Microsporidia

Fungi that lack mitochondria

• The microsporidia are a group of unicellular intracellular parasites closely related to fungi, although the nature of the relation to the kingdom Fungi is not clear.

Nosema podocotyloidis - Hyperparasitic Microsporidia

Taxonomy

• The taxonomic position of this group has been debated and revised repeatedly; historically, they were considered protozoa and often remain managed by diagnostic parasitology laboratories.

Characteristics

• Microsporidia are characterized by the production of resistant spores that vary in size (usually 1—4 µm for medically-important species).
• They possess a unique organelle, the polar tubule or polar filament, which is coiled inside the spore as demonstrated by its ultrastructure.
• Microsporidia also possess degenerated mitochondria called mitosomes and lack a conventional Golgi apparatus.
• There are more than 1400 species belonging to over 200 genera have been described as parasites infecting a wide range of vertebrate and invertebrate hosts.

Microsporidia Life Cycle

Pathogenic types

• There are at least 15 microsporidian species that have been identified as human pathogens;
• The vast majority of cases being caused by Enterocytozoon bieneusi, followed by some Encephalitozoon species (E. cuniculi, E. hellem, E. intestinalis (=Septata intestinalis)).
• Other less frequently reported agents include members of the genera Anncaliia (=Brachiola) (A. algerae, A. connori, A. vesicularum), Microsporidium (M. ceylonensis, M. africanum), Trachipleistophora (T. hominis, T. anthropophthera), Nosema ocularum, Pleistophora ronneafiei, Vittaforma corneae (=Nosema corneae), Tubulinosema acridophagus, and an unknown species likely belonging to Endoreticulatus.

Life Cycle

1. The infective form of microsporidia is the resistant spore, which can persist in the environment for months image The spore then germinates, rapidly everting its polar tubule which contacts the eukaryotic host cell membrane.
2. The spore then injects the infective sporoplasm into the host cell through the polar tubule.
3. Inside the cell, the sporoplasm enters the proliferative phase marked by extensive multiplication via merogony (binary fission or multiple fission), creating meronts.
4. The location of this developmental stage within the host cell varies by genus; it can occur either in direct contact with the host cell cytosol (Enterocytozoon, Nosema), inside a parasitophorous vacuole of unknown origin (Encephalitozoon), in a parasite-secreted envelope (Pleistophora, Trachipleistophora), or surrounded by the host cell endoplasmic reticulum (Endoreticulatus, Vittaforma).
5. Following the proliferative phase, meronts undergo sporogony in which the thick spore wall and invasion apparatus develop, creating sporonts and eventually mature spores when all organelles are polarized.
6. When the spores increase in number and completely fill the host cell cytoplasm, the cell membrane is disrupted and spores are released to the surroundings. These free mature spores can infect new cells thus continuing the cycle.

Microsporidia History

Transmission

• Mature spores of intestinal-localizing species may be shed in feces, although the route of transmission remains uncertain for many species.
• Exposure to spores in water or in soil appears to be a potentially major route, based on the finding of spores in these sources along with case histories.
• E. bieneusi and V. corneae have been identified in surface waters, and spores of Nosema sp. (and likely A. algerae) have been identified in ditch water.
• Cases of donor-derived microsporidiosis (Encephalitozoon cuniculi) following bone marrow, kidney, liver, and heart transplantation have been confirmed.

Nosema podocotyloidis - Hyperparasitic Microsporidia

Hosts

• Many domestic and wild animals may be naturally infected with various medically-important microsporidia. Enterocytozoon bieneusi is generally considered a human parasite, but has been detected in swine, primates, cattle, cats, dogs, and several other mammals. Some, but not all, of these animal-derived strains appear to represent zoonotic genotypes.
• Encephalitozoon cuniculi is endemic in several captive and wild rabbit populations.
• It has also occasionally been found in domestic dogs, cats, foxes, captive monkeys, and mink.
• Birds, especially psittacines (parrots, parakeets, love birds, budgerigars, etc.), may represent reservoirs for Encephalitozoon hellem.
• Unlike the other two important members of the genus, E. intestinalis is only rarely identified in animals other than humans.

Geographic Distribution

• Microsporidia are being increasingly recognized as opportunistic infectious agents worldwide.
• Efforts to characterize the global distribution of species and genotypes are ongoing.

Also see

Microsporidiosis

Microsporidia Resources
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