Challenge trials on the anthelmintic effect of drugs and natural agents against the monogenean Heterobothrium okamotoi in the tiger puffer Takifugu rubripes

In vitro and oral administration challenge trials were performed to search for effective agents derived from natural sources (natural agents) against infections by the monogenean Heterobothrium okamotoi in the tiger puffer Ta kifugu rubripes. First, four drugs (praziquantel, levamisole, pyrantel pamo ate and antimony sodium tartarate) were screened for their anthelmintic eff icacy against H, okamotoi to select the drug most suitable as an effective positive control for a challenge trial of natural agents. Of these, praziqu antel showed anthelmintic efficacy against H, okamotoi in both in vitro and challenge trials and the in-feed praziquantel (4 g/kg basal diet) was chos en as the positive control. Next, four natural agents (caprylic acid, orang e oil, peppermint oil and cinnamon oil) were screened. OC these, caprylic a cid, peppermint oil and cinnamon oil had an efficacy against larvae of the H. okamotoi in in vitro trials. In the challenge trials, when each natural agent (2.5 g/kg basal diet) that was effective in vitro trials were also gi ven to the fish in feed, caprylic acid and praziquantel prevented horizonta l infection. Furthermore, the survival of groups treated with caprylic acid and praziquantel were significantly higher than the negative control (basa l diet) and the other groups, Additionally, the number of matured parasites on the branchial cavity wall of fish, which was assumed to exist from the beginning of the challenge trials, decreased in the groups treated with cap rylic acid and praziquantel. Our results show that caprylic acid has an ant helmintic efficacy against H. okamotoi. (C) 2000 Elsevier Science B.V. All rights reserved.