We evaluated the prevalence of the akoya oyster disease (AOD) in Japan and the pathogenicity of the unidentified causative agent. We obtained naïve Japanese pearl oysters Pinctada fucata martensii from a site with no history of AOD. In separate experiments, the oysters were either challenged by intramuscular injection with the hemolymph of infected oysters or reared in several major pearl oyster farms throughout Japan. Both groups of oysters experienced high mortality and exhibited discoloration of the adductor muscle and histopathological changes characterizing AOD. Our data suggest that AOD is prevalent in pearl oyster culture fields throughout Japan and that the causative pathogen remains highly pathogenic to P. fucata martensii. When groups of oysters produced by repeated selective breeding against AOD were reared in different pearl oyster fields alongside uninfected naïve oysters, the selectively bred oysters had significantly lower mortality than naïve oysters. The recent decline in outbreaks of AOD can be attributed, in part, to the introduction of selectively bred oysters that are resistant to this disease.
We evaluated the protective efficacy of a combined vaccine containing formalin-inactivated Edwardsiella tarda, Streptococcus iniae, and S. parauberis using olive flounder Paralichthys olivaceus. The fish were injected intraperitoneally with 0.1 mL of the combined vaccine. The fish were then challenged by intraperitoneal injection of the three bacterial strains. Mortality was significantly lower in the vaccinated groups compared with the control groups 3 wk post vaccination. Furthermore, there was a significant increase in antibody titer up to 16 wk after vaccination. Our results suggest that the combined vaccine successfully induced humoral and protective immunity against the three pathogens in olive flounder.
We evaluated the pathogenicity of Korean and Japanese isolates of viral hemorrhagic septicemia virus (VHSV: genotype IVa) from olive flounder Paralichthys olivaceus using rainbow trout Oncorhynchus mykiss fry. The cumulative mortalities of fish challenged with FYeosu05 (Korean) and Obama25 (Japanese) isolates at 106.5 TCID50/fish were 64 and 48%, respectively. No mortality was observed among the fish challenged with either of the isolates at 105.5 TCID50/fish, or in mock-challenged fish. The affected fish exhibited darkening of the body, an expanded abdomen, pale gills, an enlarged spleen, and diffuse necrosis in the splenic and interstitial hematopoietic tissues. We re-isolated VHSV from all mortalities using cell culture and confirmed their presence by reverse transcription loop-mediated isothermal amplification (RT-LAMP). We thus concluded that Asian VHSV isolates from olive flounder are pathogenic to rainbow trout fry, although with low virulence.
We measured the concentration of F. psychrophilum in the ovarian fluid and milt of six species of salmonids that were collected from freshwater fish farms in Japan. We detected the bacteria in 39 of 61 groups of female fish (544 of 3,276 fish) and 21 of 42 groups of male fish (248 of 1,434 fish). The concentration of F. psychrophilum ranged from 10 to 107.7 CFU/mL (mean 102.0 CFU/mL) in the ovarian fluid and from 10 to 104.5 CFU/mL (mean 102.1 CFU/mL) in the milt. Our data suggest that the bacteria are widely distributed, and are present at high levels, throughout Japan.
Edwardsiella tarda causes high mortality infections in fish. To determine the efficacy of a vaccine against E. tarda, a formalin-killed preparation of atypical E. tarda FPC503 was intraperitoneally injected into red sea bream Pagrus major with or without the oil adjuvant, Montanide ISA 763 A VG. Both vaccine regimens showed protective efficacy against experimental infection with E. tarda FPC503 at 28 and 56 days post-vaccination. An enzyme-linked immunosorbent assay (ELISA) revealed the specific antibody production in vaccinated fish but the relationship between protective efficacy and antibody titer was not clear. The present results indicate that the formalin-killed vaccine is useful for the prevention of atypical E. tarda infection in red sea bream.
We evaluated the occurrence of intra-ovum infection with Flavobacterium psychrophilum, the causative agent of bacterial cold-water disease (BCWD) and rainbow trout fry syndrome (RTFS), in the eggs of six salmonid species in Japan. Flavobacterium psychrophilum was isolated from four eggs in one of 13 lots of eyed eggs (999 eggs) imported from the United States. In contrast, the bacterium was not isolated from 28 lots (1,680 eggs) or 61 lots (3,693 eggs) of domestically produced unfertilized or eyed eggs, respectively. Our results indicate there is a low likelihood of intra-ovum infection in cultured salmonids in Japan.