Answer to the Parasite Case of the Week 686: Cystoisospora belli sporulated oocyst
This case shows a single - but classic - sporulated oocyst of Cystoisospora (formerly Isospora) belli. Note that two sporoblasts are present in this case, which is unusual to see in freshly passed stool specimens. Most C. belli oocysts are shed in an immature state - either unsporulated or partially sporulated (with only one sporoblast). They mature further in the environment, with the sporoblast dividing in two so that the mature oocyst has 2 sporoblasts. The sporoblasts become sporocysts with a surrounding cyst wall, and the sporocysts divide twice so that they produce 4 sporozoites each.