Quantitative Analysis of Diplomonad Flagellate Spironucleus salmonis Infection in Intestines of Hatchery and Wild Salmonid Fishes in Hokkaido

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.