Pond water of eel farms in Yoshida, Shizuoka Prefecture was examined for the presence of fish pathogenic bacteria, especially for Aeromonas spp. and Edwardsiella tarda. One-tenth milliliter of a series of ten fold dilutions of pond water was spread over the surface of nutrient agar(Difco).Plate cultures were incuvated for 4 or 5 days at about 25°C and colonies counted. Every colonies within a given area of the plate of suitable dilutions was subcultivated for identification. The number of bacteria ranged from 102 to 105 cells/ml through the year, being at the maximum in September and at the minimum in January(Table 1 and Table 2). Aeromonads and enterobacteria were predominantly found in all samples. Aeromonads represented the majority in fall to spring and tended to decrease as the water temperature rose(Table 3 and Table 4). A total 232 aeromanads were consisted of 224 A. hydrophila and 8 A. punctata. Of 134 A. hydrophila and 5 A.punctata, which were injected to weatherfish for testing its pathogenicity, only 13 A. hydrophila proved able to kill fish without full(Table 6). Enterobacteria were classified to the rank of genus as follow:78 Citrobacter, 46 Enterobacter, 9 Edwardsiella, 4 Escherichia and 45 others(Table 5). While many eels found to be infected with Edwardsiela tarda (WAKABAYASHI et al.1973, 1975), pond water had rather few E. tarda.
Histopathological studies were made on 9 diseased eels with suppurative hepatitis caused by Edwardsiella tarda infection. Fish in advanced stages of the disease showed a reddening of fins and the body surface and a swelling of the liver region of the trunk. Anatomically abscesses were observed in the liver, some of which were followed by the ulcer formation The primary, detectable histopathological change in the liver was the appearance of micro abscesses containing macrophages phagocytizing bacteria in the sinusoids. As the disease progressed enlarged abscesses which were composed of a large number of juvenile neutrophils and necrotized hepatic cells and were thought to develop from the above-mentioned primary foci became more noticeable. Extensive lesions accompanied by liquefaction of abscesses and bacterial multiplication were found to involve large blood-vessels and in such cases metastatic necrotic lesions were observed in various parts of the liver and the hepatic cells were in an intensive fatty degeneration. While healing lesions demarcated by precipitated fibrin and newly formed granulation tissues were often observed. Peripheral extensive lesions often developed into ulcers and suppurative lesions were found in the region of the body musculature adjacent to large ulcers of the liver. In the stage of the generalized infection metastatic lesions were observed in the heart, gills and body musculature, and the spleen and the hematopoietic tissue of the kidney showed a cellular proliferativeinfiltrative reaction, and a marked hyperplastic reaction of the hematopoietic cells, respectively.
Studies were made on tissue levels of chloramphenicol(CP)in amago salmon, Onchorhynchus rhodurus which were given 50 and 100 mg of CP per kilogram of body weight. CP was incorporated into soft pellets, which were introduced into the stomach. The highest tissue levels of CP were obtained 12 and 24 hours after medication at about 20°C and at about 7°C, respectively and maximum recorded levels were somewhat higher in all tissues studied at 7°C than at 20°C. The highest levels of CP were found in liver and the lowest levels were in muscle. Past-treatment time required for drug elimination was greater at 7°C than at 20°C. It was suggested that a considerable withholding period of at least 10 to 14 days may be needed in cases in which CP was administered daily to salmonids for successive 3 to 5 days for the treatment of bacterial infections.
Glugea infection is one of the most dangerous diseases of the ayu. The present study was undertaken to find effective therapeutic agents against the infection. Nine drugs which had been reported to have protozoacidal effects were preliminarily tested for effectiveness by administering per os to juvenila ayu which had been infected artificially with the Glegea. Among the drugs tested fumagillin was found to be promising.Investigations on the effective dosage of the drug, treatment period, etc., showed that treatments with 50mg of the drug per day per kg of fish for 3 days and 12.5 mg per day per kg of fish for 10 days were effective when applied during the period of 10 to 25 day after infection.
Histological and haematological studies were made on apparently healthy eels (Anguilla japonica) collected from eel-culture ponds in Shizuoka prefecture through late August to mid-December, 1973. No epizooty was reported in those ponds during the period of study and all fish studied had no visible external abnormalities. Hematocrit values and plasma Cl- concentrations were within the normal ranges in all fish throughout the seasons and no histological alterations were observed among the fish collected in August and September. Histological changes similar to those observed in fish with “branchionephritis”, however, occurred in the fish of October and progressed to advanced stages with seasons. Epitherial hyperplasia started in the gill filaments, not in the lamellae, and formed a tong-like growth of epithelial tissue between the lamellae, and finally developed into crubbing of gill filaments. Histological changes in the kidney were rather slight. Those changes were thought to be the reactions to the coldness and/or fasting of the hibernating eels in the pond.
Recently IVASHKIN et al. (1971) reidentified a nematode of the carp in Russia, which was first classified as Philometra lusiana by VISMANIS in 1966, as a member of the genus Philometroides. The present author compared in morphological discription P. lusiana with P. cyprini (ISHII, 1931) NAKAJIMA, 1970 which has long been known as a common parasite of cultured carp in Japan and was first reported by ISHII as a Filalia of carp in 1916 and named F. cyprini by him in 1931. No essential difference could be found out in morphology and parasitism between the two species and therefore it was suggested that the former might be a synonym for the latter.