Comparative palatability and efficacy of two praziquantel isomers in rainbow trout infected by eyefluke.

Praziquantel is a widely used anthelmintic in both human and veterinary medicine, but low palatability of the commonly used compound, which is a mixture (M) of two isomers (D and L forms), may result in rejection of oral treatments. Purification of the D-isomer of praziquantel allows a comparative evaluation of both palatability and efficacy of the isomers. We showed that rainbow trout (Oncorhynchus mykiss) preferred feed containing D-PZQ feed (60% acceptance) compared to M-PZQ feed (20% acceptance) (inclusion of 400 g PZQ/kg feed). Subsequent efficacy studies using eyefluke Diplostomum pseudospathaceum cercariae (in vitro) and experimentally infected rainbow trout carrying metacercariae (in vivo) demonstrated a strong antiparasitic effect of both PZQ-forms. In vitro assays documented that both M-PZQ and D-PZQ induced rapid and concentration-dependent mortality in cercaria (100% within 4 h in 0.125 mg PZQ/L), but with a significantly higher killing rate of of D-PZQ. In vivo assays (17 days feeding medicated feed) showed significant efficacy of both D-PZQ and M-PZQ. A slightly (non-significant) faster effect of D-PZQ may partly be ascribed to a combination of higher consumption of this isomer (due to its higher palatability) and higher efficacy. Cataract formation occurred in control fish but was prevented by both drug types indicating that metabolic and/or mechanical activities of live parasites are associated with cataract induction. This rainbow trout/eyefluke infection may serve as a model for non-piscine host-parasite systems as well.