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.
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.
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.