Copper sulfate toxicity to two isolates of Ichthyophthirius multifiliis relative to alkalinity

Theronts from 2 different strains of Ichthyophthirius multifiliis (AR1 and AR5) were exposed to copper sulfate (CuSO4) in waters of different total alkalinities and observed for 4 h to determine relative toxicity and kinetics of parasite mortality. Consistent with the known solubility properties of the metal, Cu was significantly more toxic to cells maintained under low (48 mg l -1 ) com- pared with high (243 mg l -1 ) total alkalinity conditions. This was reflected in both the median lethal concentration (LC50) values and rates of mortality for both parasite strains; strain differences were also observed. The AR1 strain was significantly more resistant to copper toxicity than the AR5 strain in both high and low alkalinity waters. In general, these strain differences were more evident under conditions of low stress (i.e. low CuSO4 concentration and high alkalinity), and suggest that genetic factors are overridden under high stress conditions. The present study establishes a role for alkalin- ity in the effectiveness of CuSO4 treatment of ichthyophthiriasis and reveals differences in the sus- ceptibility of parasite populations that are clearly important for control programs.