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

Abstract

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 Takifugu rubripes. First, four drugs (praziquantel, levamisole, pyrantel pamoate and antimony sodium tartarate) were screened for their anthelmintic efficacy against H. okamotoi to select the drug most suitable as an effective positive control for a challenge trial of natural agents. Of these, praziquantel showed anthelmintic efficacy against H. okamotoi in both in vitro and challenge trials and the in-feed praziquantel (4 g/kg basal diet) was chosen as the positive control. Next, four natural agents (caprylic acid, orange oil, peppermint oil and cinnamon oil) were screened. Of these, caprylic acid, 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 given to the fish in feed, caprylic acid and praziquantel prevented horizontal infection. Furthermore, the survival of groups treated with caprylic acid and praziquantel were significantly higher than the negative control (basal 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 caprylic acid and praziquantel. Our results show that caprylic acid has an anthelmintic efficacy against H. okamotoi.