Answer to Case 686

 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. 

We can make the diagnosis based on its characteristic appearance. However, if we wanted to confirm the diagnosis, Florida Fan notes that the oocysts "autofluoresce brightly under UV light excitation and will stain acid fast too." He goes on to say that "our team members prefer the autofluorescent methodology as it is simple and less time consuming." I know of some other labs that also prefer fluorescent microscopy to modified acid fast (MAF) staining since it is more sensitive. 

You can see some great examples of MAF staining HERE, and an autofluorescent oocyst of C. belli  from Florida Fan HERE.

Thanks again to Dr. Mike Mitchell and Laura Derderian for sharing this great case!