Encyclopedia of Parasitology

2016 Edition
| Editors: Heinz Mehlhorn

Isospora belli

Reference work entry
DOI: https://doi.org/10.1007/978-3-662-43978-4_1630
  1. 1.

    Name: Greek: isos = similar, identical; sporos = spore; kystos = cyst: Latin: bellum = belly.

  2. 2.

    Geographic Distribution/Epidemiology: Worldwide occurrence; several 100 millions infected people, not occurring as true epidemics.

  3. 3.

    Morphology/Life Cycle: This organism belongs to the so-called  Coccidia, which are members of the former phylum Sporozoa/Apicomplexa in the newly created  Alveolata. It parasitizes intracellularly the epithelial cells of the intestine of humans being situated inside a  parasitophorous vacuole. Infection of humans occurs by oral uptake of oocysts of the Isospora type from remnants of human feces. The oocysts, which appear ovoid with a flattened pole (Figs. 1 and 3), are excreted non-sporulated. Outside of the host’s body, two sporocysts are developed within 2–3 days inside the oocyst finally containing each four sporozoites, which are the infectious stages after oral uptake of the oocyst by another human being (Figs. 2 and 3). These oocysts measure 25–35 × 18–20 μm and keep their infectivity for at least 1 year, even when temperatures are low or even close to the freezing point.

  4. 4.

    Symptoms of Disease: Oral uptake of these oocysts within contaminated food or drinking water leads to diarrhea called either coccidiosis, isosporiasis, or more common traveler’s disease. In many cases there occur very quickly (about 2 days after the infection) severe symptoms of disease: repeated diarrheas combined with strong abdominal pain, nausea, and vomiting which persist for several days or even weeks. On the other hand, other persons excrete oocysts without significant symptoms and may infect whole groups of persons (e.g., during common excursions in nature). AIDS patients are especially endangered, since these parasites do not only attack the intestinal cells but are also found in masses intra- and extracellularly in lymph nodes far from the intestine. In AIDS patients in addition to the increased diarrheas, other symptoms occur additionally: malabsorption and steatorrhea (Restrepo et al. 1987). In AIDS patients as well as in immunocompetent travelers, diarrheas due to these parasites may start again 2–20 weeks after the excretion of oocysts and symptoms had stopped due to a successful treatment (relapse with unknown reason).

  5. 5.

    Diagnosis: The oocysts can easily be diagnosed by microscopic investigation after the use of concentration methods:  M.I.F.C., S.A.F.C.,  flotation. At first the oocysts are unsporulated (Figs. 1 and 3), but within 2–3 days, two sporocysts each with four sporozoites have developed (Figs. 2). In biopsy material of the intestinal wall, also schizonts, merozoites, and gametocytes can be found, but their structure is not species specific. In some cases, the blood status shows a mild eosinophilia.

  6. 6.

    Infection: Oral uptake of sporulated oocysts (Figs. 2) within contaminated food or drinking water.

  7. 7.

    Incubation Period: 2–13 days, when the production of schizonts inside the intestine occurs and may lead to first symptoms.

  8. 8.

    Prepatency: 7–9 days.

  9. 9.

    Patency: 2 weeks up to 1–2 years (in AIDS patients).

  10. 10.

    Therapy: The acute isosporiasis (traveler’s disease) is often self-limiting in case the patient ingests enough drinking water. However, in severe cases and under chronically repeated outbreaks (especially in the case of immunocompromised persons), the following cure is effective: cotrimoxazole (4 × 800 sulfamethoxazole and 160 mg trimethoprim daily for 10 days followed by twice daily for 3 weeks). Pyrimethamine (50–75 mg daily) acts as well. Rezidive prophylaxis can be done (1 × 40 mg sulfamethoxazole and 80 mg trimethoprim daily). In the USA, patients were successfully cured by application of diclazuril (1 × daily 200 mg for 7 days). Also ciprofloxacin (2 × daily 500 mg for 7 days) limits the number of parasites.

Isospora belli, Fig. 1

Light micrograph of an unsporulated oocyst of Isospora belli

Isospora belli, Fig. 2

Light micrograph of a sporulated oocyst of Isospora belli showing inside two globular sporocysts within which four sporozoites are finally formed

Isospora belli, Fig. 3

Diagrammatic representations of an unsporulated (a) and a sporulated and thus infectious (b) oocyst of I. belli. N nucleus, OW oocyst wall, R residual body, SP sporozoite, SW sporocyst wall, arrow points at an apical flattening


  1. Restrepo C et al (1987) Disseminal extraintestinal isosporiasis in a patient with acquired immune deficiency syndrome. Am J Pathol 87:536–542Google Scholar

Further Reading

  1. Legua P, Seas C (2013) Cystoisospora and Cyclospora. Inf Dis 26:479–483Google Scholar
  2. Montalvo R et al (2013) Recurrent diarrhea due to Cystoisospora belli in HIV/AIDS patients receiving Haart. Rev Peru Exp Salud Publ 30:326–330Google Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 2016

Authors and Affiliations

  1. 1.Institut für ZoomorphologieZellbiologie und Parasitologie, Heinrich-Heine-UniversitätDüsseldorfGermany