The present authors have been studing on vibriosis (Vibrio anguillarum infection) in ayu (Plecoglossus altivelis) from the viewpoint of control. Prior to the main experiment, several experimental challenge methods were examined in oredr to establish a reproducible artificial challenge system of vibriosis. This paper deals with the validity of the artificial challenge system by intramuscular inoculation and a practical application of this system to testing the efficacy of oral-administered drugs for ayu. The results are summarized as follows. 1) The minimal lethal dose of a strain of Vibrio anguillarum used in this study proved to be 8.75 cells or 1×10 mg-8 in wet weight per 100 g of body weight of ayu. 2) A clear correlation was observed between inoculation dose and mean time to death in the challenge system tested. 3) A satisfactory result was obtained in the experiment of drug efficacy, and the intramuscular inoculation was found applicable to tests for drug efficacy.
Effect of Sodium. Nifurstyrenate (NFS-Na) and two therapeutic agents for experimental streptococcal infection were studied with yellowtail. Two times on in vivo antibacterial experiment were examined. Influence of challenge dose on the protective effect of NFS-Na was examined at experiment A. The protective effect of NFS-Na was compared with that of two therapeutical agents, and administration dose of NFS-Na and period of administration were examined at experiment B. NFS-Na showed a remarkable therapeutic effect at the administration for three or five days at 50 mg per kg body weight per day. Ampicillin do not showed therapeutic effect at the administration for five days at 50 mg per kg body weight per day. Chloramphenicol showed a slight therapeutic effect at the administration for five days at 50 or 25 mg per kg body weight per day.
New vibriosis of the Japanese eel broke out in Tokushima prefecture in 1975. MUROGA et al. (1976) reported that the causative bacterium had charactaristics resembling those of V. angillicida (BRUUN. 1932). The present authors histopathologically observed this vibriosis. A small red patch in early stages or a swollen lesion exhibiting hemorrhage and cutaneous necrosis in advanced cases was observed in the trunk or tail of the diseased fish. Internal gross symptoms of the advanced cases were congested liver, swollen spleen with dark-red coloration, reddish intestine with epithilial desquamation, softened kidney and vascular dilatation of the viscera. Histopathological observation defined that a main lesion was an infected lesion involved skin and lateral musculature. In the early lesion the bacterium penetrated the dermis, subcutaneous adipose tissue and red musculature and in the advanced cases they spread moreover myoseptum and lateral musculature. The lesion showed necrosis of the affected tissue, extensive vascular dilatation, serous exudation and hemorrhages. Epithelial slough from the edematous, hemorrhagic dermis was also accompanied in the affected skin. In particular in the advanced cases necrosis, circulatory derangements and bacterial multipoication became more intensive and extensive. The advanced cases underwent systemic infection. Metastatic lesions showing necrosis and circulatory derangements were observed in the spleen, liver, kidney, heart, gills and intestine which was particularly followed by desquamative catarrh.
A gliding bacterial infection was associated with the death of red seabream-and gilthead fry reared in floating net cages. The disease occurred among the fry grown to the size of 15 to 60 mm (mainly 20 to 40 mm) in total length, and caused the mortality of 20 to 30 percent of the populations. Eroded mouth, frayed fins and rotten tail were characteristic symptoms of the disease. Large numbers of gliding bacteria grew in the lesions and assumed whitish yellow color. The organism was a gram-negative, chytochrome oxydase-positive, flexble rod measuring 0.3-0.5μ× 2-6μ. It did not utilize agar, cellulose or chitine. There was no growth on Ordal's medium (modified) without seawater. An attempt artificially was made to infect red seabream-and gilthead fry by smearing the culture on the surface of mouth or caudal fin. The experimentally infected fish displayed essentially same symptoms as the naturally infected ones.
A milky condition in smoked coho salmon was observed at a food processing factory in Hokkaido, Japan. The incidence of milky condition was 7.8-11.7 percent of the fish after completion of the smoking process. This condition was due to an infection of the raw salmon, imported from Canada with a myxosporidian parasite identified as Henneguya salmonicola by its characteristic spores.
The first stage larvae of Philometroides carassii(ISHII, 1931) NAKAJIMA et EGUSA, 1977 which were released from the gravid females had very delicate bodies with adhesive sharp pointed tails. They extended and constracted actively in live but were always bent into a siclke-shape when died. They survived in water for seven days at 15°C, five days at 25°C, four days at 2-5°C and several hours at 37°C. Drying, freezing, heating and irradiating of ultraviolet rays were very effective to kill them. The larvae were killed instantly when water was evapolated or freezed. Heated larvae died within 18-20 minutes at 40°C, 8-9 minutes at 45°C and a minute at 50°C, and larvae were killed, within 6-8 minutes by irradiation of 2, 000μW/cm2, 14-16 minutes by 1, 000μW/cm2, 43-45 minutes by 400μW/cm2. Larvicidal effect of several disinfectants and anthelmintics were examined in vitro. One ppm of Acrinol, Benzetonium chloride and Sodium dichloroisocyanurate and 10 ppm of Bithionol and Dithiazanine iodide was demonstrated to kill larvae within three days.
The first stage larvae of Philometroides carassii(ISHII, 1931) NAKAJIMA et EGUSA, 1977, measured 332-404μlong in whole body length, were eaten actively by Cyclops; Cyclops strenus FISHER, C. vicinus ULJANIN and Tropocyclops prasinus(FISHER). The swallowed larvae penetrated the stomach wall of Cyclops and moved to the hemocoel within five hours. Most larvae began to grow in two days in the hemocoel under room temperature(21-26°C)and fully developed within a week when their total body length reached about 470μon the average, though the maximun size reaching 554μ. In this first one week the tail length was gradually decreased, and afterward the total body length changed no longer until after 20 days. Cyclops infected with numerous larvae lost activity and died within a week.
Quantitative and qualitative investigations of intestinal microflora were made on healthy and diseased eels collected from culturing ponds in Yoshida, Shizuoka Prefecture. The whole intestine of each eel was homogenized in sterile 0.85% NaCl solution. Tenfold dilutions were prepared and 0.1 ml of which was inoculated on the surface of nutrient agar (Difco). Plate cultures were incubated for five or seven days at 25°C and colonies were counted. Every colony within a given area of the plate of suitable dilutions was subcultured for identification. The numbers of viable bacteria per gram of intestinal tract with contents in healthy eels were ranged from 10 to 109 through the year. The percentage of Aeromonas spp. (mostly A. hydrophila) was relatively higher in winter, while that of Enterobacteriacea was higher from spring to fall. Vibrio sp. appeared only in October. Streptococcus sp. was always present 20% to 80% in the intestine of fed eels. Viable counts of intestinal bacteria of diseased eels were 10 to 106 higher than those of the healthy ones (Fig. 1 and Table 4). The percentage of A. hydrophila tended to be higher (Fig. 2), especially in the case of red disease. Moreover, virulent strains from eels suffered from red disease were greater in number than those from healthy ones or eels infected with Edwardsiella tarda (Table 5).
The larvae of Digramma alternans were found on the Japanese dace, Tribolodon hakonensis inhabited Hidaka-horobetsu River in Hokkaido. This is a first finding of D. alternans in fish inhabited rivers in Japan. It was assumed that the first intermediate host (copepods) and the final host (birds) inhabit the swamps which are located around the mouth of the river and lead to it.
The present study was carried out to reveal the characteristics of organism responsible for socalled vibriosis prevailing in cold-water season among cultured red sea bream, Pagrus major. Nine strains obtained from diseased red sea bream in some culture farms in Nagasaki Prefecture in the mid-winter of 1977 were submitted to the morphological, biochemical and physiological characterization. All the strains were gram-negative, nonsporning short rods with one polar flagellum.Cytochrome oxidase and catalase reactions were all positive. Glucose was fermented with no gas production by these strains. In consideration of these characteristics, the present isolates were tentatively ascribed to Vibrio, although many differences were observed among these strains in the other characteristics. On the basis of the decarboxylation of lysine, arginine and ornithine, it was possible to divide these stains into three groups. In general, the temperature of 20-27°C, the NaCl concentration of 2-3%and the pH of 6-8 were optimal for the growth in peptone water. Future taxonomic or genetic studies will be necessary to define the present isolates accurately.