Submitted to: Diseases of Aquatic Organisms
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/1/2006
Publication Date: 2/8/2007
Citation: Xu, D., Klesius, P.H., Shoemaker, C.A. 2007. Evaluation of a cohabitation challenge model in immunization trials for channel catfish, Ictalurus punctatus (Rafinesque), against Ichthyophthirius multifiliis. Diseases of Aquatic Organisms. 74: 49-57. Interpretive Summary: Ichthyophthirius multifiliis (Ich) is a severe fish parasite that causes heavy economic loss in aquaculture (tens of million dollars annually). Vaccination against Ich is an alternative to chemical treatments because no effective chemicals are currently available against this parasite. A method which closely mimics natural exposure is needed for evaluation of protective immunity in fish against this parasite. This study evaluated a cohabitation challenge method in immunization trials against Ich using calcein, a fluorescent dye, to mark channel catfish. Marking fish by calcein is easily performed on large numbers of fish in a short time and had no effect on the susceptibility of fish to the parasite Ich. The cohabitation method enables immunized and non-immunized fish to be compared within the same experimental unit and ensures that all the test fish are exposed to the same trial conditions, such as pathogen concentration, density of fish, water exchange, water quality and volume of water, etc. Study results from two immunization trials demonstrated that no difference was observed in fish survival for cohabitation-challenged fish and fish challenged by non-cohabitation (i.e. in individual tanks). Cohabitation challenge can be used as an alternative challenge method for parasite study since it closely mimics natural exposure and reduces the chance for variation between experimental units.
Technical Abstract: Calcein marking and cohabitation challenge 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, respectively, and then challenged with 10,000 theronts per fish. No difference was noted on fish infection level, mortality and mean day to death (MDD) caused by Ich between fish marked and non-marked with calcein or between marked with high (1500 mg L-1) and low (500 mg L-1) concentration of calcein. After ensuring that calcein marking had no effect on the susceptibility of fish to Ich theronts, two 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. Cohabitation challenge can be used as an alternative challenge method for parasite study since it closely mimics natural exposure.