Isosporiasis is caused by the protozoan Isospora belli. I. belli is also classified as a coccidia, a suborder characterized by unicellular intestinal parasites. The general term for the intestinal disease caused by coccidia is coccidiosis (2).
Species: belli (7)
Morphology and Life Cycle
Oocysts (the stage of I. belli that exists in the external environment) are ovoid in shape and measure 20-33 x 10-19 µm.
Life cycle of Isospora belli (individual figures from 1, 4, 6)
1) Immature, unsporulated oocyst is excreted through feces.
2) Sporoblast divides into two.
3) Each sporoblast develops into a sporocyst with 4 sporozoites, resulting in mature oocysts. The time spent in stages 1 through 3 is 2-3 days.
4) Mature oocyst is ingested.
5) Oocyst bursts. Sporozoites are released and lodge into the intestinal lining. Sporozoites undergo asexual reproduction to form merozoites. The merozoites mature into gametes which undergo fertilization to produce a new oocyst. (3)