Recently, Mycobacterium infection has been observed in cultured yellowtail Seriola quinqueradiata in Japan but not studied in detail. Diseased fish were lethargic, anorexic and emaciated, and showed hemorrhagic cutaneous ulceration and ascites. The necropsy and histopathological features showed that disseminated necrosis and numerous white nodules were found in the kidney, spleen, liver and heart. Numerous acid-fast bacteria were detected in the above tissues and granulomas. Myositis, hepatitis, splenitis and nephritis due to granulomas and gill inflammation were histologically observed. Almost all granulomas were classified into soft tubercle-type. All bacteria isolated from the diseased fish were Gram-positive, acid-fast, rod and non-motile. As a result, they were classified into the genus Mycobacterium. The isolates were identified as Mycobacterium marinum on the basis of biological and biochemical characteristics and the analysis of a partial 16S rRNA gene sequence. An experimental infection test showed that a representative isolate had pathogenicity to yellowtail with disease signs similar to those of naturally affected fish. This is the first report on M. marinum infection in cultured yellowtail.
A hitherto unknown leech has been found infected on crucian carps Carassius auratus langsdorfii and C. cuvieri in the Yodo River system since 2000. From the morphological characteristics, the parasite was identified as Limnotrachelobdella sinensis (Piscicolidae). Parasitological surveys on 16 species of wild fish (n = 1,661) caught from October 2000 until March 2005 revealed that the leech was found only from moribund crucian carps. It has a one-year life cycle, in which the leeches were found from December until April next year. Because of its recovery only from moribund fish, large size (up to 4.9 cm in body length) and frequent heavy infection (highest number of parasites/fish: 62), its pathological effects on the hosts were suspected. However, such effects were not made clear except for the hemorrhages and erosion on and near the inner surface of operculum, the attachment site. This is the first record of this parasite from Japanese waters. Since the geographical distribution of the leech in Japan is still restricted to the Yodo River system, the potential risk of spread of the parasite to other water bodies is discussed.
Monthly samplings of tiger puffer Takifugu rubripes, which had been cultured in China and later introduced to a farm in Kagawa Prefecture, Japan, in May 2005, were conducted over a four-month period commencing in June 2005. Blood flukes were found in the visceral vascular system with the highest number being 162 worms/fish, and their eggs accumulated in the visceral organs. The parasite was classified within the genus Psettarium based on the unique male reproductive system, and tentatively designated as Psettarium sp. TPC (= tiger puffer from China). Molecular analyses, using the ITS2 rDNA gene indicated that the blood flukes of tiger puffer from two other sources (cultured in China and subsequently in the Kyushu area, and cultured in China only) were also Psettarium sp. TPC. This suggests that the infection cycle of Psettarium sp. TPC has been established in China. On the other hand, no evidence was found that its life cycle is complete in Japanese waters. Similar blood flukes were collected in 1993 from post-spawned tiger puffer caught in a set-net in Wakasa Bay, Fukui Prefecture and maintained there for several months. This parasite was differentiated from Psettarium sp. TPC by the presence of 5-6 rows of pre-oral spines and designated as Psettarium sp. TPJ (= tiger puffer from Japan).
Streptococcus iniae causes an acute systemic disease in Japanese flounder Paralichthys olivaceus, one of the important cultured fish in Japan. Although commercial S. iniae vaccines have been developed and used in farms since 2005, the immunoprotection mechanism in vaccinated fish has not been elucidated yet. To verify the involvement of capsular polysaccharides of S. iniae in immunoprotection, we compared the protective efficacy of formalin-killed cells (FKC) of S. iniae NUF631 (capsulated) with those of its isogenic capsular-deleted mutants in S. iniae infection of Japanese flounder. As a result, high protection was achieved by immunization with NUF631 FKC but not with mutant FKC. Viable count of intravenously inoculated NUF631 decreased in the kidney of flounder immunized with NUF631 FKC, and NUF631 cells opsonized with anti-NUF631 flounder serum elevated the phagocytic activity and reactive oxygen species production of flounder peritoneal macrophages. In electron microscopic examination, electronically dense materials were observed on the capsule of NUF631 cells pretreated with the antiserum, and in western blot analysis flounder antibody was detected in the antiserum components bound to NUF631 cells. These findings indicate that capsular polysaccharides are important protective antigens and anti-capsule antibody plays a protective role as an opsonin in flounder S. iniae infection.
The susceptibility of carp Cyprinus carpio to koi herpesvirus (KHV) was examined by experimental infections. Larvae of two strains of common carp were exposed to KHV by immersing the fish in water containing the virus at a dose of 101.6TCID50/mL measured using KF-1 cells. No mortality by KHV disease was observed among any larvae (average TL: 7.5 and 8.7 mm). On the other hand, high mortalities (69% and 100%) were observed in common carp juveniles (TL: 13.8 and 29.2 mm). Koi carp larvae (TL: 6.9 mm) were challenged in the same experimental conditions as described above, in which no larva died. Two months later, the same fish (TL: 48.2 mm) were exposed to the virus again. This time, all fish died of KHV. These results suggest that carp larvae are not susceptible to KHV, and that they become susceptible as they grow up.
The virucidal effect of disinfectants against spring viremia of carp virus was assessed using EPC cells. Five kinds of disinfectants, i. e. benzalkonium chloride (BZC), alkyltoluene (AKT), chlorhexidine gluconate (CHG), cresol and ethanol, were examined at graduated concentrations of the disinfectants for different reaction periods, 30 s and 20 min. The minimum concentration of each disinfectant demonstrating >99.99% inactivation of the virus was 100 ppm (20 min) for BZC, 500 ppm (30 s) and 350 ppm (20 min) for AKT, 175 ppm (30 s) and 100 ppm (20 min) for CHG, 0.25% (30 s) and 200 ppm (20 min) for cresol and 50% (30 s) and 40% (20 min) for ethanol.
Presence of fimbrial genes among fish pathogenic and non-pathogenic strains of Edwardsiella tarda was investigated by polymerase chain reaction (PCR). Four primer sets for PCR were designed to detect the four genes (etfA, etfB, etfC, etfD) of the type 1 fimbrial gene cluster of E. tarda. All the four genes were successfully amplified with the four primer sets in all the pathogenic strains. On the other hand, any of etfA, etfB and etfC were not detected in 13 of 14 non-pathogenic strains. The results suggest that fimbriae play a role in the pathogenicity of E. tarda and that detection of etfA, etfB and etfC by PCR is useful to distinguish between pathogenic and non-pathogenic strains of E. tarda.
Disease records (25,265 cases in total) of cultured salmonids diagnosed from 1978 to 2002 at prefectural fisheries experiment stations in Japan, belonging to National Association for Technology Development of Trout and Salmon Culture, were analyzed. With the diversification of cultured species and fish size, disease problems in large-sized fish and mixed infections have become serious. Important infectious diseases were IHN, OMVD, streptococcicosis and bacterial cold-water disease.