The developmental stages of a recently described microsporidian from the nucleus of hematopoietic cells of salmonid fish were found to be unique among the Microsporida. All observed stages, including meronts, sporonts, and spores were in direct contact with the host cell nucleus (principally hematopoietic cells) of chinook salmon (Oncorhynchus tshawytscha). There is no parasitophorous vacuole and sporogony does not involve formation of a pansporoblastic membrane as with other members of the suborder Apansporoblastina. The extrusion apparatus differentiates prior to division of sporogonial plasmodia. The spores are ovoid (1 x 2 microns) and uninucleate, and possess a coiled polar tube with 8-12 turns. Developmental stages of the salmonid microsporidian are similar to those described for Enterocytozoon bieneusi as found in the intestinal mucosa of human AIDS patients. However, the intranuclear development, different cell types, and host infected clearly separate the salmonid and human parasites. Accordingly, the intranuclear parasite of salmonids is given the name Enterocytozoon salmonis n. sp. within the suborder Apansporoblastina.
Intranuclear infections of hematopoietic cells with characteristics of lymphoblasts were detected in juvenile chinook salmon Oncorhynchus tshawytscha with a leukemic condition. The rnicrosporidian infection was associated wlth an anemia secondary to the proliferat~on of hematopoietic cells in the kidney and spleen. Many of the nuclei of these lymphoid cells contained plasmod~a 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 X 2.0 km), have a thln 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 microsporidian is found only in the genus Enterocytozoon. The microsporidian stimulates an abnormal proliferation 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 microsporidlan was transmitted to prev~ously uninfected kokanee salmon 0. 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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