Edwardsiella tarda has been known as the causative bacterium of paracolo-disease (edwardsiellosis)in cultured eels in Japan. However, the ecology of the pathogen in eel culture ponds has not been fully investigated so far. Field surveys on the presence and abundance of E. tarda in water and mud of eel culture ponds in Tokushima Prefecture were made at four seasons from 1981 to 1982 by a modification of the detection method described by WYATT et al.(1979). The incidence of E. tarda in water(W)and mud(M)samples from some 30 ponds were 90%(W)and 91%(M)in summer, 97%(W)and 100%(M)in autumn, 48%(W)and 25%(M)in winter, and 73%(W)and 75%(M)in spring. The number of the organism also increased during the warmer seasons. From the results of intramuscular injections into Japanese eels(Anguilla japonica), 37 strains out of 159 strains of E. tarda isolated from water and mud samples were proved virulent. Although these virulent strains shared common O-antigens with each other, they were divided into 2 serotypes based on a bsorption tests. About one-third of the avirulent strains also agglutinated with rabbit O-antisera against the virulent strains.
Susceptibility in fry of several salmonid to Oncorhynchus masou virus (OMV) was studied. Each 100 fry of chum salmon (Oncorhynchus keta; 0, 1, 2, 3, 4, 5, 6 and 7-month-old), masu salmon (O. masou; 0, 3 and 5-month-old), kokanee salmon (O. nerka; 0 and 1-month-old), coho salmon (O. kisutch; 0 and 1-month-old) and rainbow trout (Salmo gairdneri; 1.6-month-old) were used. The fry were immersed for one hour in 10°C water containing 100 TCID50/ml of culture grown OMV, and then held in running water at 10°-15°C. The cumulative mortality of just hatching chum salmon, observed in ensuing 4 months was 35%, but between 1-month-old and 5-month-old fry, the cumulative mortality was more than 80%, in particular, at 3-month-old, the fry exhibited 98%mortality, the greatest sensitivity, At 6-and 7-month-old, the frys'susceptibility was reduced and only 7 and 2% fish had succumbed. On the other hand, masu salmon fry, at 1-month-old, was the most sensitive and the cumulative mortality reached 87%. In 3-month to 5-month-old fry, cumulative mortality decreased from 65% to 24%. Comparing the five different salmonid fry, at the age of 1-month-old, for relative sensitivity to OMV, kokanee salmon exhibited the greatest sensitivity, and 100% of them died. Masu salmon and chum salmon exhibited high sensitivity at 87% and 83% mortality respectively. Coho salmon and rainbow trout were shown less sensitive to OMV infection at 39% and 29% mortality respectively.