A bacterial gill disease named “Eragusare-byo” occurred among pond-cultured eels in a eel farm in Fukui Prefecture during March 1974. The pond-cultured eels with the disease included two species, Japanese eels (Anguilla japonica) and European eels (A. anguilla), and the water temperature of the ponds was above 20°C throughout the year. Histopathological studies on these diseased eels were done by the author and the following results were obtained. The eariest histopathological changes were observed on the gill lamellae of the two species of eels as inflammatory edema, swelling of the epithelial cells and excessive secretion of mucin. Column-like protrusions formed by a number of slender, rod-shaped Gram-negative bacteria were often found on the surfaces of the gill lamellae in these eels. In the subsequent inflammatory processes, there were marked differences regarding histological changes between Japanese and European eels. In the case of Japanese eels, there were three types of processes in the formation of “clubbing” of gill filaments. The type I was caused by hyperplasia of the lamellar epithelium at the distal tips of the lamellae. The type II was caused by hyperplasia of the epithelium in all the regions of the lamellae. The type III was caused by hyperplasia of the lamellar epithelium at the distal tips and basal parts of the lamellae. In many cases these three types of processes of the clubbing formation were recognized in the same individual. In a more aggravated case the clubbed filaments had gangrenous inflammation owing to combination of circulatory failure and putrifying bacterial infection, and finally they fell off from the gill arches. In European eels, the clubbing of the filaments was mainly caused by hyperplasia of mucous cells, but necrosis or defect of the clubbed filaments was never recognized.
Serological relationships among the strains of pathogenic Vibrio isolated from cultured eels and between the eel isolates and the species of the genus Vibrio were studied by means of Ouchterlony immunodiffusion techniques and agglutinin adsorption tests with rabbit antisera prepared against representative strains of the present isolates. The present eel isolates were antigenetically very homologous : All representative five isolates produced four lines of identity in Ouchterlony plates, and possessed agglutinins (flagellar and at least one heat-stable antigens) in common. The species of the genus Vibrio exhibited slight relationships to the eel isolate : V. anguillarum strains, V. parahaemolyticus strain, and V. alginolyticus strain shared one precipitin band with the present eel isolate, and V. anguillarum strains, V. fischeri strains, V. parahaemolyticus strain, and V. alginolyticus strain had agglutinins, most likely flagella antigen, common to the present eel isolate. These results well agree with those of the previous DNA studies, and present another piece of evidence supporting that the present eel isolate is a new member of the genus Vibrio.
A search was made for the diagnostic tests for the disease of cultured eels due to the present bacterium. It is considered impossible to differentiate the disease of eels caused by the present bacterium, by means of external signs of diseased fish, from that caused by other eel pathogens such as Aeromonas hydrophila, Edwardsiella tarda, Vibrio anguillarum, and Pseudomonas anguilliseptica. This necessitates the isolation and identification of the infectious agent for diagnosis. Positive proof was obtained that the diluted rabbit antisera prepared against representative strains of the present bacterium can be used for diagnostic purpose. In addition, a simplified identification method by the use of API 20E was shown. It was proposed that the disease of cultured eels caused by the present vibrio be named as vibriosis of eels Type B ; that due to V. anguillarum as Type A.
Cidal effect of ultraviolet irradiation on 10 strains of fish pathogenic aquatic fungi and 6 specises of bacteria including fish pathogens were examined. The results obtained are summarized as follows : 1) The punched agar medium disk covered with fungus hyphae was employed for the measurement of susceptibility to ultraviolet irradiation of aquatic fungi. The hyphae of 10 strains of aquatic fungi, i. e. Saprolegnia parasitica IFO 8978 etc. show relatively lower susceptibility to ultraviolet irradiation compared with the bacteria. The minimal fungicidal dosage of ultraviolet irradiation, inhibition of growth of hyphae, was 154-252×103μW·sec/cm2. 2) This dosage was about 50 times higher than the minimal bactericidal dosage of ultraviolet irradiation for destruction, at least 99.9% of viable bacterial cells.
The effect of ultraviolet disinfection (22-63×103μW·sec/cm3) of water supply on hatching rate of salmon eggs, masu salmon (Oncorhynchus masou) and chum salmon (O. keta) were examined. The results obtained are summarized as follows: 1) Viable bacterial counts were decreased about 90% in ultraviolet treated water, but viable fungi counts did not decrease to the same degree as the bacterial counts. 2) The eying rate and hatching rate were about 20% higher in the ultraviolet treated hatching tray than the untreated control, the fungi infection rate of the egg decreased about 19%. 3) Utilization of disinfection of hatchery water supply by ultraviolet irradiation was expected to increase the eying rate and the hatching rate.
An outbreak of Caligus orientalis (parasitic Copepoda) occurred in a fish farm of carp, Cyprinus carpio, at Tottori Prefecture in summer 1978, and many carp over 1 age in addition to the all individuals of 0 age were led to die. The farm has used the water of lake Togo-ike which communicates with the Sea of Japan through a passage of about 2 km long. In the year 1978, chlorinity of the water increased from March to September; it was measured 1380 ppm Cl- concentration in March, 4060 ppm in July and 7300 ppm in August in 0.5 m layer at the center of the lake. It is evident that the outbreak of the caligid on carp is attributed to a rise of chlorinity of the water as in a case of crucian carp in Lake Shinzi (Suzumoto, 1974). This is the first record of an outbreak of Caligus orientalis on carp.