Protection of rainbow trout, <i>Oncorhynchus mykiss</i> (Richardson), against <i>Diplostomum spathaceum</i> (Digenea): the role of specific antibody and activated macrophages

Abstract

Abstract. The degree of protection in immunized rainbow trout against the infective larval stages of Diplostomum spathaceum was investigated. Immunized rainbow trout demonstrated a 70–80% reduction in the number of metacercariae establishing in their eye‐lenses following challenge with live cercariae compared with unimmunized fish. Both normal trout serum and anti–diplostomule serum weTC cytotoxic to diplostomules in vitro . However, there was no correlation between the degree of protection and the level of serum antibody within individual fish. Reduced establishment of migrating diplostomules, indicative of in vivo killing, occurred in fish possessing activated macrophages and challenged with cercariae opsonized with specific antibody. An antibody–dependent cell–mediated cytotoxic (ADCC) mechanism is proposed, conferring protection in immunized fish. This is supported by the finding that cercarial and diplostomule antigens are capable of activating macrophages in immunized rainbow trout. The role of macrophage activation and specific antibody in conferring protection in immunized fish is discussed.