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Microsporidia is a nontaxonomic term referring to an extensive group of unicellular organisms classified in the phylum Microspora. Previously considered to be primitive protists, they have been reclassified as fungi.1 All microsporidia lack mitochondria, are obligate intracellular parasites, and possess a characteristic coiled extrusion apparatus consisting of a polar tubule anchored to an anterior disk within the spore. This apparatus is capable, upon extrusion, of penetrating the host cell membrane and injecting the infectious sporoplasm material into the cytoplasm where the life cycle begins (Fig. 302-1). Of over 1200 species known, at least 14 species of microsporidia have been described that are capable of infecting humans (Table 302-1).2 The pattern of infection varies depending on the species of microsporidia and the immune status of the host.

Figure 302-1.

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) Asexual reproduction then takes place either in the host cell cytoplasm or within a parasitophorous vacuole, depending on the species involved. (5) After undergoing multiple divisions, meronts undergo a sporogonic phase, which, after several more divisions, results in infectious spores. (6) Host cell rupture then releases these spores into the lumen or microenvironment. While the gastrointestinal tract is the most common site of infection, certain species can disseminate to other sites as shown in Table 302-1, which essentially only occurs in the setting of severe immunosuppression.

(Source: Reprinted from Centers for Disease Control and Prevention; National Center for Zoonotic, Vector-Borne, and Enteric Diseases; Division of Parasitic Diseases. Life cycle of Microsporidia [from image library]. Accessed May 28, 2009.)

Table 302-1. Microsporidia Capable of Infecting Humans

Controversy existed as to the pathogenic potential of ...

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