Microsporidiosis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Ogheneochuko Ajari, MB.BS, MS [2]

Overview

Microsporidia are a group of obligate intracellular parasitic fungi with more than 1,200 species belonging to 143 genera that infect a wide range of vertebrate and invertebrate hosts. They are characterized by the production of resistant spores that vary in size, depending on the species.

Pathophysiology

Microsporidiosis is acquired through the ingestion or inhalation of the spores (the infective form) of microsporidia. Sexual transmission have been suggested from previous studies that isolated Encephalitozoon species in the urinary tract of those with disseminated disease.

Microbiology

At least 15 microsporidian species have been recognized as human pathogens, spread across eight genera:

• Anncaliia (formerly Brachiola)
  • A. algerae, A. connori, A. vesicularum
• Encephalitozoon
  • E. cuniculi, E. hellem, E. intestinalis (formerly Septata intestinalis)
• Enterocytozoon
  • E. bieneusi
• Microsporidium
  • M. ceylonensis, M. africanum
• Nosema
  • N. ocularum
• Pleistophora sp.
• Trachipleistophora
  • T. hominis, T. anthropophthera
• Vittaforma
  • V. corneae.
• Tubulinosema
  • T. acridophagus

Life Cycle

The infective form of microsporidia is the resistant spore and it can survive for an exteneded period of time in the environment.

1. The spore extrudes its polar tubule and infects the host cell.
2. The spore injects the infective sporoplasm into the eukaryotic host cell through the polar tubule.
3. Inside the cell, the sporoplasm undergoes extensive multiplication either by merogony (binary fission) or schizogony (multiple fission).
4. This development can occur either in direct contact with the host cell cytoplasm (E. bieneusi) or inside a vacuole called a parasitophorous vacuole (E. intestinalis). Either free in the cytoplasm or inside a parasitophorous vacuole, microsporidia develop by sporogony to mature spores.
5. 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.
6. These free mature spores can infect new cells thus continuing the cycle.