The survival rate of larval Japanese catfish (Silurus asotus) during seedling production is usually very low due to cannibalism and mass mortality caused by unknown factors. Mass mortalities with the rate of 90-100 % occurred frequently among larvae reared at Gunma Prefectural Fisheries Experimental Station just before the advent of cannibalism. Aeromonas hydrophila and other species of bacteria were isolated from moribund 7-day-old catfish larvae, and examined for their bacteriological characteristics and sensitivity against 7 drugs. Experiments on intra-abdominal injections of the isolates into catfish and carp showed that the isolates of A. hydrophila killed 80-100 % of catfish and carp, and the bacterium was successfully re-isolated from the kidney of the experimental fish. A. hydrophila was, thus, suspected to be a causative agent of the mass mortality in catfish larvae.
The toxicity of the herbicide paraquat (methyl viologen) was evaluated in various developmental stages of common carp, Cyprinus carpio L., namely yolk, post-yolk, fry, and fingerling. Bioassay was performed for a period of 96 h, and median lethal concentrations (LC50) were determined for all stages. The LC50 values at 96 h for yolk, post-yolk, fry, and fingerling stages were 11.3, 33.3, 67.5, and 134.1 ppm, respectively. These results show that the tolerance of carp to paraquat increases with the development of fish. Severe damage was observed in various organs of the fish exposed to 178 ppm paraquat for 96 h. Among histopathological changes, nucleic pyknosis in epithelial cells of renal tubules and in parenchymal cells of the liver was most prominent.
Detection survey on Aeromonas salmonicida was conducted from the coelomic fluid of 420 mature chum (Oncorhynchus keta), 120 pink (O. gorbuscha), and 180 masu (.O. masou) salmon which have no clinical signs of furunculosis, in 1989 in Hokkaido. A. salmonicida was isolated from 22 out of 120 chum salmon from the Tokachi River. The number of A. salmonicida in the coelomic fluid ranged from 101 to 106 CFU/ml in the samples from Shibetsu and Teshio rivers, respectively. It is suspected that the coelomic fluid contaminated with A. salmonicida flows out from the fish at the time of stripping eggs or during maturation in the pond. Since most hatcheries in Hokkaido drain untreated sewage directly to rivers, A. salmonicida in the coelomic fluid from mature fish may contaminate the river water and infect other anadromous salmon with A. salmonicida.
Enzyme linked immunosorbent assay (ELISA) using the anti-masu Ig rabbit serum was applied to detect the antibody against Aeromonas salmonicida in the sera of salmonid fish. This ELISA method could detect the antibody in sera of masu (Oncorhynchus masou), chum (O. keta) and coho salmon (O. kisutch) immunized by A. salmonicida and could be applied to detect the antibody against A. salmonicida in sera of matured masu, pink (O. gorbuscha), chum, and kokanee salmon (O. nerka). In 1989, 89 masu and 250 chum salmon, and 59 kokanee were examined to detect antibody against A. salmonicida by ELISA and agglutination test. The number of the fish showing the positive reaction by ELISA, agglutination test and isolation of A. salmonicida were 262, 48, and 33, respectively. In 1990, 200 masu and kokanee salmon fry released from hatchery to river were tested and antibody against A. salmonicida was detected from 111 fish by ELISA, and the incidence of the fish showing the positive reaction was ranged from 4 to 90 %.
Methods for artificially inducing erythrocytic inclusion body syndrome (EIBS) in cohosalmon, Oncorhynchus kisutch, were studied. Healthy coho salmon (B.W.7-8g) were injectedintraperitoneally or bath challenged with filtered homogenates of blood, spleen, head kidney, or excrement from naturally infected fish. The syndrome was induced by both injection andbath challenges. Artificial infections were obviously induced at temperatures of 8-10°C and there was a high incidence of fish with erythrocytic inclusions and low hematocrits. At 8-10°C, inclusion bodies appeared most frequently 3 weeks after injection, while they were most prevalent 4 weeks after bath challenges. Hematocrits decreased to 8-12% approximately 4-5weeks after injection when fish were held at 8-10°C. Blood homogenates of substantiallyinfected fish with red blood cells with inclusions were more effective than those from moribund fish without inclusions and were able to infect fish when diluted 10, 000 times by injectionor 1, 000 times by bath challenges.
Erythrocytic inclusion body syndrome (EIBS) is a major contributor to mortality of coho salmon reared in seawater net-pen culture in Japan. In this paper, a study of disease progression after artificial infection was carried out to better understand the disease and its causative agent. The five stages described by Piacentini et al. were recognized. Stage II and III were divided into two substages, respectively. A substage (Stage II-a) in which inclusions appeared only in immature erythrocytes was characterized. Stage III which was characterized by appearence of lysed erythrocytes was divided into two substages; Stage III-a (18-21 days postinjection), during which the prevalence of inclusions in mature erythrocytes increased rapidly and incidence of immature erythrocytes decreased rapidly, and Stage III-b (by 24 days postinjection), when the Ht values were lowest and no inclusions were observed in erythrocytes. Our result showed that a viral agent which causes high mortality in sea-cultured coho salmon in Japan has the same characteristic disease progression as the EIBS virus of North America.
We attempted to isolate the causative agent of Kuchijirosho of farmed tiger puffer using 8 established cell lines and a primary cell culture derived from the gonad of tiger. puffer (PFG). The inoculation of the brain or kidney filtrate prepared from diseased fish produced conspicuous CPE on PFG cells. However, no CPE was found in any of the cell lines used, i.e., BB, CHH-1, , CHSE-2l4, EPC, FHM, MCT, RTG-2 and SSE-30. Neither toxicity of the culture medium on PFG cells nor bacterial/mycoplasmal contamination was found, and the observed CPE was thus confirmed to be due to virus. The inoculation of the isolated virus to the tiger puffer resulted in high mortality and experimentally infected fish showed similar characteristic behavior and symptoms to those of naturally infected fish with Kuchijirosho. Furthermore, all experimental fish died after the inoculation of the viral fluid passed through a membrane filter with a pore size of 50 nm. Taken together the results of viral isolation, experimental infection and filtability test of isolated virus, it is concluded that Kuchijirosho is an infectious disease caused by a virus with a size less than 50nm in diameter.
A mass mortality due to an infectious disease occurred in juvenile striped jack (Pseudocaranx dentex) reared at a station of the Japan Sea-Farming Association in Nagasaki Prefecture in February, 1991 and a total of about 10, 000 juveniles (34%) were lost for about one month. Administrations of oxytetracycline and oxolinic acid with food were effective to decrease the mortality.One species of bacteria was purely or dominantly isolated from diseased juveniles and identified as Pasteurella piscicida based on its morphological, biochemical, genetical (G+C contents : 40.7-41.0 mol%), and serological characteristics. An experimental infection revealed that a selected isolate was pathogenic to juvenile striped jack and yound red sea bream, the LD50 by intraperitoneal injection for the two fish species being about103 and 107 CFU/fish, respectively. It was also demonstrated that the extracellular products (ECP) of the isolate were lethal to both fishes when it was injected intraperitoneally.
In vitro antimicrobial activity of bicozamycin (BCM), a cell wall synthesis inhibitor, was investigated against Pasteurella piscicida. Minimum inhibitory concentrations (MIC) of BCM against 263 clinical isolates of P. piscicida ranged from 1.56 to 6.25μg/ml. High incidence of drug-resistant strains agains other drugs such as β-lactamam antibiotics and pyridonecarboxylic acids were recognized. No cross resistance between BCM and other antibacterial agents were observed. BCM displayed bactericidal activity at its MIC level. P.piscicida strains showed a slowly increasing and stepwise pattern of artificial developmen of resistance against BCM. From the results obtained, it may be suggested that BCM would be applied as a new type of therapeutic agent among the cell wall synthesis inhibitors for the treatment of pseudotuberculosis.