Comparative pathology of microsporidiosis.

The obligate intracellular protozoan parasites belonging to the phylum Microspora are ubiquitous. They parasitize insects and all five classes of vertebrates. Only one genus infects mammals and birds but at least four genera affect humans. Two genera have been isolated from human specimens and both infect experimental animals. Some genera (eg, Enterocytozoon and Pleistophora) seem to be limited to a very few tissues but others (Encephalitozoon and the Encephalitozoon-like genus Septata) can infect multiple organs. Lesions range from classic microgranulomas to foci of infected cells unaccompanied by any inflammatory response. The most commonly occurring microsporidia infection of man (Enterocytozoon bieneusi) is characterized by infection of enterocytes of the villus tips of the small intestine, accompanied by villus blunting, crypt hyperplasia, sloughing of infected villus tip cells, and increased numbers of intraepithelial lymphocytes.