The anthelmintic effect of medium-chain fatty acids against the monogenean Heterobothrium okamotoi in the tiger puffer Takifugurubripes: evaluation of doses of caprylic acid at different water temperatures

Abstract

The anthelmintic effect of short-chain (carbon numbers C 2 and C 4) and medium-chain (carbon numbers C 6–C 10) fatty acids against the monogenean Heterobothrium okamotoi was examined using in vitro trials. The effective dose for the oral administration of the fatty acid that was most effective in the in vitro trials was examined using challenge trials with H. okamotoi in the tiger puffer Takifugu rubripes at different water temperatures (15°C, 20°C and 25°C). The fatty acids between C 6 and C 10 had an effect of enfeebling the H. okamotoi larvae and a parasiticidal effect against the oncomiracidia in in vitro trials. Among these, C 8 (caprylic acid) had the strongest anthelmintic activity. In the challenge trials, when fish were fed the same amount of diets with different doses of caprylic acid to give 0 (control), 25, 50, 100 and 200 mg caprylic acid/kg B.W./day, the in-feed caprylic acid had an effect at all doses when they were tested at 15°C and 20°C. The effect at 15°C was higher at low doses (25 and 50 mg/kg B.W./day) but the effect at 20°C was higher at high doses (100 and 200 mg/kg B.W./day). At 25°C, a dose of more than 100 mg/kg B.W./day was required to obtain an apparent effect. Also, the effect at 25°C was weak compared with that of 15°C and 20°C. The number of infected parasites on the gills and the branchial cavity wall in the groups treated with caprylic acid at the most effective dose for each water temperature was significantly fewer than the control group. Our results show that in-feed caprylic acid has an anthelmintic efficacy against H. okamotoi at these water temperatures that is applicable at almost all seasons in districts where tiger puffers are cultured.