Life Cycle and Transmission

 

Apicomplexans are intracellular parasites with complex life cycles. The generalized life cycle consists of 3 phases: 1. Schizogony or merogony through which asexual division produces merozoites or meronts. 2. Gametogony or gamogony which results from merozoites or meronts differentiating into gamonts or gametocytes, each of which develops   into many micro or one macrogamete, and constitutes the sexual reproduction phase, and 3. sporogony through which sporozoites are produced by asexual reproduction. The invasive  (and extracellular) stages are the merozoite and the sporozoite, and contain the apical organelles.

          

 

 

 

Generalized apicomplexan life cycle

 

I. belli lives in the epithelial cells (enterocytes) of the small intestine.  In fresh feces immature oocysts are shed. Mature oocysts develop in 48hr if kept below 37^oC, and within each of the 2 sporocysts are 4 sporozoites  

 

Isospora belli oocysts

a.sporoblast; b two sporoblasts;

c, two sporocysts with 4 sporozoites

 

---

 

Transmission is by the fecal-oral contaminative route.

 

Life cycle of Isospora belli

 

After ingestion with water or food the sporozoites are released and invade enterocytes. Within the enterocyte the sporozoite  de-differentiates into a feeding form, the trophozoite, then  becomes a schizont (or meront)  which undergoes   asexual division (schizogony or merogony)  to produce merozoites; the merozoites leave the cell, enter other  enterocytes, and undergo a second round of asexual reproduction. The resultant merozoites then invade fresh enterocytes where they either continue asexual reproduction or enter gametogony differentiating into either microgametocytes or macrogametocytes. Each macrogametocyte develops into a macrogamete, and each microgametocyte develops into many flagellated microgametes. The microgametes swim to the macrogamete, fertilize it  in situ, and the resultant zygote  develops into an oocyst  containing a sporoblast.  The time from oocyst ingestion to oocyst production is 21 days. Sporulation (i.e. sporozoite formation) requires moist conditions and also requires gaseous oxygen and low temperature. Therefore,  I. belli oocysts ordinarily must have a time outside the body to become infective.