Practical Parasitology

Isospora belli

Life Cycle

As with other Coccidia, Isospora belli has both a sexual and an asexual cycle. Oocysts are passed in the feces unsporulated. Sporulation takes approximately 24 hours and the oocyst becomes infective. Upon ingestion of the infective oocyst the sporozoites present in the oocyst excyst and invade epithelial becoming trophozoites. As with other Coccidia these trophozoites undergo schizogony. The mature cell ruptures and releases merozoites to invade other cells furthering the process of infection. A male and female gametocyte is formed (sexual cycle). The male gametocyte fertilizes the female gametocyte forming the oocyst that is soon released in the intestinal lumen and passed with the feces, completing the cycle.