Fine structure of trophozoites and cysts of the pathogenic diplomonad Spironucleus muris

Using light and electron microscopy, we studied the ultrastructure of Spironucleus muris (syn. Hexamita muris) from spontaneously and experimentally infected normal and athymic mice. Kinetosomes and cytoskeletal fibers arranged in two-fold rotational symmetry are similar to the species Spironucleus elegans from amphibians. Taxonomic proposals and a possible evolutionary scheme for diplomonad genera accepted at the last International Congress of Parasitology are given. The genus Hexamita should be divided into two new genera: Hexamita (usually free-living) and Spironucleus (exclusively parasitic). We consider previous descriptions of hexamitiasis in rodents as dealing with spironucleosis. We distinguish a fresh and an old cyst of the parasite on the basis of structure, light refraction, location in the host, and infectivity. The transition from a fresh to an old cyst parallels the dessication of feces. The trophozoites can damage the microvilli and penetrate into the epithelium. We explain the differences in findings concerning intestinal lesions and forms of the disease (from acute to chronic to latent) by differences in the virulence of different parasite strains and by differences in inherent host resistance.