Figure 1. Size, shape, and staining morphologic features of control materials containing coccidians using a modified acid-fast stain (oil, x100). Left, The oocysts of Cyclospora cayetanensis measure 7.7 to 10.0 µm in diameter. Staining quality varies dramatically from no stain uptake by the oocysts (arrow) to bright pink staining. Center, Morphologic features of Cryptosporidium parvum. Oocysts stain faintly (arrows) to intensely pink with the stain. Compared with the Cyclospora oocysts, the C parvum oocysts are smaller, measuring 4 to 6 µm in diameter, and are round to irregularly shaped. Right, Isospora belli from cat feces demonstrates the characteristic large, thick-walled immature oocyst containing the modified acid-fast sporoblast. Variation in staining quality is seen with some sporoblasts taking up little or no stain (top 2 images). The oocysts of I belli measure 20 to 33 x 10 to 19 µm. The coccidian might appear refractile and, thus, might be identified in formalin-ethyl acetate concentrate wet preparations (not shown). Modified acid-fast staining enhances detection, however, particularly if small numbers of oocysts are present.