Immunity in rainbow trout, Oncorhynchus mykiss, against the monogenean Discocotyle sagittata following primary infection

Rainbow trout (Oncorhynchus mykiss) were experimentally infected by continuous or single exposure with the monogenean Discocotyle sagittata. To determine whether immunity follows primary infection, fish were exposed to a secondary challenge by one of two modes: (1) primary infections were cleared with praziquantel (PZQ) and hosts re-infected with 100 oncomiracidia; (2) parasites were allowed to reach maturity and hosts super-infected with 100 oncomiracidia. Fish challenged after initial continuous exposure developed significant partial resistance to re-infection, carrying burdens 35% lower than controls. PZQ treatment controls demonstrated that the drug did not account for the protection observed. Single exposure did not significantly modify the outcome of secondary challenges. Super-infection experiments suggested that no concomitant immunity develops. No correlation was found between initial burdens and the outcome of secondary challenges in the same individual. Significantly elevated anti-D. sagittata antibodies were detected in infected fish, but there was no correlation between immunoglobulin levels and parasite burdens.