In vitro characteristics of the microsporidian: Enterocytozoon salmonis.

Enterocytozoon salmonis, as intranuclear microsporidian of salmonid fish, was propagated in vitro using chinook salmon mononuclear leukocytes. Characteristic morphology and infectivity of the cultured parasites were evaluated to determine the effect of in vitro maintenance and passage on the parasites. Cultured parasites developed through several stages from meronts to infectious spores. Parasites obtained from in vitro passages tested up to the 17th subculture, retained their morphological characteristics and pathogenicity for chinook salmon (Oncorhynchus tshawytscha). The disease induced by experimental infections with parasites from in vitro cultures was identical to that observed in naturally infected chinook salmon. An examination of supernatants obtained from the infected cultures revealed evidence of soluble factor(s) produced by E. salmonis-infected cells that stimulated uninfected target cells in vitro. This observation may explain in part the proliferative disease of hematopoietic tissues which characterizes the disease in infected chinook salmon.