DAO 74:49-55 (2007)  -  doi:10.3354/dao074049

ABSTRACT: 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 15000 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.

KEY WORDS: Calcein · Cohabitation challenge · Immune response · Infection level · Ichthyophthirius · Channel catfish