Evaluation of an antiparasitic compound extracted from Polygonum cuspidatum against Ichthyophthirius multifiliis in grass carp.

•An active compound, emodin, was isolated from Polygonum cuspidatum by bioassay-guided isolation.•Emodin had significantly parasiticidal efficacy against Ichthyophthirius multifiliis.•Emodin has the potential to be a new and effective drug to control I. multifiliis in fish farming industry.