Answer to Parasite Case of the Week 617: Cystoisospora (formerly Isospora) belli
The diagnostic features in this case include the size of the oocyst (~25 micrometers), oval shape, internal structure (single sporoblast in the wet prep) and acid fast positivity on the iron hematoxylin Kinyoun stain. The acid fast positivity allows us to rule-out Sarcocystis sp. as the oocysts of this parasite are not acid fast.
Kamran and Florida Fan noted that the oocysts of C. belli autofluoresce beautifully when examined with an excitation filter of 330 to 365 nm. As Blaine will tell us, they also exhibit a less intense fluorescence when examined with an excitation filter of 450-490 nm. Here is a great example of this autofluorescence from Florida Fan:
Note that this oocyst from Florida Fan has 2 sporoblasts, 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. Here are some great photos from Idzi showing this (you can see the sporozoites in the last image):