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Nucleospora salmonis is a microsporidian parasite that infects the hematopoietic cells of salmonid fishes causing a leukemia-like condition. N. salmonis can cause direct mortality but often leads to a chronic disease and immune suppression. Clinically infected fish are typically lethargic and anemic with swelling of the kidney, spleen and posterior kidney. Infected fish may be more susceptible to secondary infections with other pathogens. The parasite can be horizontally transmitted and there is no evidence of an alternate host. There is some evidence that the parasite is vertically transmitted. N. salmonis has been reported from N. America, S. America and Europe. Natural infections have been reported in a range of salmon and trout species, including Chinook salmon (Oncorhynchus tshawytscha), coho salmon (O. kisutch), rainbow trout and steelhead (O. mykiss), cutthroat trout (O. clarkii), golden trout (O. aquabonita), lake trout (Savelinus namaycush) and brook trout (S. fontinalis). Recent studies at the Western Fisheries Research Center found significantly higher prevalence of N. salmonis in juvenile steelhead reared in hatcheries relative to wild steelhead in the Columbia River Basin. The contribution of N. salmonis to mortality of juvenile salmon entering saltwater is not known.