Partial cross protection against Ichthyophthirius multifiliis in Gyrodactylus derjavini immunized rainbow trout

Partial cross protection against a skin-parasitic ciliate has been recorded in rainbow trout previously immunized with an ectoparasitic platyhelminth. The susceptibility to infection by Ichthyophthirius multifiliis differed s ignificantly between naive and Gyrodactylus derjavini immunized rainbow tro ut. Fish partly immune to the ectoparasitic monogenean G. derjavini became less infected and experienced lower mortality than naive fish when exposed to I. multifiliis infections. In vitro studies on immobilization of theront s using decomplemented (heat-inactivated) serum from G. derjavini immune or non-immune hosts showed no immobilization. However, untreated serum from b oth immune and non-immune fish containing intact complement immobilized the ronts (titre 128-256). Ln addition, non-specific priming of the host respon se with interleukin (IL-1), bacterial lipopolysaccharide (LPS), concanavali n A (Con A) or mannan did confer a partial resistance to I. multifiliis inf ection. This will suggest that nonspecific factors including complement cou ld be partly responsible for the host response against infections with this ciliate.