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Protozoan Diseases - MICROSPORIDIOSIS

Protozoa of the order Microsporidia (Phylum: Microspora) are represented by about 700 species parasitic in hosts of many invertebrate and vertebrate groups. These protozoa have only recently been recognized as "opportunistic" parasites in patients with AIDS. In veterinary medicine, Microsporidia are well known as causative agents of certain animal infections, such as nosemosis in bees, encephalitozoonosis in rabbits and of numerous fish diseases. Microsporidia are obligatory intracellular parasites (macrophages, histiocytes, endothelial cells, kidney tubular cells, etc.) with a characteristic spore stage and a unique mode of infecting host cells. Spores of Microsporidia species infecting mammals are ovoid or piriform, 1.5-5.0 şm long with mostly a thick wall consisting of 2 main layers (exospore, endospore). The spore contains a coiled, minute tube (polar tube or polar filament) connected with a complex extrusion apparatus and a nucleated, infective sporoplasm. After ingestion by a suitable host, the physiological conditions of the digestive tract stimulate the spores to avert with force the coiled polar filament. The tip of the filament penetrates the host cell membrane, the sporoplasm migrates through the tube and enters the cytoplasm where asexual multiplication (merogony) and spore formation (sporogony) occur. Sporoplasms usually infect the gut epithelium either to develop there or to be transferred, probably by the action of phagocytic cells, to the circulation and their preferred site of development in various organs. The spores, released to the environment from the intestinal or urinary tract, are ingested by a new host. All Microsporidia examined so far have a direct developmental cycle. Up to now 5 genera of Microsporidia have been identified as causative agents of human infections.


Diagnosis is based on the direct detection of Microsporidia by histology after hematoxylin-eosin, Giemsa, Gram or other staining (spores are Gram-positive!) or by electron microscopy in autopsy or biopsy material. Serum antibody detection provides another tool for the in vivo diagnosis of encephalitozoonosis.


Information on chemotherapy of microsporidian infections is scanty. In experimental infections chloroquine and oxytetracycline reduced harvests of E. cuniculi spores only by 69% and 58%, respectively. Treatment of an AIDS patient suggested that either metronidazole or ganciclovir may have inhibited the microsporidian infection.

  • Microsporidiosis