Genotype identication of Flavobacterium psychrophilum isolated from ayu Plecoglossus altivelis altivelis in rivers of Kochi Prefecture

Abstract

We collected 28 and 10 Flavobacterium psychrophilum strains isolated from dead and diseased ayu Plecoglossus altivelis altivelis, respectively, from several rivers of Kochi Prefecture. A total of 38 strains were identified from 10 gene regions by using the on/oŠ switch assay, polymerase chain reaction-restriction fragment length polymorphism, and multilocus sequence typing. Genotypes of F. psychrophilum isolated from dead ayu were mostly G-C/ A/S/52 (22/28) whereas genotypes of F. psychrophilum isolated from diseased ayu were G-C/A/S/52 (6/10) and A-C/A/R/45 (4/10). The number of outbreaks of bacterial cold-water disease (BCWD) caused by F. psychrophilum in ayu in the rivers of Kochi Prefecture increased in 2015 and peaked in 2018. Most F. psychrophilum isolates from dead ayu in 2015-2018 were identified as having genotypes of G-C/A/S/52 (22/24). These results suggest that the outbreak of BCWD in ayu in the rivers of Kochi Prefecture was caused by a genotype of G-C/A/S/52.