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Microsporidia is a nontaxonomic term referring to an extensive group of unicellular, spore-forming eukaryotic organisms now reclassified in the kingdom Fungi. Microsporidia lack mitochondria, are obligate intracellular parasites, and have no metabolically active stages outside the host cell. The phylum Microsporidia encompasses more than 160 genera and 1300 species that are pathogenic in nearly all animal phyla and even some protists. To date, at least 16 species of microsporidia have been implicated in human infections (Table 297-1). Mature microsporidial spores possess a characteristic coiled extrusion apparatus consisting of a polar tubule anchored to an anterior disk within the spore. The polar tubule is capable of penetrating the host cell membrane, after which the infective spore contents (sporoplasm) can be injected into the cytoplasm and the reproductive life cycle begins (Fig. 297-1).

Figure 297-1

Microsporidian life cycle. The intracellular development of Enterocytozoon bieneusi and Encephalitozoon intestinalis are shown in sequence. (1) The infective spore stage is ingested by a susceptible host. (2, 3) Extrusion of the polar tubule permits injection of the infectious material (sporoplasm) directly into the host cell cytoplasm. (4) The microsporidia develop by sporogony either in the host cell cytoplasm (E bieneusi) or inside a parasitophorous vacuole (E intestinalis). (5) After undergoing multiple divisions, meronts undergo a sporogenic phase, which, after several more divisions, results in infectious spores that are resistant to adverse environmental conditions. (6) The host cell membrane is disrupted and ruptured as the spores increase in number, releasing spores into the lumen or microenvironment. (Reproduced with permission from Centers for Disease Control and Prevention. Life cycle of microsporidia. Accessed August 30, 2017.)


Microsporidia are ubiquitous in the environment with an extremely broad host range. Human infections have been documented worldwide. Serosurveys in humans have demonstrated a high prevalence of antibodies to various microsporidia, suggesting that asymptomatic infection may be common. The incidence, reservoirs, and sources of human infection are not well defined, but ingestion of the environmentally highly resistant spores is probably the most important mode of transmission, particularly ...

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