Edwardsiellosis in fish caused by Edwardiella tarda and E. ictaluri is reviewed. Genus Edwardsiella belongs to the Enterobacteriaceae family, and E. tarda produces both hydrogen sulfide and indole, whereas E. ictaluri does neither. E. tarda was first isolated from diseased eel in Japan, and infects freshwater fish as well as marine fish. E. ictaluri has been detected in only freshwater fish after described as a new species from diseased channel catfish in the United States. The two pathogens survive in phagocytic cells, and hosts affected with edwardsiellosis die from septicemia. There were no reports about E. ictaluri infection in Japan before E. ictaluri was isolated from diseased wild ayu in 2007. Recently, the vaccine against edwardsiellosis of Japanese flounder was officially approved in Japan.
Fish blood flukes (FBFs) are the most important digenean parasites in marine finfish aquaculture due to their high pathogenicity. Numerous eggs accumulate in gill lamellae and capillary vessels in various organs, interfere the blood flow. This causes fish to suffocate to death or leads to other fatal health problems. In Japan, important culture fish, such as amberjacks, bluefin tuna and tiger puffer are affected by FBFs of different taxa; namely Paradeontacylix spp., Cardicola spp. and Psettarium spp., respectively. FBFs are relatively host specific and utilize a complex two-host lifecycle. To date, the lifecycles have been elucidated for only a handful marine FBF species and all use terebellid polychaetes as the intermediate host. Chemotherapy with the oral treatment of praziquantel is effective and commonly used in fish farms as a sole control measure against FBFs. The development of prevention method is expected with the recent advances in knowledge on the biology of FBFs.
Koi herpesvirus (KHV) disease caused by cyprinid herpesvirus 3 (CyHV-3) affects carp Cyprinus carpio and its varieties. The disease is listed by the OIE and is also designated as “Specific diseases” in a Japanese law. In Japan, the disease first occurred in 2003 and has quickly spread to all prefectures by distributing infected fish. This review describes basic information on KHV/KHV disease such as its pathogenicity, diagnostic methods, control measures and status of virus distribution in Japan.
The 5.8S rRNA gene of C. irritans in water and mud substrate samples were quantified at 6 or 7 stations in Nomi Bay, Kochi, Japan twice a month from the late June to the middle December, using a TaqMan real-time PCR assay developed in the present study. The number of copies of the gene in water peaked at 4:00 am over a 20-h sampling period in October. The mean numbers of the gene copies equivalent to 4.465-6,590 theronts/L were detected from water sampled at a station close to cages containing great amberjack Seriola dumerili affected with cryptocaryoniasis in early October and late November, when cryptocaryoniasis outbroke. The frequency of detection of the gene in the substrate mud was highest at a station far from the affected area, suggesting that the distribution of tomonts of C. irritans were influenced with tidal currents. The 18S-ITS1 region of C. irritans was PCR-amplified from a water sample, sequenced and compared with the sequences previously obtained from the parasite of various localities. The present sequence was contained in a clade mainly comprising the sequences obtained in China, suggesting that the greater amberjack juveniles imported from southern China was a source of C. irritans in Nomi Bay.
We investigated the development of the macronucleus of Cryptocaryon irritans, and the ingestion and digestion of host cells by the parasite. All developmental stages of the parasite except the theront (trophont, protomont and tomont) were examined with histological staining and/or whole-mount staining. The sections were also subjected to in situ hybridization targeting the 18S rRNA gene of the host fish. The macronucleus developed negligibly, maintaining its four-segmented shape in trophonts, elongated and formed a massive nucleus in tomonts before cell division, and subsequently underwent repeated divisions to generate theronts. Denatured host cells, examined in situ hybridization, were accumulated in trophonts, and disappeared in tomonts by the beginning of cell division. Denatured host cells were also observed in the host tissue surrounding trophonts. They had condensed nuclei. Protomonts filled with denatured host cells presented a 180-bp DNA ladder in gel electrophoresis, suggesting apoptosis of the host cells. These results indicate that DNA synthesis occurs exclusively in the early stage of the tomont and that host cells are fed and accumulated, probably as apoptotic cells, in the trophont and are digested in the early tomont stage.
Plecoglossus altivelis poxvirus-like virus (PaPV) is suspected as the causative agent for atypical cellular gill disease in ayu P. altivelis. The virus was detected by PCR from wild juvenile ayu and Japanese sardine Sardinops melanostictus caught in coastal areas, but not detected from the larval ayu caught in freshwater. No histopathological changes were observed in the gills of juvenile ayu positive for PaPV. When the positive juvenile population was reared in freshwater, the detection rate of the virus gradually decreased and reached below the detection limit 40 days after the stocking.
Caecognathia coralliophila is known as a pathogenic ectoparasite infecting tiger grouper Epinephelus fuscoguttatus in a hatchery in Sabah, Malaysia. The effects of copper sulfate, formalin, trichlorfon, and hydrogen peroxide on the survival of C. coralliophila larvae were tested in vitro. The larvae were exposed to different concentrations of each chemical for 10, 20, 30, 60 min, or 24 h. Trichlorfon was found to be the most effective, killing the parasites within 24 h at 0.2 ppm. Consequently, the toxicity of trichlorfon to tiger grouper was tested. Fish were exposed to trichlorfon at 0.2 ppm for 24 h or 3.2 ppm for 60 min. No fish died in the experiment. Thus, our data suggest that trichlorfon is effective for treating C. coralliophila infection in E. fuscoguttatus.
We determined the minimum ultraviolet (UV) irradiation dose to influent water to prevent Kudoa yasunagai infection, using microscopy and qPCR. The infection prevalence in Seriola lalandi reared in untreated water reached 45% while that in fish reared in UV-treated water at 5 mJ/cm2 remained below 10%. Additionally, 5 mJ/cm2 UV irradiation significantly reduced spore formation in the brain. No infection was detected when the water was treated with UV at the doses 15 and 30 mJ/cm2. These results indicate that K. yasunagai actinospores are relatively vulnerable to UV irradiation and the minimum effective dose lies between 5 and 15 mJ/cm2.