​This review focuses the recent occurrence of K. septempunctata-induced food poisoning in Japan for 10 years, the typical symptoms of the foodborne disease in humans, parasitology, diagnoses, countermeasures and the remaining issues. ​The numbers of K. septempunctata-induced cases and patients have peaked in 2014, followed by a gradual decrease in recent years. ​The symptoms of humans are characterized by transient vomiting and diarrhea. ​Also, onset of the disease is dose-dependent of fresh spores (> 1 × 107). ​The host fish for K. septempunctata is mainly limited to P. olivaceus. ​The geographical distribution is Japan and Korea. ​The two locality isolates are easily differentiated by a ratio of spores possessing 7 polar capsules. ​The light-microscopy, conventional or real-time PCRs, LAMP, immunochromatography have been developed for the diagnostic tools. ​To prevent the infection with K. septempunctata in juvenile stage of cultured flounder, a sand-filtration combined with ultraviolet irradiation (> 46 mJ/cm2) is effective. ​At the time of shipment of adult flounder, light-microscopic examination for spores is recommended to remove the infected fish group. ​The remaining problems include the disease mechanisms in humans, the host range, and the harmful Kudoa species other than K. septempunctata.

The present study investigated the genotype of Ichthyobodo spp. that caused six cases of ichthyobodosis in various salmonid species cultured in freshwater, in Hokkaido, Japan, through molecular phylogenetic analysis of the parasite ribosomal RNA gene (rDNA). ​Polymerase chain reaction (PCR) and scanning electron microscopy (SEM) were performed to distinguish the genotypically differentiated Ichthyobodo spp. ​The parasites were identified as genotypes I (I. necator sensu stricto), II (I. salmonis), or III (Ichthyobodo sp.). ​The present PCR method could completely distinguish between genotypes I, II and III using the electrophoresis pattern of PCR products from the parasite rDNA. ​Our SEM observations were able to SEM observations were able to morphologically distinguish genotype I from other two genotypes, but not between genotype II and III.

Mycobacteriosis causes serious economic damages to yellowtail aquaculture in Japan. ​From October to November 2020, mycobacteriosis occurred in yellowtail cultured in Owase Bay, Mie Prefecture, Japan. ​In this study, we observed the gross pathology and histopathology of the diseased fish. ​We identified mycobacteria isolated from the diseased fish and investigated their drug sensitivities against 12 antimicrobial agents. ​Diseased yellowtail exhibited abdominal swelling and many nodules in their kidney and spleen. ​The head kidney and trunk kidney presented typical granulomas with a necrotic core surrounded by epithelioid cells and fibroblasts. ​The isolates were identified as Mycobacterium pseudoshottsii based on phylogenetic analysis using the 16S rRNA gene, hsp65 and rpoB. ​Low concentrations of amikacin, ciprofloxacin, kanamycin and lincomycin inhibited growth of the isolates. ​Two of three isolates showed sensitivity to erythromycin; the other isolate was resistant to it. ​Erythromycin and lincomycin that are permitted for therapeutic use in Perciformes in Japan may be useful for treating yellowtail infected with M. pseudoshottsii. ​However, emergence of antibiotic-resistant strains of M. pseudoshottsii due to antibiotic use requires attention in yellowtail aquaculture.

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.

ブリ属魚類のべこ病に対するフェバンテル (FBT) の経口投与による効果を養殖場での野外試験により調べた。日間投与量 1~10 mg/kg bwのFBTをシスト形成前のブリ稚魚に5日間連続投与した結果,10 mg/kg bw投与区において,死亡率,シスト検出率,シスト数およびM. seriolae遺伝子量が対照区と比較して有意に減少した。また,カンパチへ同濃度を5日間投与した場合も,ブリと同様にシスト検出率等が有意に減少した。FBT 10 mg/kg bwを5日間投与,2日間休薬を1クールとして15回の反復投与を行った結果,シスト形成が完全に抑制された。以上のことから,FBT濃度10 mg/kg bwの5日間投与により,べこ病のシスト形成が大幅に軽減され,さらに反復投与により発症を完全に防げることが示された。

Beko disease, caused by the Microsporidium seriolae infection, has been a problem in yellowtail aquaculture in western Japan. ​In recent years, severe cases of this disease have been confirmed, resulting in a significant decrease in product value due to mass mortality and residual cysts. ​Only polymerase chain reaction (PCR) assays have been reported so far as a detection method for the disease. ​In this study, quantitative real-time PCR (qPCR) and loop-mediated isothermal amplification (LAMP) methods have been optimized for M. seriolae detection in order to establish a more sensitive and rapid diagnosis. ​Target regions for each detection method were selected based on the nucleotide sequences obtained by the gene analysis of cysts in diseased fish. ​Primer sets for the qPCR and LAMP methods were designed, and the gene amplification efficiency of each method was evaluated. ​The results showed that the newly developed qPCR method could detect 1.4 copies of the target gene, and the LAMP method detected 100 copies within 15 minutes. ​In this study, the newly developed qPCR and LAMP assays were shown to be rapid and highly sensitive methods for quantitative detection of M. seriolae.