Isospora belli  is a coccidian protozoan of cosmopolitan distribution, occurring especially in warm regions of the world. 

Life cycle

The life cycle of Isospora belli involves an asexual (schizogonic stage) and a sexual (sporogonic stage)

Infection with I. belli occurs in both immunocompetent and immunocompromised patients and begins when the mature oocyst is ingested in water or food. .  The mature oocyst contains 2 sporocysts, each containing 4 sporozoites.  The sporulated oocyst is the infective stage of the parasite and they excyst in the small intestine releasing sporozoites which penetrate the epithelial cells, thus initiating the asexual stage of the lifecycle.  The sporozoite develops in the epithelial cell to form a schizont which ruptures the epithelial cell containing it, liberating merozoites into the lumen.  These merozoites will then infect new epithelial cells and the process of asexual reproduction in the intestine proceeds.  Some of the merozoites form macrogametes and microgametes (sexual stages) which fuse to form a zygote which matures to form an oocyst.

Pathogenesis

In the immunocompetent, infection is generally asymptomatic or a self-limiting gastro-enteritis.  However, in chronic infections, severe diarrhoea and fat malabsorption can occur. 

Infection in immunocompromised individuals ranges from a self-limiting enteritis to severe diarrhoeal illness resembling that of cryptosporidiosis.

Laboratory diagnosis

Oocysts are thin walled, transparent and ovoid in shape. They can be demonstrated in faeces after formal ether concentration where they appear as translucent, oval structures measuring 20-33m by 10-19m.

Alternatively, oocysts can be seen in a faecal smear stained by a modified Ziehl-Neelsen method , where they  stain a granular red colour against a green background, or by phenol-auramine.

An oocyst of Isospora belli stained with modified Ziehl-Neelsen method

An oocyst of Isospora belli showing 1 sporocyst

An oocyst of Isospora belli showing 2 sporocysts

Oocysts of Isospra belli stained with phenol auramine