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

The anthelmintic effect of short-chain (carbon numbers C-2 and C-4) and med ium-chain (carbon numbers C-6-C-10) fatty acids against the monogenean Hete robothrium okamotoi was examined using in vitro trials. The effective dose for the oral administration of the fatty acid that was most effective in th e in vitro trials was examined using challenge trials with H. okamotoi in t he tiger puffer Takifugu rubripes at different water temperatures (15 degre esC, 20 degreesC and 25 degreesC). 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 ac id) had the strongest anthelmintic activity. In the challenge trials, when fish were fed the same amount of diets with different doses of caprylic aci d to give 0 (control), 25, 50, 100 and 200 mg caprylic acid/kg B.W./day, th e in-feed caprylic acid had an effect at all doses when they were tested at 15 degreesC and 20 degreesC. The effect at 15 degreesC was higher at low d oses (25 and 50 mg/kg B.W./day) but the effect at 20 degreesC was higher at high doses (100 and 200 mg/kg B.W./day). At 25 degreesC, a dose of more th an100 mg/kg B.W./day was required to obtain an apparent effect. Also. the e ffect at 25 degreesC was weak compared with that of 15 degreesC and 20 degr eesC. The number of infected parasites on the gill?, and the branchial cavi ty wall in the groups treated with caprylic acid at the must 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 a gainst H. okamotoi at these water temperatures that is applicable at almost all seasons in districts where tiger puffers are cultured. (C) 2001 Elsevi er Science B.V. All rights reserved.