Microsporidia - Introduction, Classification, Morphology, Life Cycle, Pathogenesis, Pathology

Introduction of Microsporidia

Microsporidia are spore-forming, obligate eukaryotic intracellular parasites. The spores contain extrusion apparatus which has a coiled polar tube ending in an anchoring disc at the apical end. They have a unique mechanism to infect host cells such as:

• resistant spores which vary in size depending on the species

• has a polar tubule or polar filament while a unique organelle is present on the side of the spore

The phylum Microspora contains more than 1,200 species from 143 genera. At least 13 species have been known to cause disease in humans:

• Enterocytozoon bieneusi

• Encephalitozoon hellem

• Encephalitozoon intestinalis

• Encephalitozoon hominis

• Pleistophora

• Trachipleistophora hominis

• Trachipleistophora anthropophthera

• Nosema connori

• Nosema ocularum

• Brachiola vesicularum

• Vittaforma corneae

• Microsporidium ceylonensis

• Microsporidium africanum

Classification of Microsporidia

The classification of Microsporidia can be done by:

(unranked): Obazoa

(unranked): Opisthokonta

Clade: Holomycota

Kingdom: Fungi

Subkingdom: Rozellomyceta

Phylum: Rozellomycota

Class: Microsporidea

History of Microsporidia

Historically, they were classified as protozoans or protists and now it has been to be fungi or a sister group of fungi.

Habitat of Microsporidia

Microsporidia inhabit the small intestine of human hosts.

Morphology of Microsporidia

Spore is one of the important morphological features of Microsporidia.

Spores

• spore is the infective form of the parasite

• size of the spores depends on the species of Microsporidia

  Encephalitozoon spp, Vittaforma corneae, Nosema spp – 1.5 μm to 4 μm

  Encephalitozoon biemeusi – 0.8 μm to 1.4 μm

• resistant forms of Microsporidia

• can survive several months in the environment

Life Cycle of Microsporidia

Humans acquire microsporidiosis after ingestion or inhalation of Microsporidia spores, thus starting its life cycle.

• The polar tubules erupt from the apical end of the spore which injects the infective sporoplasm into the host cell

• the sporoplasm multiplies inside the cell either by merogony (binary fission) or schizogony (multiple fission)

• merogony and schizogony can take place directly inside the host cell cytoplasm (in Encephalitozoon bieneusi) or inside a parasitophorous vacuole (in Encephalitozoon intestinalis)

• Microsporidia undergo sporogony to develop into mature spores

• maturation of spores also includes thickening of cyst wall

• the host cells after completely filled with spores, stretch and finally ruptures to release the spores

• thick spore wall is able to protect the parasite during unfavorable environmental conditions

Pathogenesis, Pathology of Microsporidia

Microsporidia are invasive intracellular parasites with Encephalitozoon intestinalis being more invasive.

• they invade the intestinal mucosa and are usually found only in enterocytes

• in the intestinal mucosa, infected enterocytes are distributed in patches

• the infected enterocytes show pathological changes such as villous atrophy, elongated crypts, and depletion of goblet cells

• in AIDS patients, the biliary tract is also infected which results in cholangitis and cholecystitis

• cells of lamina propria, macrophages, and enterocytes are also infected

• disseminated infections can also occur by invading the small intestine, large intestine, gallbladder, urinary tract, respiratory tract, and epithelial cells of the respiratory and urinary tract