Antiparasitic effect of medium-chain fatty acids against the ciliate Cryptocaryon irritans infestation in the red sea bream Pagrus major

The antiparasitic effect of short-chain (carbon numbers C, and C,) and medi um-chain (carbon numbers C-6-C-10) fatty acids against the ciliate Cryptoca ryon irritans was examined in in vitro trials. A challenge trial was conduc ted using the most effective fatty acid from in vitro trials to control C. irritans infestation on red sea bream Pagrus major at two temperatures (17 degreesC and 24 degreesC). The in vitro results showed that C, (caprylic ac id) had the strongest antiparasitic effect against C, irritans theronts. In challenge trials, uninfected fish were divided into six groups, 30 fish in each group (three groups for each temperature), and fish were fed the same amount of experimental diet (expanded pellet) with different doses of capr ylic acid at 0 (control), 37.5 and 75 mg caprylic acid/kg B.W./day during t he experiment. acid then 2000 theronts were placed into each of the six tan ks for 5 days after initiating the feeding of the experimental diets. Five fish of each group were randomly sampled periodically. The number of parasi tes on the gills and the eye surface in the treatment groups (caprylic acid ) were significantly fewer than in the control group at 17 degreesC. Mortal ity of fish did not occur in treatment groups during the trial, although al l control fish died. At 24 degreesC, mortality of fish occurred in all grou ps on the same day but the number of parasites on the gills and the eye sur face in the group fed 75 mg caprylic acid/kg B.W./day was significantly few er than in the control group. Our results indicate that caprylic acid has a n antiparasitic effect against C. irritans. (C) 2001 Elsevier Science B.V. All rights reserved.