Effect of the microsporidian Enterocytozoon salmonis on the immune response of chinook salmon

The protozoan parasite Enterocytozoon salmonis is an intranuclear microsporidian of salmonid mononuclear leukocytes. Experimental infections were initiated in chinook salmon to determine the effects of parasitism on selected host immune functions. The humoral antibody response to dinitrophenylated-keyhole limpet hemocyanin and the in vitro blastogenic responses of isolated mononuclear leukocytes to mitogens (concanavalin A, lipopolysaccharide and phytohemagglutinin-P) were evaluated. The humoral response as detected by enzyme-linked immunosorbent assays was suppressed following infection. The degree of suppression increased as the severity of the infection progressed. Additionally, the response to mitogen-induced lymphoproliferation was impaired. These results suggest that infection with E. salmonis may cause suppression of host cell immune functions, thus increasing the susceptibly of infected fish to other pathogens.