Heterobothrium okamotoi infects the gills and branchial cavity wall of Japanese pufferfish Takifugu rubripes, feeding on blood from the gills. Infection of this monogenean can cause anemia, and adult worms on the branchial cavity wall induce inflammation and tissue necrosis by the action of haptoral clamps. Eggs deposited as a long string easily entangle on the culture net, which is prone to heavy infection of pufferfish cultured in net cages, as the oncomiracidium, hatched larva, can encounter the host fish at much higher chances than expected in natural waters. Prevention of infection is practically impossible as its life cycle has been established in culture farms. Chemotherapy with hydrogen peroxide or febantel, a benzimidazol, is effective but repeated treatment may be required, as immunity acquired by infected fish is not perfect. Biological data on H. okamotoi should be effectively incorporated in its control measures.
Streptococcosis caused by the genera Streptococcus and Lactococcus have occurred in cultured freshwater, brackish water and marine fish species owing to the worldwide development of intensive aquaculture. In Japan, four pathogens, Streptococcus iniae, S. parauberis, S. dysgalactiae and Lactococcus garvieae, have been observed in aquaculture. Infections of these pathogens cause high mortality rates in the species important for aquaculture such as yellowtail, red sea bream and flounder. Recently, commercial vaccines have been licensed against these pathogens to prevent the infections in some aquaculture species. This review describes the outlines and topics of the diseases.
The first outbreak of bacterial kidney disease (BKD) of salmonids occurred in the autumn of 1973 in Hokkaido, Japan. Subsequently, outbreaks of BKD were confirmed in Hokkaido and main land of Japan. Most of the cases were caused by imported eyed eggs of coho salmon Oncorhynchus kisutch. In this review, pathogenicity of the causative agent R. salmoninarum (Rs), culture methods of Rs, serological and Rs specific gene detection methods for diagnoses, disease signs of infected fish, and distribution of BKD in Japan are described. Also, preventive measures of vertical transmission and control methods of BKD are introduced.
Candidatus Xenohaliotis californiensis, a Rickettsia-like organism (RLO) that causes ‘withering syndrome’ (WS) in abalone, has been recently detected in Japan. We analyzed partial nucleotide sequence (113 bp) of 16S rRNA of WS-RLOs (n = 335) from Japanese black abalone Haliotis discus discus, Ezo abalone H. discus hannai, giant abalone H. gigantea, tokobushi abalone H. diversicolor aquatilis and fukutokobushi abalone H. diversicolor diversicolor. All the sequences from Japanese black, Ezo and giant abalone were identical, but different from those from tokobushi and fukutokobushi at one nucleotide position. We also conducted cohabitation challenge to determine whether the WS-RLO in fukutokobushi infects Japanese black and giant abalone or the agent in Japanese black abalone infects fukutokobushi. Twenty fukutokobushi naturally infected with WS-RLO were cohabited with 10 healthy individuals of each of Japanese black, giant, and fukutokobushi abalone for a total of 84 days. At the end of the experiment, surviving fukutokobushi abalone were positive in PCR test for WS-RLO, but negative for that of Japanese black and Ezo abalone. In the reverse combination experiment, in which naturally infected Japanese black abalone were cohabited with these three species, WS-RLO transmissions were found in Japanese black and Ezo, but not in fukutokobushi abalone. These results suggest that two genetic variants of WS-RLO have a different host specificity.
Pathogenicity of a rainbow trout isolate RtNa-0010 of Oncorhynchus masou virus (OMV = salmonid herpesvirus 2) was investigated in salmonids and cyprinids. Immersion method was used to challenge three species of salmonid fish with RtNa-0010 and OMV reference strain OO-7812 at a dose of 100 TCID50/mL for 60 min. Cumulative mortality of rainbow trout was 3 and 51 of 51 fish when challenged with OO-7812 and RtNa-0010 respectively. When six salmonids and three cyprinids were challenged with RtNa-0010 (103 TCID50/fish) by intraperitoneal injection, mortality rates were 85%, 10% and 0% for rainbow trout, whitefish Coregonus lavaretus maraena and other tested fish respectively.
We examined the infection of Japanese eel endothelial cells-infecting virus (JEECV), which is the agent of viral endothelial cells necrosis (VECNE), in Japanese eels caught from the southern part of West Mariana Ridge in 2013 to know the infection status of the virus in their spawning area. JEECV was detected in the gills from a female out of five mature male and female fish by both quantitative and conventional PCRs. Additionally, the predicted polyomavirus large T like protein of the detected virus was different by one amino acid from that of the virus from farmed eels with VECNE.
Mass mortality of dotted gizzard shad Konosirus punctatus occurred at the Otagawa River estuary in Hiroshima Prefecture in autumn of 2007. Gram-negative bacteria were purely isolated from the kidney and brain of moribund fish. Based on the results of conventional characterization tests and genetic analysis, the bacterial isolates were identified as Vibrio harveyi. Intraperitoneal injection experiments revealed that the bacterial isolates were highly pathogenic to ayu Plecoglossus altivelis and amago salmon Oncorhynchus masou ishikawae. These results suggest that V. harveyi infection was associated with the mass mortality of dotted gizzard shad in 2007.