EXPERIMENTAL INFECTIONS WITH ENTEROCYTOZOON-SALMONIS CHILMONCZYK, COX, HEDRICK (MICROSPOREA) - AN INTRANUCLEAR MICROSPORIDIUM FROM CHINOOK SALMON ONCORHYNCHUS-TSHAWYTSCHA

Infections were induced in juvenile chinook salmon Oncorhynchus tshawytscha by intraperitoneal injections of cells from the kidney of fish with naturally-occurring infections of Enterocytozoon salmonis. A mild hyperplasia of the hematopoietic tissues of the kidney and spleen were observed 3 wk after injection at water temperatures of 15-degrees-C. By 5 wk postinjection, populations of lymphoblasts, some of which contained intranuclear forms of the parasite, were abundant in the kidney and spleen but also occurred in the leptomeninges, skeletal muscle, coelomic mesenteries, hepatic sinusoids and periorbital connective tissues of the eye. Plasmodial stages were abundant within the nucleus of blast cells in stained kidney imprints from infected fish but spores were only rarely detected. At 7 wk postinjection, the infections had progressed to a severe systemic disease with features similar to naturally-occurring infections with the microsporidium. These included anemia accompanied by a leukemic condition in which lymphoblastic cells were found disseminated throughout most tissues including prominent accumulations in the lamina propria of the posterior intestine. Mortalities occurred among infected fish between 53 and 60 d postinjection and infections were detected in most survivors at the end of the study at 9 wk postinjection. Moderate numbers of spores were found most often in dead fish and survivors examined at the end of the study (63 d). The potential for treatment of infections with fumagillin DCH was demonstrated by the absence of mortalities and microscopic signs of clinical infections and a marked decrease in detection of the parasite among chinook salmon fed the drug (0.1 g kg-1 food at 1.5% body weight d-1) between 3 and 7 wk postinjection.