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

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 longand 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.