Microsporidia are obligate intracellular (cytoplasmic or nuclear) parasites of a variety of animals (vertebrates and invertebrates) including man. Recent molecular biology research has clarified their taxonomic classification. These studies support the hypothesis that microsporidia are true fungi (not protozoa) descended from a zygomycete ancestor and suggest that microsporidia reproduce sexually.

The life cycle of microsporidia includes two distinct phases: the first (merogony) is proliferative and the second (sporogony) leads to the formation of spores that are released into the external environment and that can infect a new host. The term “spore” is used to define both the infectious form of microsporidia and the form that permits environmental resistance. Microsporidian spores are not only extremely resistant in the external environment (they remain infective for long periods of time), but also unaffected by many chemical disinfectants.

Microsporidia have a cosmopolitan distribution and many species can infect man. Many different animal species can probably act as reservoirs for infection. Human infection occurs through inhalation or ingestion of infectious spores from contaminated food and drink or by contact with infected animals. The most common species infecting the intestinal tract and other organs and tissues of man are: Enterocytozoon bieneusi (formerly Septata intestinalis) and Encephalitozoon intestinalis.

Microsporidiosis is an opportunistic infection that may become serious as it can cause chronic malabsorption, diarrhea and wasting in immunocompromised individuals, especially those with HIV infection and full-blown AIDS. In immunocompetent individuals, a few cases have been reported of non-serious, self-limiting infection.


Size: spores of Enterocytozoon bieneusi are 1.08-1.64 μm long and 0.70-0.98 μm wide. Spores of Encephalitozoon intestinalis are 2.0 μm long and 1.5 μm wide.

Morphology: oval. Due to their very small size, it is extremely difficult to observe under a light microscope the morphological details of spores as they are often described in literature (granules, bands, polar filament), even with appropriate stains.

With the modified Weber's trichrome staining, spore walls stain bright pink or pinkish; some spores appear empty, while others show a red band that traverses either equatorially or diagonally. With Giemsa, spores stain blue and it is difficult to differentiate them from other fecal elements of the same color. Fluorochrome-based techniques can also be used (e.g. Uvitex 2B, calcofluor white); however, it is still difficult to differentiate microsporidian spores from yeast and other elements in the same sample. For any staining technique, positive and negative control specimens should be used.

Standard concentration methods are not useful for the enrichment of microsporidian spores in stool samples. Given the difficulty of identifying these parasites, it is not sufficient to perform standard analytical procedures with care: the search for microsporidia should be carried out only by experienced microscopists working in specialized diagnostic laboratories.


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