Emerging New Serotype Lactococcus garvieae Infection in Farmed Marine Fish in Japan

Abstract

Mortalities due to Lactococcus garvieae-like infection have been observed in farmed greater amberjack Seriola dumerili previously immunized with a commercial L. garvieae vaccine containing serotype I and II strains in Miyazaki Prefecture since August 2021. ​The isolates from diseased fish were not agglutinated by the anti-serotype I and II sera, and showed genetic similarity to L. garvieae serotype I in multi-locus sequence (16S rRNA gene, pheS, recA, rpoA and rpoB) analysis. ​The pathogenicity of isolates was confirmed by intraperitoneal injection in greater amberjack, and the lethal dose fifty was less than 1.6 × 10³ CFU/FISH. ​In a vaccine trial, greater amberjack, which had been immunized with formalin-killed cells (FKC) prepared from both serotype I and II strains, was not effectively protected against an isolate from this study, whereas those immunized with FKC prepared using the isolate were effectively protected against the infection with an isolate exhibiting the same antigenicity. ​Based on these findings, we propose a new serotype: serotype III, which is not agglutinated with both anti-serotype I and II serum and not protected against a serotype I and serotype II-combined vaccine.