Partial cross protection against Ichthyophthirius multifiliis in Gyrodactylus derjavini immunized rainbow trout K. Buchmann * , T. Lindenstrøm and J. Sigh Department of Veterinary Microbiology, Section of Fish Diseases, Royal Veterinary and Agricultural University, 13 Bu ¨ lowsvej, DK-1870 Frederiksberg C, Denmark Abstract 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 significantly between naive and Gyrodactylus derjavini immunized rainbow trout. 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 theronts using decomplemented (heat-inactivated) serum from G. derjavini immune or non-immune hosts showed no immobilization. However, untreated serum from both immune and non-immune fish containing intact complement immobilized theronts (titre 128–256). In addition, non-specific priming of the host response with interleukin (IL-1), bacterial lipopolysaccharide (LPS), concanavalin A (Con A) or mannan did confer a partial resistance to I. multifiliis infection. This will suggest that non- specific factors including complement could be partly responsible for the host response against infections with this ciliate. Introduction Freshwater fish in most parts of the world are sus- ceptible to the ubiquitous freshwater ciliate Ichthyophthirius multifiliis, which continues to represent a threat to both wild fish populations (Elser, 1955; Allison, 1963; Wurtsbaugh & Tapia, 1988; Jessop, 1995; Traxler et al., 1998) and cultured hosts (Fouquet, 1876; Buschkiel, 1910, 1937; Butcher, 1947; Bauer, 1958; Johnson, 1961; Wootten & Smith, 1980; Valtonen & Kera ¨nen, 1981; Mellergaard & Dalsgaard, 1987; Buchmann & Bresciani, 1997). Several studies have documented that a number of fish species are able to develop acquired resistance to challenge infections with this skin parasitic ciliate (Buschkiel, 1910; Bauer, 1958; Hines & Spira, 1974; Houghton & Matthews, 1993; McCallum, 1986; Clark et al., 1988; Cross & Matthews, 1992; Clark & Dickerson, 1997) which makes immunoprophylactic measures such as vaccination feasible (He et al., 1997). However, the responsible immune mechanisms are still subject to some dispute. Both cellular factors (Graves et al., 1985; Cross & Matthews, 1993; Cross, 1994; Sin et al., 1996) and humoral factors (Hines & Spira, 1974; Wahli & Meier, 1985; Clark & Dickerson, 1997) have been suggested to be involved in the host response but their relative impor- tance are poorly defined. In addition, some studies have indicated that the protection is based on non-specific mechanisms whereby reactions to other ciliates offer protection against I. multifiliis (Goven et al., 1981; Wolf & Markiw, 1982; Dickerson et al., 1984; Ling et al., 1993). Rainbow trout do also develop acquired resistance against the ectoparasitic monogenean G. derjavini (Buch- mann & Bresciani, 1998; Lindenstrøm & Buchmann, 1998) and the present study describes a partial cross protection against I. multifiliis in rainbow trout immunized against G. derjavini. In addition, a number of studies conducted in vitro and in vivo are described which indicate that Journal of Helminthology (1999) 73, 189–195 189 * Fax: 45 35 28 2711 E-mail: kurt.buchmann@vetmi.kvl.dk