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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).
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PATHOGENESIS AND EPIDEMIOLOGY
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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 ...