No 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.

Outbreaks of beko disease caused by the microsporidian Microsporidium seriolae have been a serious problem in Seriola aquaculture in Japan. Recent laboratory experiments demonstrated that the oral administration of a probenzimidazole febantel (FBT) at the early stage of infection effectively suppresses the formation of the microsporidian cysts in the trunk muscle. This opens the possibility of drug therapy for the beko disease. Here, we conduct a series of field trials to assess the efficacy and practicality of FBT treatment for beko disease in farmed Seriola quinqueradiata and Seriola dumerili. The trial tested FBT doses of 0, 1, 3, 5, and 10 mg/kg bw for 5 days resulted in significantly lower beko disease-related mortality, cyst detection rate, and progression of cyst formation in the 10 mg/kg group of S. quinqueradiata. This 10 mg/kg bw for 5 days FBT administration was also effective to suppress cyst formation in S. dumerili. Another trial used fifteen repeated administrations of 10 mg/kg FBT for 5 days with 2 days interval in between completely prevented the cyst formation in juvenile S. quinqueradiata, in comparison to the cyst detection rate of as high as 83.3% in non-drug treated fish. No adverse effects of FBT on the fish were observed in any of the trials. Our field trials indicate that the oral administration of FBT at the dose of 10 mg/kg bw is highly effective and practical chemotherapy for beko disease in Seriola fish.

Capsalid skin flukes are major parasites of amberjacks Seriola spp. worldwide (Ogawa et al., 1995;Ogawa, 2015). They attach to the fish body surface

Beko disease, caused by the Microsporidium seriolae infection, has been a problem in yellowtail aquaculture in western Japan. In recent years, severe cases of this disease have been confirmed, resulting in a significant decrease in product value due to mass mortality and residual cysts. Only polymerase chain reaction (PCR) assays have been reported so far as a detection method for the disease. In this study, quantitative real-time PCR (qPCR) and loop-mediated isothermal amplification (LAMP) methods have been optimized for M. seriolae detection in order to establish a more sensitive and rapid diagnosis. Target regions for each detection method were selected based on the nucleotide sequences obtained by the gene analysis of cysts in diseased fish. Primer sets for the qPCR and LAMP methods were designed, and the gene amplification efficiency of each method was evaluated. The results showed that the newly developed qPCR method could detect 1.4 copies of the target gene, and the LAMP method detected 100 copies within 15 minutes. In this study, the newly developed qPCR and LAMP assays were shown to be rapid and highly sensitive methods for quantitative detection of M. seriolae.