2020 Volume 55 Issue 3 Pages 71-79
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.
