Persistent diarrhea caused by Isospora belli: therapeutic response to pyrimethamine and sulfadiazine.

A 54-year-old human immunodeficiency virus (HIV)-positive homosexual man developed overwhelming watery diarrhea and marked weight loss over a 3-week period. Although Entamoeba histolytica and other nonpathogenic enteric protozoa were observed along with Isospora belli in this patient's stool specimens, they were promptly eradicated after metronidazole (flagyl) treatment. The presence of I. belli oocysts in various stages of development in the stool and clinical symptoms related to Isospora infection persisted for 10 more months despite treatment with combined chemotherapeutic agents. Clinical and parasitiological resolution was ultimately achieved through an 8-week course of pyrimethamine and sulfadiazine.