A case of Encephalitozoon cuniculi peritonitis in a patient with AIDS.

A 45-year-old man with acquired immune deficiency syndrome (AIDS) presented with an unusual peritonitis resulting from a microsporidian infection. An inflammatory mass formed by the omentum magnum was revealed at partial autopsy. This mass was infested with parasites measuring 2.5-2.6 microns by 1.2-1.5 microns, which appeared gram positive and stained dark blue with Giemsa stain. These organisms were pale pink with Kinyoun stain, and occasionally one of their poles was periodic acid-Schiff positive. Sporoblasts and spores contained in parasitophorous vacuoles were revealed by electron microscopic examination. These spores were mononucleated, without mitochondria, and had a polar filament with four or five coils. They were identified as Encephalitozoon cuniculi. Peritonitis resulting from this parasite has not been reported previously, however, it should be suspected among rare infections resulting from unusual parasites in patients with AIDS.