Causal Agents:
The term microsporidia is also used as a general nomenclature for the obligate intracellular protozoan parasites belonging to the phylum Microsporidia.  To date, more than 1,200 species belonging to 143 genera have been described as parasites infecting a wide range of vertebrate and invertebrate hosts.  Microsporidia, are characterized by the production of resistant spores that vary in size, depending on the species.  They possess a unique organelle, the polar tubule or polar filament, which is coiled inside the spore as demonstrated by its ultrastructure.  The microsporidia spores of species associated with human infection measure from 1 to 4 µm and that is a useful diagnostic feature.  There are at least 14 microsporidian species that have been identified as human pathogens: Brachiola algerae, B. connori, B. vesicularum, Encephalitozoon cuniculi, E. hellem, E. intestinalis, Enterocytozoon bieneusi Microsporidium ceylonensis, M. africanum, Nosema ocularum, Pleistophora sp., Trachipleistophora hominis, T. anthropophthera, Vittaforma corneae.  Encephalitozoon intestinalis was previously named Septata intestinalis, but it was reclassified as Encephalitozoon intestinalis based on its similarity at the morphologic, antigenic, and molecular levels to other species of this genus.  Based on recent data it is now known that some domestic and wild animals may be naturally infected with the following microsporidian species: E. cuniculi, E. intestinalis, E. bieneusi.  Birds, especially parrots (parakeets, love birds, budgies) are naturally infected with E. hellem.  E. bieneusi and V. corneae have been identified in surface waters, and spores of Nosema sp. (likely B. algerae) have been identified in ditch water.

Life Cycle:

The infective form of microsporidia is the resistant spore and it can survive for a long time in the environment .  The spore extrudes its polar tubule and infects the host cell .  The spore injects the infective sporoplasm into the eukaryotic host cell through the polar tubule .  Inside the cell, the sporoplasm undergoes extensive multiplication either by merogony (binary fission) or schizogony (multiple fission) .  This development can occur either in direct contact with the host cell cytoplasm (e.g., E. bieneusi) or inside a vacuole termed parasitophorous vacuole (e.g., E. intestinalis).  Either free in the cytoplasm or inside a parasitophorous vacuole, microsporidia develop by sporogony to mature spores .  During sporogony, a thick wall is formed around the spore, which provides resistance to adverse environmental conditions.  When the spores increase in number and completely fill the host cell cytoplasm, the cell membrane is disrupted and releases the spores to the surroundings .  These free mature spores can infect new cells thus continuing the cycle.

Geographic Distribution:
Microsporidia are being increasingly recognized as opportunistic infectious agents worldwide.  Cases of microsporidiosis have been reported* in developed as well as in developing countries, including: Argentina, Australia, Botswana, Brazil, Canada, Czech Republic, France, Germany, India, Italy, Japan, The Netherlands, New Zealand, Spain, Sri Lanka, Sweden, Switzerland, Thailand, Uganda, United Kingdom, United States of America, and Zambia.

*These data account for infections caused by at least one of the microsporidian species listed in the causal agent section.