Studies on Metacercaria, of Centrocestus formosanus (NISHIGORI, 1924)-I

Parasitism of Metacercariae in Gills of Young Rearing Eels, and Abnormal Deaths of the Hosts

Abstract

It has been a serious problem of eel culturists in Tanegashima, Kagoshima Prefecture that mass mortality of eel elvers was brought about by an uncertain disease.
The introduction of elvers (50 kg) of eels, Angilla japonica into a pond was conducted for initial investigation on the disease in February, 1980. Intermittent deaths (number in mortality: about 500-1000 per day) lasted till April resulting in mortality over one half of all the elvers for about two months. Most of surviving elvers also died at the end of August, showing abnormal and retarded growth.
The second investigation to determine the cause was done using new elvers since January, 1981. By microscopic examination on various tissue sections of the elvers, we found that metacercariae of Centrocestus formosanus (NISHIGORI, 1924) infected in the gills of them. We considered that the parasitism of metacercariae in gills was the primary phase of the disease and the following mass mortality was caused by high cultur temperature (25°C) and secondary bacterial infection.
Further, abnormal behaviour of the host eel elvers (weighing 0.25 g in average), which was characterized by such signs as (1) abnormal swimming (2) non-swimming and reposing at the bottom, and (3) climbing up pond walls, was also observed with a notable inter-relationship to the degree of parasitism.