2026 Volume 61 Issue 3 Pages 89-95
Bacterial cold-water disease (BCWD), caused by Flavobacterium psychrophilum, is a major infectious disease of ayu Plecoglossus altivelis in Japan. In this study, serological and multiplex PCR (mPCR) analyses were conducted on 11 isolates from BCWD-affected ayu. The results revealed the coexistence of two distinct types, Type-1 and Type-3 in ayu. Type-1 strains, previously reported only in salmonids, were identified in ayu for the first time. The PCR-RFLP typing pattern of the Type-1 strain isolated from ayu corresponded to the A/S/QS/C types commonly detected among ayu isolates in previous studies, suggesting that infection originating from other fish species is unlikely. Vaccine efficacy was evaluated using formalin-killed cell (FKC) preparations. Monovalent vaccines provided strong protection against homologous strains but showed limited protection against heterologous strains, indicating serotype-specific immunity and limited cross-protection. In contrast, a bivalent vaccine containing both Type-1 and Type-3 strains conferred high protection against both types. These findings demonstrate that multiple mPCR types of F. psychrophilum are involved in BCWD in ayu and highlight the importance of multivalent vaccines for effective disease control.
