Epidemiological Study on Newly Emerging Lactococcus garvieae Serotype II Isolated from Marine Fish Species in Japan

Abstract

Lactococcus garvieae serotype II, a newly emerging causal pathogen of lactococcal infection, has spread to fish farms since 2012, and its damage to aquaculture has increased. A total of 161 strains were isolated from four fish species, yellowtail Seriola quinqueradiata, amberjack S. dumerili, kingfish S. lalandi, and striped jack Pseudocaranx dentex, in eight prefectures between 2012 and 2017. This is the first epidemiological study on L. garvieae serotype II, by using agglutination titers against antisera for diagnosis, biased sinusoidal field gel electrophoresis (BSFGE) analysis, and antimicrobial susceptibility test. Almost none of these strains were agglutinated with anti-Ia serum (anti-KG serum), except for two strains agglutinated with both anti-Ia and II sera. BSFGE analysis revealed that genetically homogeneous L. garvieae serotype II has spread to fish farms since the first outbreak in 2012, and the strains resistant to lincomycin were already found in 2015.