The ciliated protozoan parasite, Cryptocaryon irritans, and protective immunity in marine fish

The ciliated protozoan parasite, Cryptocaryon irritans , is the main parasitic threat to marine aquaculture fishes in large parts of the world. Although this white-spot disease of marine fish has significant economic interest, and the development of vaccines against C. irritans is needed, the immune mechanisms involved in these host responses are not fully understood. This study investigated the immune responses in marine fish against C. irritans.;Experimental infection with C. irritans was standardized and effects of infectious dose level on the immune responses examined in tilapia. This study demonstrated that direct enumeration of trophonts on the pectoral fin was useful to quantitatively assess immune protection against C. irritans. The number of trophonts on a pectoral fin was positively correlated with infectious dose of live theronts. Moreover, fish immunized by direct exposure under controlled laboratory conditions allowed for in depth examination of the effects of the degree of infectious dose on immune response. There was no significant positive correlation between the initial infectious dose and degree of immune responses. Mozambique tilapia initiated a strong immune protection by direct exposure with even a small number of parasites (e.g. 300 theronts per fish).;This is the first report that showed immunization by direct exposure with different C. irritans serotypes elicited cross-protection against both serotypes. Fish directly exposed to the corresponding live theronts of C. irritans produced antiserum that immobilized only the homologous serotype. However, the immunization resulted in cross-protection against subsequent challenges by both homologous and heterologous serotypes.;Furthermore, fish were immunized by intraperitoneal (i.p.) injection with either of two serotypes or a mixture of two serotypes followed by challenge exposures to determine whether the i.p. injection elicited the production of serotype-specific antibodies and cross-protection. This study provided evidence for the first time that the immunization by i.p. injections of sonicated C. irritans elicited specific antibodies against only the homologous serotype. However, the present study demonstrates that immunization with i.p. injection alone does not elicit the immune protection in tilapia against challenge exposure with live C. irritans.