Various compounds have been applied for control of Ichthyophthirius multifiliis (commonly known as Ich) which is the parasitic pathogen, responsible for the white spot disease, in freshwater aquaculture worldwide. Available methods are based on disinfection of fish tank water, targeting infective free-swimming theronts and escaped tomonts. No legal drug is available and licensed for treatment or prevention of the disease. The presently reported study was performed to test the potential of toltrazuril (Baycox® vet.)—a drug licensed for treatment of coccidial infections in other animal species—to reduce Ich infections when administered orally. Commercial pelleted feed containing 5.0 or 2.5 mg toltrazuril per 1 g of feed was offered to rainbow trout, Oncorhynchus mykiss (Walbaum, 1792) (Actinopterygii: Salmoniformes: Salmonidae), during a three day feeding regime (1% of biomass offered per day). Two trials were performed: one with feeding before exposure to parasites and one where already infected fish were treated. Before the treatment it was tested if fish would eat feed containing different drug concentrations and it was found that feed with the high drug concentration was not eaten by the fish whereas all feed containing 2.5 mg per 1 g feed was eaten. Following the exposure to infective I. multifiliis theronts it was found that the fish treated with toltrazuril before challenge obtained a significantly lower parasite burden (number of trophonts in the skin) compared to untreated control. On the other hand, toltrazuril proved to be ineffective when administered to the fish which were already infected before the treatment.
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