Morphology and Invasion Apparatus
Causative agents of microsporidiosis (Microsporidia) are primitive round microorganisms combining features of both protozoa and fungi. Modern taxonomy classifies them in the kingdom Fungi (phylum Zygomycota, phycomycetes). To date, over 1,300 species have been described, of which representatives of 8 genera are recognized as pathogenic to humans, including Enterocytozoon, Encephalitozoon, Nosema, and Pleistophora.
The main morphological feature of microsporidia is a unique extrusion apparatus necessary for infection. The spore contains sporoplasm with a nucleus, as well as a specialized tubular filament with a polar disc. When a spore encounters favorable conditions—such as the lumen of the duodenum—this filament instantly 'fires'. It pierces the target cell membrane like a syringe needle and injects the sporoplasm directly into the host cell.
Life Cycle
Microsporidia are obligate intracellular parasites. Following successful cell entry, a complex three-stage developmental cycle is initiated:
- Merogony — repeated binary fission of the parasite.
- Schizogony — multiple fission, producing a large number of new individuals.
- Sporogony — formation and maturation of spores.
The parasite's localization depends on the species. For example, Enterocytozoon bieneusi replicates directly in the cytoplasm, whereas Encephalitozoon intestinalis forms a specialized parasitophorous vacuole. A thick wall forms around the maturing spores, protecting them from environmental factors. Ultimately, the overloaded host cell ruptures, spores are released, infect neighboring tissues, and cause local inflammation.
Epidemiology and Clinical Presentation
The infection is widespread among animals, which serve as the primary reservoir for the pathogen. Resistant spores are shed in the feces and urine of infected animals. The main mechanism of human infection is fecal-oral (via ingestion of spores). Less common routes include aerogenous (via the respiratory tract) and contact (leading to conjunctivitis).
The disease runs a severe course in immunodeficient individuals. Several forms of microsporidiosis are distinguished based on the pathogen:
- Intestinal and systemic infections (caused by Enterocytozoon bieneusi and E. intestinalis). Accompanied by debilitating chronic diarrhea. Purulent-inflammatory processes may also occur, including sinusitis, bronchitis, cystitis, urethritis, nephritis, and acalculous cholecystitis.
- Ophthalmic manifestations. Keratitis and keratoconjunctivitis are characteristic of Encephalitozoon hellem, Vittaforma corneae, and Nosema ocularum.
- Myositis and disseminated forms. Systemic involvement of internal organs and muscle tissue is caused by representatives of the genera Pleistophora, Trachipleistophora, and Nosema-like microorganisms.
Microbiological Diagnostics and Treatment
Research specimens include intestinal and bladder biopsies, smears of cerebrospinal and bronchoalveolar fluid, and urinary sediment. The diagnosis is confirmed by identifying spores (1–2 µm in size) via microscopy.
Given the Gram-positive and acid-fast properties of the spores, specific staining methods are applied. The optimal choice is the Goodpasture method:
- Staining the preparation with carbol fuchsin.
- Decolorizing with a 37% formaldehyde solution.
- Counterstaining with picric acid.
Standard Gram staining is also used.
Etiotropic therapy relies on metronidazole. For ocular forms (keratoconjunctivitis), topical fumagillin may be used. Specific prophylaxis with vaccines is currently unavailable. Non-specific sanitary and hygienic measures are completely analogous to the prevention rules for cryptosporidiosis.