Basic studies on the control of blood fluke infection of Pacific bluefin tuna

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Firstly, a second blood fluke species from Pacific bluefin tuna was found from the heart ventricle and described as Cardicola opisthorchis n. sp. Secondly, new methods were developed to distinguish eggs of C. orientalis from those of C. opisthorchis morphologically, and to count the number of eggs in the gills. Thirdly, infection cycle was partly elucidated. Namely, both species of blood flukes were found in juvenile tuna 45 days after transfer to outside net cages; one week later, flukes were matured and laid eggs after one more week. Fourthly, C. opisthorchis was eradicated from tuna with oral administration of praziquantel (PZQ) at 7.5 mg/kg fish body weight or more for 3 consecutive days. Reinfection was confirmed 1 month after medication. PZQ concentration in tuna decreased to the undetectable level（0.02 μg/L）24 h after administration. Lastly, SEM observation of the two species of blood flukes shows swelling and contraction of the tegument and formation of many minute furrowed ridges in C. opisthorchis, while in C. orientalis, many minute holes were characteristically formed on the tegument.