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

Abstract

The antiparasitic effect of short-chain (carbon numbers C 2 and C 4) and medium-chain (carbon numbers C 6–C 10) fatty acids against the ciliate Cryptocaryon irritans was examined in in vitro trials. A challenge trial was conducted 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°C and 24°C). The in vitro results showed that C 8 (caprylic acid) 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 caprylic acid at 0 (control), 37.5 and 75 mg caprylic acid/kg B.W./day during the experiment, and then 2000 theronts were placed into each of the six tanks for 5 days after initiating the feeding of the experimental diets. Five fish of each group were randomly sampled periodically. The number of parasites on the gills and the eye surface in the treatment groups (caprylic acid) were significantly fewer than in the control group at 17°C. Mortality of fish did not occur in treatment groups during the trial, although all control fish died. At 24°C, mortality of fish occurred in all groups on the same day but the number of parasites on the gills and the eye surface in the group fed 75 mg caprylic acid/kg B.W./day was significantly fewer than in the control group. Our results indicate that caprylic acid has an antiparasitic effect against C. irritans.