Stationary metal sheets (copper, zinc or brass) in fish tanks prevent Ichthyophthirius multifiliis Fouquet, 1876 infection of rainbow trout: In vivo and in vitro effects

Infections with the parasitic ciliate Ichthyophthirius multifiliis Fouquet, 1876 challenge health of freshwater fishes worldwide by eliciting white spot disease (ichthyophthiriosis). A series of chemicals, pharmaceuticals and herbal extracts are currently in use to control the infections by regular or continuous administration to aquaculture systems. Recently application of brass (copper/zinc alloys) metal sheets in marine fish tanks was shown to eliminate tomonts of a related ciliate, Cryptocaryon irritans Brown, 1951 causing cryptocaryonosis (marine white spot disease). The present work documents that brass, and each of its pure constituents (copper and zinc), have in vivo and in vitro antiparasitic effects in freshwater. We used an experimental model with I. multifiliis infected rainbow trout (Oncorhynchus mykiss Walbaum, 1792). In the in vivo study metal sheets of either brass (alloy of copper and zinc), pure copper or pure zinc, were placed in fish tanks with trout experimentally infected with I. multifiliis (duplicated trials). All three metals (pure or combined) effectively blocked the life cycle of I. multifiliis and inhibited reinfection and morbidity. In control tanks with infected trout, but no metal sheets, we observed significantly elevated infection levels and development of disease. In vitro assays, exposing three life cycle stages (tomont, tomocyst and theront) to the same metal sheets in Petri dishes, confirmed the antiparasitic effects. Tomonts were sensitive to pure copper and brass (100% and 66% mortality within 15 min, respectively) and zinc (100% mortality within 30 min). Tomocysts, with enclosed tomites, were less sensitive, as even 12 h exposure to brass, pure copper and zinc merely led to mortality rates of 44%, 50% and 60%, respectively. Theronts were killed within 75 min when exposed to brass and pure copper plates, whereas zinc plate exposure eliminated all theronts within 60 min. Microscopical observations suggested that direct contact between the parasite and the metal sheet increased the parasiticidal effect, although the released metal ions may contribute may play a role. The use of stationary metal sheets in aquaculture settings may be a possible way to control ichthyophthiriosis. However, we measured the concentration of free metal ions in the fish tanks with metal sheets, which suggested release of metal to the water. This calls for additional studies on toxicology and environmental effects of metal sheet use before implementation at fish farms level.