Antiparasitic efficacy of procyanidins against the fish parasite, Ichthyophthirius multifiliis: In vivo and in vitro

Ichthyophthirius multifiliis is a globally distributed pathogenic ciliate that causes white spot disease in freshwater fish, inflicting severe economic losses on the aquaculture industry. In this study, we investigated the efficacy of the natural product, procyanidins, against I. multifiliis both in vitro and in vivo. It was demonstrated that procyanidins exhibited remarkable antiparasitic activity, achieving 100 % mortality of theronts, protomont, and tomonts at concentrations of 0.20 mg/L, 4.00 mg/L, and 8.00 mg/L, respectively, with median effective concentrations of 0.10 mg/L, 2.99 mg/L, and 4.65 mg/L. Morphologically, treatment with procyanidins induced significant changes in I. multifiliis, including ciliary disruption and the formation of numerous cytoplasmic vacuoles. Transmission electron microscopy revealed that procyanidins could induce ciliary loss in the parasites, as well as severe mitochondrial damage and vacuolization. In vivo, a cohabitation experiment was conducted to investigate the effect of procyanidins (0, 1, 2, 4, and 8 mg/L) on the transmission of ichthtophthiriasis between diseased and healthy goldfish. By day 4, the infection intensity in diseased fish ranged from 158.51 to 211.78 ind/cm², with no significant differences across groups. However, by day 8, the infection intensity in the control group of healthy goldfish was 165.14 ind/cm², while it was significantly reduced to 79.88, 62.56, and 24.66 ind/cm² in the 2.0, 4.0, and 8.0 mg/L groups, respectively. Acute toxicity tests showed that the 96-h median lethal concentration of procyanidins for goldfish was 30.22 mg/L. These findings suggest that procyanidins are a potentially safe and effective compound for combating ichthyophthiriasis.