Evaluation of a cohabitation challenge model in immunization trials for channel catfish Ictalurus punctatus against Ichthyophthirius multifiliis

Calcein marking and cohabitation challenges have not been investigated in fish parasite research. This study evaluated a cohabitation challenge method in immunization trials against Ichthyophthirius multifiliis (Ich) using calcein, a fluorescent dye, to mark channel catfish Ictalurus punctatus (Rafinesque). Fish were marked by calcein immersion at 0, 500, and 1500 mg l(-1), and then challenged with 15 000 theronts fish(-1). No difference was noted in fish infection levels, mortality, and mean days to death (MDD) caused by Ich between unmarked and marked fish or between fish marked with high (1500 mg l(-1)) and low (500 mg l(-1)) concentrations of calcein. After ensuring that calcein marking had no effect on the susceptibility of fish to Ich theronts, 2 immunization trials were conducted to evaluate the cohabitation challenge model using calcein-marked catfish. Fish mortality, relative percent survival (RPS), and MDD were compared between cohabitation-challenged fish and fish challenged by non-cohabitation. No significant difference was observed in RPS for cohabitation-challenged fish and fish challenged by non-cohabitation. A cohabitation challenge can be used as an alternative challenge method in parasite studies, since it closely mimics natural exposure.