We conducted a series of experiments to understand the behavioral characteristics of oncomiracidium of the monogenean Heterobothrium okamotoi and compared them with those of two other monogenean species, Heteraxine heterocerca and Neobenedenia girellae. Behavior of H. okamotoi oncomiracidium consisted of two alternate phases: a swimming phase with strong ciliary beatings and a stationary phase with ciliary beatings too weak to generate any movements. The duration of one swimming-stationary cycle considerably varied among individuals but tended to increase with larval age. During the 5-day monitoring, length of the swimming phase decreased while that of the stationary phase tended to increase as the larva became older. The locomotion patterns differed significantly among individuals, but as a whole, larvae showed random horizontal movement and no phototactic reaction. The oncomiracidium tended to move downward because it was heavier than seawater and also swam downward at the start of the swimming phase because its anterior part pointed downward when the larva was not swimming. Those behavioral characteristics of larval H. okamotoi are likely to have evolved in order to maximize the chance of encountering its benthic fish host, tiger puffer Takifugu rubripes.
Bacterial cold-water disease (BCWD) causes heavy mortality of ayu Plecoglossus altivelis in Japan. To control BCWD of ayu, warmed water treatment at 28°C has been used. However, the mechanism of this treatment is unknown. Hence, we investigated the growth/survival of Flavobacterium psychrophilum at 15-28°C under two nutritional conditions: in modified cytophaga (MCY) broth and sterilized underground water. Within 2 days at 28°C in both the conditions, F. psychrophilum totally lost its colony-forming ability. We also studied distributions of F. psychrophilum in infected fish organs and in rearing water after various treatments. For bacteria detection, the colony-counting method and loop-mediated isothermal amplification (LAMP) method were used. For warmed water treatment, the rearing water was warmed from 18°C to 23°C or 28°C for 3 days. The treatments were started 1 day after immersion infection. In the experimentally infected fish, F. psychrophilum was detected neither in any fish organs nor in the rearing water after the 28°C treatment for 3 days. These results indicate the effectiveness of the warmed water treatment at 28°C against BCWD of ayu.
Minimum inhibitory concentrations (MICs) of 12 chemical reagents were determined for four strains of Tenacibaculum maritimum. Bronopol and stabilized chlorine dioxide showed the lowest MIC (1.0 μg/mL). With 6-h treatment, minimum bactericidal concentrations (MBCs) of bronopol and stabilized chlorine dioxide were 4.0 and ≤8.0 μg/mL, respectively. Both chemicals were non-toxic to five marine fishes in 6-h exposure at concentrations 5 times higher than the MBCs. The two chemicals are the most suitable candidates for antibacterial agents for treatment of T. maritimum infection.
Amami Station of Fisheries Research Agency, Japan has experienced mass mortalities of Pacific bluefin tuna Thunnus olientalis in the process of seed production since around 2000. The mortalities mostly occurred at larval stages. In some cases of the mortalities, but not all cases, the diseased fish was characterized by vacuolation in the central nervous systems and retina, and a betanodavirus (RGNNV genotype) was detected in the affected fish by PCR and FAT methods and culture isolation. The present results suggest that VNN can be a cause of larval mortality of Pacific bluefin tuna.
FBS components in viral culture fluid interfere with the detection of specific antibody in immunized fish. In the present study, we developed a method to eliminate the interference of FBS using a model of red sea bream iridovirus (RSIV) vaccine. In an experiment using yellowtail antiserum against BSA, which is a major component of FBS in cell culture media, antibodies against BSA were completely absorbed via incubation of antiserum in 50% FBS at 25°C for 1 h. In antibody detection, ELISA with the sera of yellowtail surviving RSIV-challenge, antibodies against RSIV were detected, but those against FBS disappeared via the pre-treatment of the sera. Therefore, antibodies against RSIV are effectively detected using ELISA with the pre-treatment of fish sera.
Mass mortality occurred in Manila clam Ruditapes philippinarum under a hanging cultivation condition from April to July 2008 in Japan. A bacterium was isolated from the affected clams showing brown conchiolin deposits on the inner shell surface and identified as Vibrio tapetis, the causal agent of brown ring disease (BRD) in clams, by bacteriological characterization and genetic analysis. Experimental infection of clams confirmed pathogenicity of the isolated bacterium. These results indicate that the present mortality of cultured clams was caused by BRD. This is the first report on isolation of V. tapetis from diseased R. philippinarum in Japan.
An adult neon tetra Paracheirodon innesi with a history of difficulty in feeding caused by a mass overwhelming the snout is presented. Macroscopically the fish presented a unilateral, single, nodular and well-circumscribed, perlaceous, firm to hard mass in the left snout region. Histologically, tumor tissue was formed mainly by abundant, highly cellular elliptic nests of chondrocytes lying in mature hyaline cartilage matrix. Besides this component there were bubbly appearing large cells (physaliphorous-like cells) and a minority of cells having well defined cytoplasmic boundaries, clear eosinophilic pale cytoplasm and a rounded centrally located nucleus (“fried egg-like cells”). Alcian-PAS and Masson’s trichrome stains were applied to demonstrate proteoglycans and collagen of the matrix. Based on topographic localization and histomorphological features, a diagnosis of snout chondroma was made. This case can be added to the other rare spontaneous cartilage tumors of fish described in literature.
We conducted experimental infection tests of Mycobacterium marinum NJB 0419 isolated from yellowtail Seriola quinqueradiata to amberjack S. dumerili and red sea bream Pagrus major to determine the pathogenicity of the bacterium. As a result, NJB 0419 was pathogenic to both of them. In light of concerns about zoonoses, BALB/c mice were intravenously or subcutaneously injected with NJB 0419. The histopathological findings of examined mouse tissues were normal. Acid-fast bacteria were not found in the tissues and not re-isolated from the tissues. Our data suggest that M. marinum NJB 0419 might not be a strong pathogen of mammals, although more intensive studies are required for confirmation of the pathogenicity.
In 2009, a disease with cumulative mortality of 52.5% was observed in a captive population of sailfin sandfish Arctoscopus japonicus reared at an aquarium in Japan. The affected fish had no apparent lesions on the body surfaces, with the exception of skin abrasions on the lower jaw. The trunk kidney was swollen and bore white miliary nodules. The most prominent histopathological feature of the diseased fish was numerous bacterial colonies in the internal organs. However, no inflammatory cellular response was apparent. Single species of bacteria was isolated from the trunk kidney and spleen, and identified as atypical A. salmonicida based on the biochemical and serological characteristics.