Oral treatment of praziquantel as an effective control measure against blood fluke infection in Pacific bluefin tuna (Thunnus orientalis)

Aquaculture of tuna has shown exponential growth in the recent years. In association with the global development of tuna aquaculture, problems with parasitic infections have arisen. Among the parasites found in cultured tuna, blood flukes are considered to be a significant threat as their eggs accumulate in the gills and interfere with the gas exchange process. To date, two species of blood fluke from the genus Cardicola have been found in Pacific bluefin tuna (Thunnus orientalis, PBT) cultured in Japan. We conducted an experiment to develop a drug treatment against the blood flukes of cultured tuna. Artificially produced juvenile tuna that were naturally infected with blood flukes were orally administered with anthelmintic praziquantel (PZQ) of 150, 15, 1.5 and 0 mg kg−1 BW for three consecutive days, and differences in the abundance of adults and eggs in the gills were assessed. In the fish given 150 and 15 mg kg−1 BW PZQ, adult worms in the heart ventricle were eradicated by day 8 post treatment with the number of eggs in the gills gradually declining over time. However, small numbers of adults re-appeared by week 3 or 5 post treatment. The drug does not seem to affect eggs as viable miracidia were observed throughout the experiment. Mortalities in the two lower dose groups were significantly higher than the groups in which PZQ doses were shown to be effective. The study empirically demonstrated the efficacy of PZQ on Cardicola blood flukes and that the parasite significantly affects the survival of juvenile PBT. The oral PZQ treatment may be an essential practice for sustainable aquaculture of PBT.

► Praziquantel (≥ 15mg/kg BW) is effective to eradicate Cardicola blood flukes in tuna. ► The drug kills C. opisthorchis in the heart, but did not kill the eggs in the gills. ► Cardicola spp. cause mortality in juvenile PBT and can be a threat to tuna culture.