Efficacy of five peroral therapeutics against Ichthyophthirius multifiliis infestation in salmonids

Ichthyophthirius multifiliis is a widespread parasitic ciliate causing significant losses in intensive freshwater aquaculture. This study evaluated the efficacy of medicated feed containing amprolium hydrochloride (2 mg/kg b.w./day), clopidol (2 mg/kg b.w./day), praziquantel (20 mg/kg b.w./day), toltrazuril (50 mg/kg b.w./day), and doxycycline (40 mg/kg b.w./day) administered at different stages of infection in experimentally infected brook trout fingerlings. Doxycycline and praziquantel demonstrated high protective efficacy (0-5% mortality) when applied during parasite exposure or the preclinical stage of infection. In contrast, amprolium hydrochloride, toltrazuril, and clopidol showed limited or no protective effect. The timing of administration was critical, and prophylactic use of effective compounds via feed may offer a practical strategy for controlling I. multifiliis outbreaks. These findings support the potential of doxycycline and praziquantel as oral chemotherapeutics in aquaculture, warranting further investigation for therapeutic use in advanced infection stages.