Fish Pathology
Online ISSN : 1881-7335
Print ISSN : 0388-788X
ISSN-L : 0388-788X
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Research Articles
  • Norihiko Komatsu, Naoki Itoh, Kazuo Ogawa
    Type: Research Article
    2020 Volume 55 Issue 3 Pages 53-60
    Published: September 15, 2020
    Released: October 22, 2020
    JOURNALS RESTRICTED ACCESS

    In August 2013, metacercarial infection in the abdominal cavity of Japanese dace Tribolodon hakonensis fingerlings occurred at a hatchery in Nagano Prefecture, Japan. ​All fish examined in this study (n = 45) were infected, and heavily infected fish showed an abnormally distended abdomen and ascites, with the number of metacercariae reaching 66. ​The metacercariae were distinctly bipartite, representing a Neascus type and classified as a member of Diplostomidae, and phylogenetical analyses based on ITS2 and cox1 sequences tentatively classified them in the genus Posthodiplostomum. ​When they were excysted, only a few were alive and intact, suggesting that dace is not a suitable host. ​Histopathologically, cysts containing dead worms and empty and vestigial cysts were in the abdominal cavity, detached from the host visceral organs and surrounded by host inflammatory cells. ​Diplostomid cercarial larvae were not found in the three species of snails (n = 613) sampled from ponds in the station. ​At the end of rearing in September, the survival rate was 0.96% in a pond with a high infection rate, whereas it was 12.5% in another pond where fingerlings showed no clear disease signs. ​Oral administration of praziquantel at 150 mg/kg fish body weight for 3 consecutive days effectively killed metacercariae in the abdominal cavity.

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  • Shinya Mizuno, Shigehiko Urawa, Yoshitomo Katsumata, Takumi Morishita, ...
    Type: Research Article
    2020 Volume 55 Issue 3 Pages 61-70
    Published: September 15, 2020
    Released: October 22, 2020
    JOURNALS RESTRICTED ACCESS

    Diplomonad flagellate Spironucleus salmonis infection in hatchery-reared juvenile chum salmon Oncorhynchus keta and masu salmon O. masou has been previously reported in Hokkaido, northern Japan. ​We established a quantitative real-time PCR (qPCR) assay for the S. salmonis ribosomal RNA gene (rDNA) using a fish intestine nucleic acid template, which we then use to investigate the infection intensity of S. salmonis in hatchery-reared, farmed, and wild salmonids throughout Hokkaido. ​Our new qPCR assay enabled the measurement of rDNA from 1.0–1.0 × 108 copies/μL. ​Our study reported a significantly positive correlation between parasite trophozoite cell number and its rDNA copy number, and presented this as a simple regression (parasite rDNA copy number = -768 + 338 × parasite cell number). ​By reporting S. salmonis infection in juvenile and/or adult chum and masu salmon collected from three hatcheries, one fish farm, and one river in Hokkaido, we demonstrate the utility of our qPCR assay for the surveillance of S. salmonis infection in hatchery-reared, farmed and wild fish.

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  • Takahiro Nagai, Toshihiro Nakai
    Type: Research Article
    2020 Volume 55 Issue 3 Pages 71-79
    Published: September 15, 2020
    Released: October 22, 2020
    JOURNALS RESTRICTED ACCESS

    The susceptibility of ayu Plecoglossus altivelis to infection by Flavobacterium psychrophilum differs among hatchery stocks. ​We challenged two ayu stocks, domesticated stock (DS) and amphidromous stock (AS), with 18 F. psychrophilum isolates derived from diseased ayu using intraperitoneal (IP) injection and immersion methods. ​In both methods, 6 isolates were more virulent to DS than to AS (designated as the Do pathotype), whereas the other 12 isolates were more virulent to AS than to DS (designated as the Am pathotype). ​The LD50s (CFU/fish) of the representative isolates, PH-1034 (Do pathotype) and PH-1037 (Am pathotype) by IP injection in DS and AS were 1.4 × 102 and 5.0 × 105, and 5.9 × 105 and 4.6 × 102, respectively. ​The difference in virulence between pathotypes was supported by the in vivo bacterial counts in the blood and organs of IP-challenged fish, but not by the in vitro growth ability in the serum of ayu, which is a virulence factor of F. psychrophilum. ​Additionally, the 18 F. psychrophilum isolates could be classified into any one of the previously reported PCR-RFLP-based genotypes. ​However, no clear correlation was found between these genotypes and pathotypes.

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