Ichthyophthirius multifiliis Fouquet, 1876
infections on intensively reared fish stocks can increase rapidly, which if left unmanaged, can result in the heavy loss of stock. The present study explores the efficacy of long duration, low dose (1, 2 and 5 mg L(-1)) treatments of
bronopol (marketed as Pyceze™, Novartis Ltd.) in reducing the number of trophonts establishing on juvenile Oncorhynchus mykiss held under small scale culture conditions. The effect of
bronopol on the colonisation success of infective theronts was also investigated by adding 2 mg L(-1)
bronopol to the water prior and during the
infection process. The number of parasites surviving on fish treated this way was compared to groups of fish that only received treatment after
infection had occurred. The effect of
bronopol on exiting trophonts throughout their external development to the point of theront release was also assessed through the delivery of 1 mg L(-1), 2 mg L(-1) and 5 mg L(-1)
bronopol for up to 27 days consecutively (days 9-36 post-infection). The trial showed that a nominal dose of 2 mg L(-1)
bronopol administered prior to
infection significantly reduced the number of theronts surviving in the water column at the time of the initial challenge by 35-40% (P<0.05). Similarly, doses of 2 and 5 mg L(-1)
bronopol administered as the first wave of mature I. multifiliis trophonts exited fish (i.e. day 11 onwards) to develop externally, reduced the number of trophonts establishing on fish as the second cycle of
infection by 52-83%. Continuous application of 2 and 5 mg L(-1)
bronopol throughout the second and third cycles of I. multifiliis
infection gave further reductions of between 90 and 98%. The number of trophonts on the fish in the control tanks and those treated with 1 mg L(-1) and the 2 mg L(-1) dose at the time of initial
infection, by comparison, were observed to increase with successive cycles of
infection. From these small scale tank trials, this study demonstrates that the strategic, long duration, low dose delivery of drugs like
bronopol can significantly reduce the number of trophonts establishing on fish suggesting the potential of this
drug at managing I. multifiliis
infections.