Microsporidia infect a wide range of invertebrates and vertebrates and were only rarely observed in humans until the mid‐1980s when these organisms became recognized as causes of opportunistic infections in persons with acquired immunodeficiency syndrome (AIDS) associated with persistent diarrhoea and wasting. Nowadays, microsporidioses are being identified more extensively among human populations that include children, travellers, the elderly and others who may be immunocompromised.

Keywords: microsporidia; opportunistic infection; emerging infection; Enterocytozoon; Encephalitozoon

Figure 1.

Ultrastructure of a microsporidian spore. The representative diagram (left panel) and transmission electron micrograph (right panel) of an E. cuniculi spore show an outer electron dense exospore (Ex) and an inner electron lucent endospore (En). The plasma membrane surrounds the cytoplasm. The anchoring disk, polarplast (Pp) and polarfilament (Pf) are components of the extrusion apparatus. The cytoplasm contains ribosomes and a posterior vacuole and depending on species one (monokaryon) nucleus (Nu) or two (diplokaryon) nuclei.

Figure 2.

Life cycle of the most common microsporidia that infect humans. Primary sites of infection commonly occur in the intestinal and respiratory tracts. Enterocytozoon bieneusi and Encephalitozoon intestinalis typically cause intestinal infections and Encephalitozoon species typically disseminate. Organisms are shed with faeces, urine and respiratory secretions and can then infect new hosts. Reproduced by permission of the DPDx: CDC's website for parasite identification; http://www.dpd.cdc.gov/DPDx/HTML/ImageLibrary/Microsporidiosis_il.htm

Figure 3.

Histopathology of Enterocytozoon bieneusi infection in rhesus macaques (Macacca mulatta). Top panel–Gallbladder (Animal DD52). The hyperplastic mucosa forms large folds and cysts lined by normal columnar epithelium. The lamina propria contains a mild‐to‐moderate lymphoplasmacytic infiltrate. Haematoxylin–eosin stain, bar=200 μm. Middle panel–Gallbladder (Animal DD52). Some but not all polychromatic 1×1.5 μm microsporidia spores are present in basal portions of the epithelial cytoplasm in two adjacent cells. One cell with a missing nucleus is in the process of sloughing into the lumen. Brown–Brenn Gram stain, bar=10 μm. Bottom panel–Gallbladder (Animal CA81). Polychromatic microsporidial spores free in the cytoplasm of an epithelial cell sloughing into the lumen and still containing remnants of the nucleus. An eosinophilic polar vacuole is prominent in basophilic staining spores. Brown–Brenn Gram stain, bar=10 μm (courtesy of Peter J. Didier, DVM, PhD).



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Further Reading

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Vossbrinck CR (2002) Microsporidians. Encyclopedia of Life Sciences. West Sussex, UK: John Wiley and Sons, Ltd, DOI 10.1038/npg.els.0001982.

Wittner M and Weiss LM (1999) The Microsporidia and Microsporidiosis. Washington DC: ASM Press.

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Hale‐Donze, Hollie, and Didier, Elizabeth S(Jul 2007) Microsporidioses. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001930]