Eimeria organisms develop in the epithelial cells of equine small intestine.

Histopathologic and immunohistochemical examinations were performed to determine the origin of host cells parasitized by Eimeria in the small intestines collected from five foals. Eimeria organisms at various stages (mainly microgametes and macrogametes) were frequently found in the cytoplasm of hypertrophied host cells in the lamina propria at the tips of villi of the jejunum and ileum. The cytoplasm of the host cell was immunohistochemically positive for cytokeratin AE1/AE3 and cytokeratin 13 and was negative for vimentin, desmin, alpha-smooth muscle actin, chromogranin A, neuron-specific enolase, and factor VIII. The host cells parasitized by Eimeria species had the immunostaining characteristics of epithelial cells but not of mesenchymal cells, endothelial cells of lacteals or capillaries, smooth muscle cells or neuroendocrine cells. These results suggest that the host cell of Eimeria species is possibly derived from intestinal epithelial cells and then displaced into the lamina propria of the small intestine.