The Isospora belli parasite is 30 micrometers long by 12 micrometers wide. Immature oocysts are ellipsoid or spindle shaped with tapered ends. An undivided mass of protoplasm exists inside the immature oocysts. This protoplasm divides to form two sporoblasts. The oocyst forms heavy cyst walls and the sporoblasts are now called sporocysts. These are 11 by 9 micrometers and within each sporocyst develop four curved, sausage shaped sporozoites. Throughout the oocyst’s development, the sporoblast and sporocysts are surrounded by a two-layered, colorless, smooth cell wall.

Isospora belli oocysts 1,2,3

1. A mature oocyst. Note the two sporoblasts inside the oocyst.

2. Immature oocyst. Note only one sporoblast. This is the stage it would be in the feces.

3. Immature oocyst. This is the stage it would be in the feces.


Images contributed by Georgia Division of Public Health on CDC Website.