The sensitivity of yellowtail Seriola quinqueradiata and red sea bream Pagrus major to Nocardia seriolae was investigated. The LD50 for yellowtail and red sea bream after being injected with N. seriolae was 1.1 × 103 CFU/100 g B.W. and 3.4 × 104 CFU/100 g B.W., respectively. In an immersion challenge, the LD50 for yellowtail was 1.2 × 104 CFU/mL, while no mortality was observed in red sea bream at 1.0 × 105 CFU/mL for 26 days. These data indicate that yellowtail is more sensitive to the bacterium than red sea bream. Serum bactericidal activity against N. seriolae was not observed in both fish species. Production of superoxide anion in kidney phagocytes isolated from red sea bream significantly increased in response to addition of live N. seriolae. However, such increase of superoxide anion production was not detected in yellowtail kidney phagocytes. This suggests that superoxide anion from phagocytes is one of important factors in protection against nocardiosis.
In this study, we made liposome vaccines in which formalin-inactivated koi herpesvirus (KHV) was entrapped within the liposomal membrane compartment for oral vaccination in common carp Cyprinus carpio. In vaccination experiments, two fish groups (mean body weight, 30 g) received the liposome vaccine (protein concentration of 20 μg/mL using an isolate of NKC03 from Nara Prefecture or an isolate of IKC03 from Ibaraki Prefecture) at a dose of 20 μL/fish/day for 3 days. Neutralization titers of vaccinated fish sera were significantly higher than control at 22 days after last vaccination. In addition, 22 days after last vaccination, the fish were challenged with NKC03 at 102.3 TCID50/100 μL/fish or IKC03 at 102.2 TCID50/100 μL/fish by dropping the inoculum onto their gills, and resulted RPS was 74.4% and 65.0%, respectively. These results proved the efficacy of oral vaccination with the KHV liposome vaccine against KHV infection in carp.
We examined the efficacy of febantel against the infection of the diclidophorid monogenean Heterobothrium okamotoi in cultured tiger puffer Takifugu rubripes. Febantel is a prodrug of fenbendazole, a benzimidazole drug, which is widely used in the field of veterinary medicine. Febantel was metabolized to fenbendazole in tiger puffer just like veterinary animals for oral administration. At the doses of 12.5 mg/kg and 25 mg/kg for 5 days, and 50 mg/kg for 3 days, 88%, 96% and 94%, respectively, of mature worms, and 97%, 95% and 97%, respectively, of immature worms were eradicated at 22-23°C. These results suggest that febantel has high anti-H. okamotoi activity.
The survivability of koi herpesvirus (KHV) in environmental water and sediment was evaluated using CCB cells. Samples were collected from Ibaraki prefecture, Kanagawa prefecture and Hakodate, Hokkaido. Significant reduction in the infectious titer of KHV was observed within 3 days in intact environmental water or sediment. However, KHV infectivity remained for more than 7 days in autoclaved or filtered (0.45 μm) water. In the autoclaved water containing sediment, KHV infectivity dropped below detectable limits within 7 days after inoculation. Ten of the 147 bacterial strains from rivers in Kanagawa, and two of the 62 bacterial strains from water from, Hakodate showed anti-KHV properties. The results suggest that in the absence of hosts, KHV can be rapidly inactivated in environmental water.
Kudoa megacapsula was found in the somatic muscle of yellowtail Seriola quinqueradiata, which were raised from wild seedlings (juveniles) imported from South Korea. Numerous parasitic foci, 2-3 mm in size, were recognized as filamentous ‘black cysts’ in the skeletal muscle, while no muscle liquefaction was observed. The spores (34.6 × 28.9 μm in mean size) were cruciform to stellate and had four distinct winglike projections, in which one extremely large and three small polar capsules were present. These morphological characteristics and small subunit ribosomal DNA sequences of the myxozoan were consistent with those of K. megacapsula, which was originally described from red barracuda Sphyraena pinguis caught off the coast of China. Histopathological observation revealed that plasmodia developed intracellularly within the myofibrils, followed by encapsulation by host's fibroblast layers, resulting in granulomatous inflammation.
The adherence of Edwardsiella tarda strains to HEp-2 cells was studied by the viable cell count and direct microscopic count methods. Three selected E. tarda strains having different hemagglutination activities against guinea pig erythrocytes were cultured in a peptone-yeast extract broth supplemented with 3% NaCl (3%-NaCl culture) or without NaCl (0%-NaCl culture). Strain FK1051 showed significantly higher adherence in the 3%-NaCl culture than in the 0%-NaCl culture. A similar adherence pattern was observed in strain KG8401, though the adherence of the 0%-NaCl culture was higher than that of FK1051. In contrast, both cultures of strain SU166 were low in the adherence. These adherence patterns to the HEp-2 cells correlated well with the previously reported hemagglutinating activities of the strains. N-acetylneuraminic acid markedly inhibited the adherence as well as the hemagglutination. This NaCl-enhanced adherence may be involved in pathogenesis of E. tarda infection particularly in marine fish.
Streptococcus parauberis (stain SNUFPC-050803), isolated from diseased olive flounder Paralichthys olivaceus in Jeju Island, Korea, was evaluated for its pathogenicity to healthy juvenile flounder (29.3 g in average body weight). When challenged with the isolate by intraperitoneal injection with tenfold serial dilutions of 4.5 × 10-106 CFU/fish, the cumulative mortality ranged from 10% to 80% within 14 days except for 4.5 × 10 CFU/fish and control with no mortality. Disease signs were hemorrhage around the mouth, eyes and pectoral fins, pale and friable liver with hepatomegaly and ascitic fluid in the peritoneal cavity. These signs were similar to those of naturally affected fish. S. parauberis was reisolated and identified by PCR method, which confirmed the pathogenicity of the bacterium to olive flounder.
Detection of antibodies against Renibacterium salmoninarum (R.s.), a causative agent of bacterial kidney disease (BKD), was conducted among apparently healthy fingerlings and broodstock of masu salmon Oncorhynchus masou. Antibodies against R.s. were detected from ≥50% of fingerlings in the hatcheries with BKD-histories and R.s. antigens were detected in some of antibody-positive fish, but not from those in the hatchery without a BKD-history. Although returning broodstock examined within two weeks after introduced into the hatchery showed all negative for anti-R.s. antibodies, 33% of those in the hatchery with BKD-histories became positive after two months of rearing for sexually maturation. Monitoring specific antibodies suggests that R.s. infection-cycle is completed between fingerlings and broodstock in hatcheries having BKD-histories.
In April 2004, a fungal infection occurred in cultured young striped jack Pseudocaranx dentex at a fish farm in Ehime Prefecture, Japan. The cumulative mortality reached about 25% in one month after the disease was first recognized. Moribund fish showed disease signs such as abdominal swelling and distended kidney. A fungus was purely isolated from the kidney of the fish using PYGS agar. The colony was pale brown in color, and the conidia were two-celled, cylindrical to oblong with rounded ends and smooth-walled. From these morphological characteristics, the fungus was identified as Ochroconis humicola. This infection of marine fishes has been reported in the skin of juvenile fish, but not known in young fish. This paper describes the first case of O. humicola infection in visceral organs of young striped jack.