Seasonal Changes of Bacterial Infections among Pond-Cultured Eels (Anguilla japonica)

Abstract

Monthly investigations of bacterial infections among pond-cultured eels in the Yoshida area of Shizuoka prefecture were carried out in 1972 to 1973 (Table 1). Eels examined were collected from the ponds where mass mortalities had occurred just before each investigation. Isolation of bacteria from blood, liver, kidney, spleen and/or gill was attempted, using nutrient agar medium (Difco) and Cytophaga agar medium (modified).
Among five established eel-pathogenic bacteria, Aeromonas hydrophila, Edwardsiella tarda, Pseudomonas anguilliseptica and Chondrococcus columnaris were isolated in these investigations, but not Vibrio anguillarum. Total incidences of the four bacterial infections and the mixed infections were calculated (Table 2 and 3). Seasonal distribution of each bacterial infection was shown in Fig. 1. Distribution of the eels of which any bacteria were not isolated from internal organs had a partiality for winter (Fig. 2). Most of the diseased eels in winter had symptoms of “branchionephritis”, from gill of which a myxobacterium was frequently isolated.