Hematopoietic intranuclear microsporidian infections with features of leukemia in chinook salmon Oncorhynchus tshawytscha

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Abstract
Intranuclear infections of hematopoietic cells with characteristics of lymphoblasts were detected in juvenile chinook salmon (Oncorhynchus tshawytsha with a leukemic condition. The microsporidian infection was associated with an anemia secondary to the proliferation of hematopoietic cells in the kidney and spleen. Many of the nuclei of these lymphoid cells contained plasmodia and sporogonic stages of the microsporidian. Infected cells occurred in the kidney and spleen but were also found in the blood, eye, brain, muscle, liver, pancreas, intestine, peritoneum and gill. Spores develop from multinucleated sporogonial plasmodia which contain polar tube precursors. Spores are ovoid (1.0 .times. 2.0 .mu.m), have a thin exospore and poorly developed endospore surrounding a complex of membranes (polaroplast), a posterior vacuole, nucleus and cytoplasm containing a polar tube with 4 to 5 turns. The characteristic sporogony and spore morphology of the salmonid microsoporidian is found only in the genus Enterocytozoon. The microsporidian stimulates an abnormal profileration of host lymphoblasts and the subsequent migration and invasion of these infected host cells into various tissues resulted in a leukemic condition. A similar disease has recently been described among adult chinook salmon reared in seawater net-pens in British Columbia, Canada. The microsporidian was transmitted to previously uninfected kokanee slamon O. nerka by intraperitoneal injections of cells obtained from kidney homogenates of naturally-infected chinook salmon. These kokanee salmon also developed a similar leukemic condition to that observed in chinook salmon.

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