Practical Parasitology

Isospora belli

• Size of oocyst: 20 - 23 microns
• A pathogenic parasite of humans
• Infection is a result of the ingestion of a infective or sporulated oocyst.
• I. belli infections are more common in area of the tropics including Haiti, Mexico, tropical Africa and the middle east.
• I. belli causes serious infections in immunocompetent hosts with symptoms including, diarrhea, vomiting, steatorrhea, general malaise and weight loss. The infection often becomes chronic lasting from months to years.(36)
• Infection with I. belli in patients with a compromised immune system are at risk of developing a disseminated infection.(36)
• Diagnosis is made by the finding of oocysts in patient stools. A number of concentration techniques have proven to be effective including Sheather sugar flotation and the routine ether-formalin concentration method.
• Intestinal biopsies and the string test may also be helpful.

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.

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