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NO55 - 1

Prevalence of Infection with Nocardia seriolae in Juvenile of Yellowtail Seriola quinqueradiata Cultured in Owase Bay, Japan

Goshi Kato1*, Kensuke Oka1, Megumi Matsumoto1, Motohisa Kanemaru2, Masato Yamamoto2 and Motohiko Sano1

1 Department of Marine Biosciences, Tokyo University of Marine Science and Technology, Tokyo 108-8477, Japan
2 OWASEBUSSAN Co., Ltd., Mie 519-3612, Japan

(Received August 30, 2019)

ABSTRACT―Nocardiosis caused by Nocardia seriolae makes severe losses of fish production of Seriola spp. in Japan. In this study, we investigated prevalence of N. seriolae infection in yellowtail Seriola quinqueradiata cultured in an open sea net-cage in Owase Bay, Japan. The gills, serum and spleen were sampled from juvenile fish (n = 5) on May, June, July, August, November 2017 and January 2018. Serum antibody titer against N. seriolae were significantly increased on November 2017 and January 2018. N. seriolae 16S rRNA gene were detected by PCR in the spleen and/or gills of 1-4 individuals sampled on May, July, August, November 2017 and January 2018, while the bacterium was not isolated from any individuals on Ogawa medium. Granulomatous inflammation which is the typical characteristic of pathological feature of nocardiosis was not observed in any spleen sections examined. However, fluorescent immunohistochemistry with anti-N. seriolae rabbit serum detected the long- and short-rod shaped bacteria in the spleen of all fish examined in this study. These data suggest that the juvenile yellowtail cultured in Owase Bay had been already infected with N. seriolae even in May. In addition, N. seriolae may be in viable but non-culturable state (VBNC), resulting in latent infection in cultured yellowtail.

Key words: Nocardia seriolae, Nocardiosis, Seriola quinqueradiata, yellowtail, prevalence, VBNC, latent infection

Ultrastructural and Molecular Phylogenetic Identification of the Diplomonad Flagellate Spironucleus salmonis Infecting Hatchery-reared Salmonid Fishes in Hokkaido

Shinya Mizuno1*, Shigehiko Urawa2, Yoshitomo Katsumata1, Takumi Morishita3 and Masatoshi Ban2

1 Salmon and Freshwater Fisheries Research Institute, Hokkaido Research Organization, Hokkaido 061-1433, Japan
2 Hokkaido National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Hokkaido 062-0922, Japan
3 Chitose Field Station, Hokkaido National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Hokkaido 066-0068, Japan

(Received September 19, 2019)

ABSTRACT―Systemic infections caused by diplomonad flagellates belonging to the genus Spironucleus commonly result in heavy mortalities in salmonids. Diplomonad flagellates were found in the intestinal lumen of hatchery-reared juvenile chum Oncorhynchus keta and masu salmon O. masou in Hokkaido. We performed ultrastructural observation and molecular phylogenetic analysis using the small subunit ribosomal RNA gene (SSUrDNA) of the flagellates to clarify their taxonomy. The flagellates showed elongated and tapered nuclei, and sub-apical location of the kinetosomes relative to the nuclei: which are the distinguishing characters of the genus Spironucleus among three genera of parasitic diplomonads. In addition, the flagellates had electron dense plaques at their posterior end, a posterior sac of dense free ribosomes, an electron dense body, and bowl-shaped membranous structures, identifying them as S. salmonis among four Spironucleus species that affect fish. The ultrastructure of the flagellates showed no variations between the two host species of salmon. All the isolated flagellate SSUrDNAs had the same 1,031 bp sequence, and no difference in the sequence. The SSUrDNA showed the highest homology (99%) with S. salmonis among 11 diplomonad flagellates. Thus, this study successfully used ultrastructure diagnosis and molecular phylogenetic analysis to confirm S. salmonis as the diplomonad flagellate that has infected hatchery-reared chum and masu salmon in Hokkaido.

Key words: Spironucleus salmonis, diplomonad flagellate, chum salmon, masu salmon, molecular phylogeny, electron microscopy, SSUrDNA, ultrastructure

Comparative Analysis of Plasmid DNAs from Two Strains of Photobacterium damselae subsp. piscicida Isolated from Japan and the United States

Akito Tsuchiya1, Aki Nishihara1, Ayumi Saeki1, Yuki Teru1, Takashi Aoki2, Tomoya Kono1, Masahiro Sakai1 and Jun-ichi Hikima1*

1 Department of Biochemistry and Applied Bioscience, Faculty of Agriculture, University of Miyazaki, Miyazaki 889-2192, Japan
2 Integrated Institute for Regulatory Science, Research Organization for Nao and Life Innovation, Waseda University, Tokyo 162-0041, Japan

(Received November 12, 2019)

ABSTRACT―Pseudotuberculosis pathogen, Photobacterium damselae subsp. piscicida (Pdp) shows some differences in virulence between Japanese and US strains. In this study, we analyzed plasmid DNAs derived from Japanese and US strains of Pdp, and found two homologous regions in these plasmids, pOT-51443-4/p91-197-1 and pOT-51443-1/p91-197-2. All genes in the p91-197-1 from US strain were observed in the pOT-51443-4 from Japanese strain. This event might have occurred during transmission of the US plasmid to Japanese strain. Inhibitor of vertebrate lysozyme (ivy), a known pathogenicity factor was encoded in the homologous region of pOT-51443-1/p91-197-2. Thus, the genomic region containing ivy may have been horizontally transmitted between the two Pdp strains.

Key words: Photobacterium damselae subsp. piscicida, plasmid, inhibitor of vertebrate lysozyme (ivy)