Laboratory Identification of the Microsporidia

Abstract

The microsporidia belong to the phylum Microspora within the taxonomic group Protozoa; there are more than 140 genera and 1,200 species that are parasitic in all major animal groups. Already, seven genera (Enterocytozoon, Encephalitozoon, Nosema, Pleistophora, Vittaforma, Trachipleistophora, and Brachiola) and some that are unclassified have been confirmed to cause human infection. Septata intestinalis, an organism that was identified in 1994 as belonging to a new genus, i.e., Septata (Encephalitozoon-like), has been reclassified as Encephalitozoon intestinalis, although there is not universal agreement on this generic change. The genus Microsporidium is essentially a catch-all genus for organisms that have not yet been classified (or may never be classified due to a lack of specimen). Depending on acceptance of these generic designations, there may be seven or nine microsporidial genera. The microsporidia are obligate intracellular, spore-forming protists with no active metabolic stages outside of the host cell. The life cycle involves a proliferative merogonic stage followed by sporogony, which results in spores containing a tubular extrusion apparatus (polar tubule) for injecting infective spore contents into the host cell (Fig. ​(Fig.11). FIG. 1. Life cycle of microsporidia. Reprinted from reference 9. Because microsporidia have a membrane-bound nucleus, an intracytoplasmic membrane system, and chromosome separation on mitotic spindles, they are considered to be true eukaryotes. However, they are somewhat unusual in that they have 70S ribosomes, have no mitrochondria or peroxisomes, and have simple Golgi membranes. Also, the microsporidial genome is smaller and less complex than those of other eukaryotes. Several characteristics, including the presence of chitin in the spore wall, suggest a potential link to the fungi. Microsporidiosis, a disease resulting from infection with these important emerging opportunistic organisms, has been noted in human immunodeficiency virus (HIV)-infected patients, as well as in other immunocompromised patients, including transplant recipients. The first human case was reported in 1959; since then, cases of microsporidiosis in both those who are immunocompromised and those who have no underlying immune problems and are considered to be immunocompetent have been reported (Table ​(Table1)1) (2, 7, 21). TABLE 1. Microsporidia causing human infection