Answer to Case 451

Answer: Cystoisospora belli (formerly Isospora belli) oocysts

As with Cyclospora cayetanensis and Cryptosporidium oocysts, the oocysts of C. belli stain red/pink using modified acid fast and modified safranin stains (below, right), with unstained "ghost" cells (below, left) also commonly seen. The oocysts are immature when shed in stool and contain only a single sporoblast. Oocysts will then mature in the soil, during which time the sporoblast divides into 2 sporoblasts  and develop cyst walls, becoming sporocysts. Each sporocyst will contain 4 sporozoites - the form that invades the intestinal epithelial cells.