In vitro and in vivo efficacies of ionophores against Cryptocaryon irritans

We assayed the effects of a variety of antiprotozoal compounds against trophonts of Cryptocaryon irritans, the causative agent of ‘white spot disease of marine fish’ in vitro using the double layered media that we developed previously for the culture of the parasite. In the assay, ionophores, particularly sodium salinomycin, showed apparent killing and growth-suppression effects against the parasite. As there was no mortality in Japanese flounder Paralychthys olivaceus that were fed a diet containing sodium salinomycin (200 ppm) for two weeks, we evaluated the efficacy of 200 ppm sodium salinomycin against C. irritans in Japanese flounder. We fed Japanese flounders a medicated diet for 5 d prior to and 3 d after challenge with C. irritans. In the experimental group, the number of protomonts recovered from the fish and the size of tomonts that were transformed from the protomonts were significantly reduced, when compared to the control group. Furthermore, in a different experiment, the fish that were fed a diet medicated with sodium salinomycin survived longer than those fed an unmedicated diet after challenge. Sodium salinomycin can be a good candidate drug for chemotherapy and control of Cryptocaron irritans infection.

► Sodium salinomycin (SS) showed a killing effect on Cryptocaryon irritans in vitro.
► Oral administration of SS reduced recovery of the parasite in Japanese flounder.
► Flounder administered with SS survived longer than control flounder after challenge.