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The annual incidence of microsporidiosis is unknown and the geographical distribution of microsporidia remains to be defined. Immunodeficient patients (individuals with HIV, and patients who have undergone a bone marrow or organ transplant) are the main target.

The infection results in chronic diarrhea that leads to severe weight loss.

Microsporidia are unicellular eukaryotic parasites without mitochondria and are responsible for opportunistic infections. Microsporidia are strictly intracellular parasites. The initial proliferative asexual stage (merogony) is followed by a disseminating stage (sporogony) that leads to spore development. Small spores (1 to 3 µm depending on the species) represent the most resistant and disseminating form and are characterized by a polar filament, which allows them to perforate the cell wall and inject nuclear material. They develop mainly in intestinal cells but may also develop in adipocytes, epithelium cells and blood cells. Several genera have been reported in humans, among which Encephalitozoon and Enterocytozoon are the most common. Contamination probably occurs after ingesting spores contained in water or food. Direct interhuman contamination is also likely.

Diagnosis is based on the identification of spores through laboratory testing.

Differential diagnoses include the other genera of microsporidia.

Treatment with albendazole is well tolerated and successful, except for patients with Enterocytozoon bieneusi infections, for which only fumagillin is effective but hematotoxic.

The prognosis may be severe in immunodeficient patients.