We studied the epidemiology of Flavobacterium psychrophilum in ayu from Lake Biwa for establishing control measures to bacterial cold water disease (BCWD). During 1998-2011, 12,743 wild ayu were collected from coastal (set net and gill net), offshore (offshore scoop net) and inflow river (fishing weir) areas of Lake Biwa. We employed a nested-PCR technique targeting 16S rRNA gene for the detection of F. psychrophilum in the gills and kidney of ayu. Prevalence of the bacterium was greatest when the water temperature of Lake Biwa was 18-21°C, which coincides with the optimum temperature for multiplication of F. psychrophilum. The prevalence of F. psychrophilum in the gills was higher in fish from fishing weirs and set nets maintained in net cages than in fish just after catch with the other fishing techniques. This suggests that rearing ayu in net cages at high densities following catch by fishing weirs and set nets can advance infection with the bacterium among the captured fish which will be used for seedlings in aquaculture or releasing in rivers. Control measures for the captured ayu must be conducted in reliable ways to prevent spread of the disease after release and in aquaculture.
The blood fluke Cardicola opisthorchis (Trematoda: Aporocotylidae) is a significant pathogen of cultured Pacific bluefin tuna Thunnus orientalis in Japan. We conducted monthly surveys of the intermediate polychaete host Terebella sp. (Polychaeta: Terebellidae) for one year in 2014 to elucidate seasonality in the C. opisthorchis infection at a tuna farm in Tsushima Island, Nagasaki Prefecture, Japan. The intermediate host, collected from ropes and floats attached to culture cages, was abundant from January to March and in November and December but quite scarce from April to October. The sporocysts of C. opisthorchis were found from January to March and from September to December, with the prevalence of infection ranging from 2.0% to 18.4%. Development of sporocysts was observed in the body of the intermediate host Terebella sp. Sporocystogenous sporocysts and cercariogenous sporocysts co-occurred in the body cavities of the polychaete host. This is the first detailed description of asexual multiplication process of sporocysts among marine aporocotylid species. Additionally, sporocystogenous sporocysts and released daughter sporocysts survived in L-15 medium for up to 63 and 66 days, respectively. The highest number of sporocysts in one polychaete host was about 1,800 and the highest number of cercariae in one sporocyst was 75, suggesting that as many as >105 cercariae can be released from one infected polychaete.
This study describes the histopathological features of a cutaneous leiomyoma in a goldfish. A mass, measuring 3.5 × 2 × 1 cm, located dorsolaterally along the cranial lateral line above the right operculum. The mass was sessile and golden orange with hemorrhagic foci. Histological examination revealed interlacing bundles of spindled to elongated tumor cells with cigar-shaped nuclei with no malignancy. The tumor cells showed red color with Masson’s trichrome stain. Immunohistochemically, the tumor cells were positive for vimentin, desmin and α-smooth muscle actin, but not for S100. Based on these findings, this case was diagnosed as a cutaneous leiomyoma.
Six Japanese fish species were intraperitoneally inoculated with an infectious salmon anemia virus (ISAV) isolate (106.4 TCID50/fish). In experiment #1, the cumulative mortality in Atlantic salmon and ayu was 100% and 40%, respectively. In experiment #2, Atlantic salmon, sockeye salmon and Japanese char had cumulative mortalities of 95%, 25% and 0%, respectively. ISAV RNA was detected from all injected Japanese char. In experiment #3, no mortality was observed in Biwa gudgeon, common carp and goldfish. These results suggest that ISAV can be pathogenic to ayu and sockeye salmon and, however, the susceptibilities of both fishes to ISAV are lower than that of Atlantic salmon.
In this study, we have developed a method for detecting Kudoa amamiensis using loop-mediated isothermal amplification (LAMP). After initial cloning and sequencing of the internal transcribed spacer (ITS) region between 18S and 28S rRNA genes of K. amamiensis, a set of four primers consisting of two inner and two outer was designed based on ITS1 and 5.8S rRNA sequence for use in the LAMP reaction. Reaction time and temperature were optimized for 60 min at 60°C, respectively. No cross-reaction of the LAMP method was observed with K. iwatai. Therefore, this LAMP technique was considered to be a rapid and simple detection method of K. amamiensis.
Juvenile Japanese flounder Paralichthys olivaceus were immersion-challenged (2.0 × 106 CFU/mL, 60 min) with Edwardsiella tarda NUF806 strain. From the following day, diets containing fosfomycin (FOM) calcium were administrated orally at concentrations of 20, 40 or 80 mg potency/kg body weight/day for 6 consecutive days. Cumulative mortality of fish during the observation period of 21 days after the drug administration was significantly lower in all groups of FOM-treated fish than that in the control group. In addition, NUF806 strain and recent clinical isolates (n = 30) of E. tarda examined were all susceptible to FOM, with MIC of 2-4 μg/mL when estimated using the glucose-6-phosphate-containing Mueller-Hinton broth. The results suggest that FOM is a promising candidate for the treatment of edwardsiellosis in cultured Japanese flounder.