Cystoisospora belli, previously known as Isospora belli, is a parasite that causes an intestinal disease known as cystoisosporiasis. This protozoan parasite is opportunistic in immune suppressed human hosts. It primarily exists in the epithelial cells of the small intestine, and develops in the cell cytoplasm. The distribution of this coccidian parasite is cosmopolitan, but is mainly found in tropical and subtropical areas of the world such as the Caribbean, Central and S. America, India, Africa, and S.E. Asia. In the U.S., it is usually associated with HIV infection and institutional living. Morphology A fully mature (sporulated) oocyst of genus Isospora is a spindle-shaped body that has two sporocysts that contain four sporozoites each. The oocysts of Cystoisospora belli are long and oval shaped. They measure between 20 and 33 micrometers in length and between 10 and 19 micrometers wide. Life cycle An oocyst with one sporoblast is released in stool of infected person After the oocyst has been released, the sporoblast matures further and divides into two After the sporoblasts divide they create a cyst wall and become sporocysts The sporocysts each divide twice, resulting in four sporozoites Transmission occurs when these mature oocysts are ingested The sporocysts excyst in the small intestine where sporozoites are released The sporozoites then invade epithelial cells and schizogony is initiated When the schizonts rupture, merozoites are released and continue to invade more epithelial cells Trophozoites develop into schizonts, containing many merozoites After about one week, development of male and female gametocytes begin in the merozoites Fertilization results in the development of oocysts, which are released in the stool The sporulation time of this parasite's egg is usually 1–4 days, and the entire life cycle takes about 9–10 days. The infective stage found in stool is the mature oocyst. The mature oocyst for Cystoisospora belli can remain infective in the environment for months. Symptoms Immune competent individuals are usually asymptomatic to this parasite's infection. But clinical symptoms such as mild diarrhea, abdominal discomfort, and low grade fever for approximately one week has been observed in some individuals. Immunocompromised people are more severely affected by Cystoisospora belli and can experience extreme diarrhea that can lead to weakness, anorexia, and weight loss. Other symptoms of cystoisosporiasis include abdominal pain, cramps, loss of appetite, nausea, vomiting, and fever, that can last from weeks to months. Diagnosis and treatment Cystoisospora belli is diagnosed by identification of the oocyst