Isospora belli

Isospora belli is a coccidian protozoan parasite belonging to the phylum Apicomplexa. Many species of Isospora infect a variety of animals such as dogs, cats, pigs and non-human primates. Pathogenicity in man appears to be restricted to Isospora belli since many human infections that in the past were attributed to Isospora hominis are now thought to have been caused by Sarcocystis hominis or Sarcocystis suihominis, through the accidental ingestion of raw beef or pork, respectively.

Isospora belli has a cosmopolitan distribution, but is more common in the tropics, particularly in the Caribbean and in Central and South America. In immunocompetent individuals, infection is often asymptomatic but can also cause a self-limiting enteritis with non-hematic diarrhea. In contrast, in immunocompromised persons (e.g. AIDS patients), infection causes severe symptoms chronic cholera-like diarrhea. In these individuals, “cystic” forms have recently been observed in extra-intestinal locations such as mesenteric lymph nodes, liver, spleen and lamina propria of the intestinal mucosa.

The life cycle of the parasite is direct and does not require an intermediate host. Isospora belli oocysts are excreted with the feces of infected individuals as immature, non-sporulated and non-infective forms. Sporulation, which requires several days and takes place in the external environment in particular conditions of temperature and humidity, leads to the formation of two sporocysts, each containing four sporozoites.

If oocysts are accidentally ingested through contaminated food or water, the sporozoites are released and enter the enterocytes of the small intestine (duodenum and jejunum), where they start asexual reproduction. At least one week later, sexual reproduction is also initiated, which results in the formation of oocysts each containing a single sporoblast.


Size: 20-23 μm long, 10-19 μm wide.

Morphology: elliptical, with sharpened ends. The cystic wall is smooth, thin, hyaline and split. A globular sporoblast is visible inside, containing many granules. In stool samples examined after storage at room temperature for short periods (1-2 days), oocysts containing two sporoblasts can be observed. However, in such cases, the sporulation process is generally incomplete.

The wall of the cyst and the sporont (or sporoblasts) stain red to garnet red by the modified Ziehl-Neelsen (hot) or the Kinyoun (cold) staining methods. Both methods have good specificity and thus are useful as confirmation tests. Like Cyclospora, Isospora belli autofluoresces under UV light.

Sheather’s flotation technique is an excellent method for the concentration of oocysts. The formalin-ether and formalin-ethyl acetate concentration methods also improve the sensitivity compared to direct examination of specimens.

Hematic eosinophilia and the detection of Charcot-Leyden* crystals in feces have often been noted but are not invariable features of an Isospora belli infection. When there is a suspicion of infection but the stool examination is negative, a sample of aspirated duodenal liquid can be examined.

*Charcot-Leyden crystals They consist of lysophospholipase (M.W. 17400), an enzyme synthesized by eosinophils, and are produced from the breakdown of these cells. They may be present in the stool or sputum. Size: 3-70 µm long, 0.5-8µ wide. Morphology: elongated, regular rhomb. The acute angle is 30°or more.