Since the epoch-making discovery of the life-cycle of Myxobolus cerebralis by Wolf and Markiw (1984), studies on myxozoan parasites of fishes have continued to progress rapidly. Molecular analyses of myxozoan SSUrDNA have been successful in clarifying its systematic position not as protozoans but as metazoans. However, there remain gaps in our understanding of their biology and pathology. Alternate myxosporean and actinosporean stages in the life-cycles of more than 25 species of freshwater myxozoan have been demonstrated, but many unresolved questions remain, e.g., the relationship between the parasite and its alternate invertebrate hosts and the mode of transmission in marine myxozoans. These unsolved questions have made it difficult to design an effective control strategy for myxozoan diseases. Recent studies on countermeasures have been directed towards the introduction of resistant fish strains and controlling oligochaete populations.
To explain the involvement of leukocytes in defense system of the red sea bream Pagrus major, the kinetics of leukocytes in the peripheral blood and the sequential histopathological changes in the inflammatory foci during artificial infection of Edwardsiella tarda were investigated. In peripheral blood, the lymphocyte count declined and the granulocyte count increased at 1 day post-inoculation (dpi). The monocyte count started to increase from 5 dpi to go to a peak at 15 dpi. In inflammatory foci, the formation of granulomas was characteristic. In the foci, prior to the formation of granulomas, lymphocytes and granulocytes infiltrated, followed by monocytes infiltration. There was strong correlation between the progression of granulomas and the kinetics of leukocytes in the peripheral blood.
The seed production of marbled sole Pleuronectes yokohamae has been attempted at a hatchery in Kagawa Prefecture, Japan since 1989, while a subacute disease with a considerable amount of mortality occurred in larval and juvenile fish (24-69 days old) in 1989, 1990, 1991, 1997 and 2001. Diseased fish displayed edema, an abdominal distension due to ascites, and exophthalmia. The outbreak of the disease usually continued for 1 month or longer and the cumulative mortality was often over 40%. The evident histopathological features were the formation of multinuclear syncytia and following necrosis in the liver, renal tubules, pancreas and intestinal epithelium, and hemorrhage in the brain, choriocapillaries, stomach and skin. Electron microscopy of affected hepatocytes revealed formation of inclusion bodies consisting of crystalline arrays of virus particles (75-80 nm in diameter). These results suggest that the pathognomonic sign of the present disease is necrosis accompamied by the syncytial formation which would be associated with a virus infection.
The present study was conducted to determine the virulence and virulence properties of 4 bacterial strains belonging to a newly identified species Aeromonas sp. T8 group, isolated from epizootic ulcerative syndrome (EUS) -affected fish of Southeast Asian countries. Virulence of a representative strain T8 to fish was investigated through intramuscular injection to silver barb Barbodes gonionotus, striped snakehead Channa striatus and tilapia Oreochromis sp., where the LD50 value was determined to be 1.5×105, 3.2×105 and 2.9×105 CFU/fish, respectively.However, all strains were found non-virulent for mice upon intraperitoneal or intravenous injections at a concentration of 3×107 CFU/mouse. Live bacterial cells of these strains showed lipase, protease and hemolytic activities in agar plate assays. The hemolytic activity of the extracellular products (ECPs) was found to be higher for fish blood cells than for mammalian blood cells. Heat stability, EDTA and PMSF sensitivities of β-hemolytic activity of the ECPs differed from that of A. hydrophila and A. caviae reference strains. ECP of only one strain (P2) showed cytotoxicity against both fish (RTG-2, FHM, EPC, BF-2 and SNN-1) and human (HeLa) cell lines, while none of the strains exhibited enterotoxicity to suckling mice. All of the strains also failed to amplify aerolysin and cytolytic enterotoxin (AHCYTOEN) genes by PCR.
ABSTRACT-The detection of the paramyxean parasite Marteilioides chungmuensis has been achieved only by using standard histological techniques or gross observations, which have a limited sensitivity for early diagnosis. In the present study, small subunit ribosomal DNA of the parasite was sequenced, and polymerase chain reaction (PCR) primers designed accordingly. It has been demonstrated that both the PCR and in situ hybridization techniques are specific for M.chungmuensis. Moreover, compared to traditional histological techniques and gross observation, the PCR diagnosis showed a far greater sensitivity. These new diagnostic tools based on molecular techniques can also be used for the elucidation of the parasite's life cycle and the identification of possible intermediate hosts.
Bacterial coldwater disease is considered to be one of the most serious diseases in ayu Plecoglossus altivelis in Japan. We evaluated the efficacy of three highly water-soluble adjuvants (Montanidae IMS-1311, IMS-1312 and IMS-2212) combined with formalin-killed bacterin of Flavobacterium psychrophilum (FKB) in ayu, in comparison with a minimally water-soluble adjuvant (Montanidae ISA-763A). The retention period and toxicity of these adjuvants in ayu were also examined. Experimental challenge 4 wk after vaccination showed that FKB adjuvanted with IMS-1311 and IMS-1312 had higher potency than FKB alone and significantly (p<0.05) higher potency than the control with sterile distilled water. Residues of the highly water-soluble adjuvants in ayu remained for shorter periods than that of the minimally water-soluble adjuvant, although the former adjuvants were acutely toxic at doses higher than 25μL/fish (1.7±0.5g). The high level of protection, shot retention period and lack of toxicity indicate that a dose of 5μL of IMS-1311 or IMS-1312 combined with 5μL of FKB makes a promising vaccine against bacterial coldwater disease in ayu.
Five isolates of infectious hypodermal and hematopoietic necrosis virus (IHHNV) (three from Taiwan, one from Thailand and one from Ecuador) were sequenced and compared with two other previously sequenced isolates, one from Hawaii and one from the Gulf of California, Mexico. An unrooted neighbor-joining (NJ) phylogenetic tree analysis and a pairwise comparison both show that the variation among these seven different isolates was low. Despite this sequence similarity, our data suggest that two of the three Taiwan isolates may have been imported from the Americas, while the other isolate and the Thailand isolate may be instances of a local (South East Asian) strain. This has implications for the epidemiology of IHHNV, and suggests that geographic isolation has allowed the virus to evolve into two distinct groups.
In February 2002, mass mortalities were first observed among striped catfish Pangasius hypophthalmus reared in freshwater ponds of central Sumatra, Indonesia. The affected fish showed pale gills, abdominal swelling and hemorrhage of the skin, as well as swelling of internal organs with white patches. Edwardsiella ictaluri was isolated from the internal organs of the diseased fish. The isolates showed pathogenicity to P. hypophthalmus by an artificial infection. However, the isolate preserved in a semisolid BHI agar for 6 months lost pathogenicity.
Virucidal effects of six kinds of disinfectants were examined against Oncorhynchus masou virus (OMV), and infectious pancreatic necrosis virus (IPNV), infectious hematopoietic necrosis virus (IHNV) and hirame rhabdovirus (HIRRV). At 15°C for 20 min, minimum concentrations showing 100% plaque reduction of OMV by iodophor, sodium hypochlorite solution, benzalkonium chloride solution, saponated cresol solution, formaldehyde solution and potassium permanganate solution were 40, 50, 100, 100, 3, 500 and 16 ppm, respectively. Susceptibility of OMV against these disinfectants was higher than that of IPNV, IHNV and HIRRV, suggesting that the usage of disinfectants against IPNV and/or IHNV is also effective against OMV.
A symposium entitled “Progress in fish and shellfish immunology and contribution to the fisheries” was held by the Japanese Society of Fish Pathology in Shimonoseki on 4th October, 2003. This was aimed to have an overview of the progress in fish and crustaceans immunology and its contribution to the development of aquaculture. The symposium was composed of six categories: (1) Prevention of infectious diseases of crustaceans by activation of immunodefense system, (2) Current status and prospects in fish immunology, (3) Immune relevant genes and their functions in fish, (4) Prevention of infectious diseases in fish by inducing cytokines, (5) Current status and prospects of development of fish vaccines, and (6) Potential application of recombinant cytokines and new gene ration vaccine for the control of infectious diseases in domestic animals. Outbreaks of infectious diseases, causing serious damage to cultured fish and shrimp, have been occurring in Japan. The advanced knowledge in fish and shellfish immunology will contribute to the control of infectious diseases in aquaculture.