Numerous species of the group known as Microsporidia exist, and many infect humans. Two of the more common ones are Enterocytozoon bieneusi and Encephalitozoon spp. They are a bit of a taxonomic dilemma, having several features in common with protozoa as well as features shared with the fungi. They are now usually classified as fungi; however, since their life cycles are very unlike those of fungi, and more like some of the coccidian parasites, their diagnosis often still occurs in parasitology departments.
The Microsporidia are obligate intracellular organisms. They form spores which is the only form that can survive outside of a cell. The alternation between asexual and sexual reproduction occurs here; however, unlike the coccidia, both can occur simultaneously in the same host cell. The image depicts the life cycle of two of the more common species. According to the CDC, other species that can infect humans include: Anncaliia spp., Encephalitozoon cuniculi, Encephalitozoon hellem, Encephalitozoon intestinalis (syn. Septata intestinalis), Tubulinosema acridophagus, Enterocytozoon bieneusi, Nosema spp., Pleistophora sp., Trachipleistophora spp., and Vittaforma corneae (syn. Nosema corneae).
Microsporidia life cycle. (44)
Many of these species are found worldwide and infection occurs by ingestion or inhalation of spores. Human-to-human and animal-to-human infections have also been documented. Depending on the species, they can cause severe intestinal symptoms, respiratory infections, sinusitis, keratoconjunctivitis, nephritis, cystitis, and many other organ infections. In general, disease in immunocompetent individuals is self-limiting, but in immunocompromised patients, it can be severe and life-threatening.
Laboratory diagnosis occurs through the use of special stains known as chromotrope-based stains (modified trichrome) and chemofluorescent optical brightening agents. The spores are extremely small—0.7–4 μm. A newer, quicker staining method known as "Quick-Hot Gram Chromotrope Technique" is now available. Antigen detection and molecular diagnostic techniques are available, but not widely used. If tissue is examined, periodic acid-Schiff (PAS) stains as well as acid-fast and silver stains should be used. The image at the right is a stool stained with a chromotrope 2R stain; the spores are red.
