2015 年 50 巻 3 号 p. 105-111
The blood fluke Cardicola opisthorchis (Trematoda: Aporocotylidae) is a significant pathogen of cultured Pacific bluefin tuna Thunnus orientalis in Japan. We conducted monthly surveys of the intermediate polychaete host Terebella sp. (Polychaeta: Terebellidae) for one year in 2014 to elucidate seasonality in the C. opisthorchis infection at a tuna farm in Tsushima Island, Nagasaki Prefecture, Japan. The intermediate host, collected from ropes and floats attached to culture cages, was abundant from January to March and in November and December but quite scarce from April to October. The sporocysts of C. opisthorchis were found from January to March and from September to December, with the prevalence of infection ranging from 2.0% to 18.4%. Development of sporocysts was observed in the body of the intermediate host Terebella sp. Sporocystogenous sporocysts and cercariogenous sporocysts co-occurred in the body cavities of the polychaete host. This is the first detailed description of asexual multiplication process of sporocysts among marine aporocotylid species. Additionally, sporocystogenous sporocysts and released daughter sporocysts survived in L-15 medium for up to 63 and 66 days, respectively. The highest number of sporocysts in one polychaete host was about 1,800 and the highest number of cercariae in one sporocyst was 75, suggesting that as many as >105 cercariae can be released from one infected polychaete.
