Characterization and Epidemiological Study of Newly Emerging <i>Lactococcus garvieae</i> Serotype III in Farmed Fish in Japan

Lactococcus garvieae serotype III, a newly emerging pathogen that causes lactococcal infection, has been detected in fish farms since 2021. This pathogen has caused severe damage to the striped jack Pseudocaranx dentex and greater amberjack Seriola dumerili. This study collected strains from various fish species on farms between 2021 and 2023 and conducted an epidemiological and characterization study of L. garvieae serotype III using biochemical characterization tests, agglutinating titers against diagnostic antisera, biased sinusoidal field gel electrophoresis (BSFGE), and drug susceptibility tests. Capsule-like structures were found on the cell surfaces of L. garvieae serotype III strains, and strains showed similar characteristics except for the reaction of acidification ribose in biochemical characterization test using the API 20 STREP system. All strains were agglutinated with antiserum raised against L. garvieae serotype III and lower or non-agglutinated with antiserum raised against L. garvieae serotype I. BSFGE analysis identified three types of electrophoretic pattern: A, B, and C. BSFGE types A and B spread to fish farms in 2021, while type C found in 2022. All strains analyzed in this study were thought to be resistant to lincomycin but not to erythromycin.