Isospora belli
Isospora belli and related speces are more often seen in the small intestine than the colon. They appear as perinuclear and subnuclear intraepithelial inclusions. Sometimes, organisms can be found in the lamina propria in macrophages.
The trophozoites are round with a prominent nucleolus and the motile parasites are large and curved. Both of them are rather large and have a size comparable to the nucleus of the epithelial cells.
A vacuole around the parasite can be seen in some but not all cases. These microorganisms are positive for Giemsa stain and PAS stain. The latter feature must be used with caution as mucin droplets are also positive for PAS.
Other histologic changes include villous blunting, crypt hyperplasia, and mixed inflammatory cell infiltration often associated with many eosinophils. Fibrosis of the lamina can occur in chronic cases. Infection of Isospora can be disseminated.