Immunological aspects of amoebic gill disease in salmonids

Paramoeba sp. antigens emulsified with Freund's complete and incomplete adjuvants were immunogenic in rainbow trout Oncorhynchus mykiss and Atlantic salmon Salmo salar by intraperitoneal (i.p.) injection as well as for sheep and rabbit by subcutaneous and intravenous injections respectively. Atlantic salmon were immunized passively with an i.p. injection (0.1 ml 100 g(-1) fish body weight) of sheep anti-Paramoeba sp. antibodies (APA). Sheep APA were detected in fish sera by enzyme-linked immunosorbent assay for up to 8 wk after i.p. injection. Assessments of passive and active immunization were undertaken concurrently in order to demonstrate their relative efficacies and especially to evaluate the practical potential of passive immunization Immunized fish were exposed to natural infection by cohabitation with infected Atlantic salmon 45 d post-immunization. Transmission of the disease was successful; however, unequivocal protection was not demonstrated in any of the immunized fish suggesting a minor role for systemic antibodies in protection against amoebic gill disease.