Fish Pathology
Online ISSN : 1881-7335
Print ISSN : 0388-788X
ISSN-L : 0388-788X
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Research Articles
  • Goshi Kato, Kensuke Oka, Megumi Matsumoto, Motohisa Kanemaru, Masato Y ...
    Type: Research Article
    2020 Volume 55 Issue 1 Pages 1-7
    Published: March 15, 2020
    Released: April 02, 2020
    JOURNALS RESTRICTED ACCESS

    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.

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  • Shinya Mizuno, Shigehiko Urawa, Yoshitomo Katsumata, Takumi Morishita, ...
    Type: Research Article
    2020 Volume 55 Issue 1 Pages 8-17
    Published: March 15, 2020
    Released: April 02, 2020
    JOURNALS RESTRICTED ACCESS

    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.

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Short Communication
  • Akito Tsuchiya, Aki Nishihara, Ayumi Saeki, Yuki Teru, Takashi Aoki, T ...
    Type: Short Communication
    2020 Volume 55 Issue 1 Pages 18-21
    Published: March 15, 2020
    Released: April 02, 2020
    JOURNALS RESTRICTED ACCESS

    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.

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