• Author: Venkat R Minnaganti, MD, FACP; Chief Editor: Michael Stuart Bronze, MD  more...
Updated: Dec 02, 2015


Cystoisosporiasis, which was previously known as isosporiasis, is an uncommon diarrheal illness caused by the protozoan Cystoisospora belli (formerly known as Isospora belli). C belli was first described by Virchow in 1860. The genus Cystoisospora is related closely to the generaCryptosporidium, Cyclospora, and Toxoplasma. However, Cystoisospora infection is not as common as infection with Cryptosporidium or Toxoplasma. The first case of human infection with C belli was described In 1915.[1] See the image below.

Oocyst of Cystoisospora belli with 2 sporoblasts. Oocyst of Cystoisospora belli with 2 sporoblasts. From the Image Library, Division of Parasitic Diseases at the National Center for Infectious Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, Georgia.

Humans are the only known hosts for C belli, which has no known animal reservoir. Cystoisosporiasis has a worldwide distribution, although it is more common in tropical and subtropical climates. (See Epidemiology.)

C belli infection usually causes a mild and protracted illness unless the patient is immunocompromised. (See Clinical.) Clinical presentation may mimic those of inflammatory bowel disease and irritable bowel syndrome.

The diagnosis of cystoisosporiasis is based on a combination of clinical, epidemiological, and diagnostic tests. (See Workup.) Cystoisosporiasis is an AIDS-defining illness, so an appropriate workup for HIV infection should be performed, if necessary.

Also see Common Intestinal Parasites, a Critical Images slideshow, to help make an accurate diagnosis.

Although cystoisosporiasis is generally a self-limited infection, patients who are treated tend to improve in 2-3 days, whereas those who are not remain sick considerably longer. (See Treatment.) Immunocompetent hosts generally respond very rapidly to antiparasitic therapy. Immunocompromised hosts also respond well, though less rapidly; however, they have a high relapse rate once therapy is stopped and thus typically require indefinite prophylaxis after therapy.



C belli is ingested in contaminated food or water, and its life cycle requires a stage outside the host. After mature C belli oocysts are ingested, they liberate sporozoites (possibly in response to bile in the small intestine), which invade the enterocytes of the proximal small intestine. Here, they become trophozoites, and asexual multiplication (schizogony) produces merozoites that invade previously uninfected cells.

Shortly thereafter, a sexual multiplication cycle (sporogony) begins, generating oocysts that may pass into the environment. Outside the host, oocysts mature and become infectious 2-3 days later. The oocysts of C belli are resistant and remain viable in the environment for months.

Symptoms of isosporiasis suggest a toxin-mediated mechanism, but no toxin has been identified. In humans, extraintestinal forms of cystoisosporiasis are rare; however, they have been reported in patients with AIDS.



The causative pathogen of cystoisosporiasis is C belli, a protozoan that belongs to the subclass Coccidia in the phylum Apicomplexa. The mode of transmission of isosporiasis is fecal-oral, ie, through food or water contaminated with human feces. In immunocompetent hosts, C belli infection causes a self-limited diarrheal illness. In individuals with immunocompromise, it may cause chronic life-threatening diarrhea and dehydration.

Exposure to contaminated food or water predisposes to this infection. Because an external stage in the environment is required for the oocysts to mature, direct person-to-person transmission is unlikely. Accordingly, isosporiasis is more common in areas with poor sanitation. The disease is also more common in patients with AIDS.



United States statistics

The exact incidence of cystoisosporiasis in humans is unknown. C belli has been reported as the cause of outbreaks of diarrheal illness in daycare centers and mental institutions and has been implicated in traveler’s diarrhea in endemic areas.[2]

C belli infection is distinctly rare among immunocompetent individuals. Cystoisosporiasis is more common in persons with AIDS, 0.2-3% of whom have stools positive for C belli. However this increased prevalence has been reduced by the widespread use of Pneumocystis jiroveci pneumonia (PCP) prophylaxis with trimethoprim-sulfamethoxazole (TMP-SMZ) among patients with HIV infection.

In Los Angeles from 1985 to 1992, 1% of patients with AIDS had stools positive for C belli.[3] Infection was more common in foreign-born and Hispanic residents with AIDS (eg, those from El Salvador [7.4%] or Mexico [5.4%]) and less common in those receiving TMP-SMZ prophylaxis for Pneumocystis infection.[3]

Cystoisosporiasis has also been reported in patients with lymphoma and leukemia and recipients of renal and liver transplants.

International statistics

Endemic areas of cystoisosporiasis include Africa,[4] Australia, the Caribbean Islands, Latin America, and Southeast Asia. One study found positive examination findings in up to 15% of Haitians infected with AIDS.[5] In developing countries, 8-40% of patients with AIDS are infected.

Cystoisosporiasis is the initial AIDS-defining illness in approximately 2-3% of patients with AIDS who are from Africa. Among patients with AIDS who are from South America, 10% with chronic diarrhea have isosporiasis. In patients with AIDS who are from Haiti and Africa, 7-20% with chronic diarrhea have cystoisosporiasis.

Age-, sex- and race-related demographics

People of all ages are susceptible to C belli infection, although it tends to be more serious in infants and young children, possibly as a result of the risk of dehydration in this population. C belli can cause severe diarrhea in infants.

No gender predilection for infection has been noted, aside from the gender distribution of people with AIDS, the risk factor most commonly associated with this disease.

No racial predilection for isosporiasis has been reported, other than the racial distribution of people with AIDS. Among people with AIDS in the United States, Hispanics appear to be more at risk than blacks or whites, likely secondary to importation.



Prognosis is excellent with therapy (and prophylaxis, if appropriate).

In immunocompetent patients, cystoisosporiasis is usually a transient, self-limited illness but can sometimes result in a protracted diarrheal illness. Cystoisosporiasis has also been reported as a contributor to malabsorption syndrome in immunocompetent patients.[6]

In patients with AIDS, cystoisosporiasis can range in severity from a chronic and intermittent illness to a severe and life-threatening diarrheal illness.

Contributor Information and Disclosures

Venkat R Minnaganti, MD, FACP Consulting Staff, Department of Medicine, Winthrop University Hospital; Clinical Instructor, Department of Internal Medicine, Division of Infectious Disease, SUNY Stony Brook University School of Medicine

Venkat R Minnaganti, MD, FACP is a member of the following medical societies: All India Ophthalmological Society, American College of Physicians, American Medical Association, American Society for Microbiology, Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

John W King, MD Professor of Medicine, Chief, Section of Infectious Diseases, Director, Viral Therapeutics Clinics for Hepatitis, Louisiana State University Health Sciences Center; Consultant in Infectious Diseases, Overton Brooks Veterans Affairs Medical Center

John W King, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians, American Federation for Medical Research, Association of Subspecialty Professors, American Society for Microbiology, Infectious Diseases Society of America, Sigma Xi

Disclosure: Nothing to disclose.

Chief Editor

Michael Stuart Bronze, MD David Ross Boyd Professor and Chairman, Department of Medicine, Stewart G Wolf Endowed Chair in Internal Medicine, Department of Medicine, University of Oklahoma Health Science Center; Master of the American College of Physicians; Fellow, Infectious Diseases Society of America

Michael Stuart Bronze, MD is a member of the following medical societies: Alpha Omega Alpha, American Medical Association, Oklahoma State Medical Association, Southern Society for Clinical Investigation, Association of Professors of Medicine, American College of Physicians, Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Additional Contributors

Klaus-Dieter Lessnau, MD, FCCP Clinical Associate Professor of Medicine, New York University School of Medicine; Medical Director, Pulmonary Physiology Laboratory; Director of Research in Pulmonary Medicine, Department of Medicine, Section of Pulmonary Medicine, Lenox Hill Hospital

Klaus-Dieter Lessnau, MD, FCCP is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, American Medical Association, American Thoracic Society, Society of Critical Care Medicine

Disclosure: Nothing to disclose.

Glenn Fennelly, MD, MPH Director, Division of Infectious Diseases, Lewis M Fraad Department of Pediatrics, Jacobi Medical Center; Clinical Associate Professor of Pediatrics, Albert Einstein College of Medicine

Glenn Fennelly, MD, MPH is a member of the following medical societies: Pediatric Infectious Diseases Society

Disclosure: Nothing to disclose.

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Oocyst of Cystoisospora belli with 2 sporoblasts. From the Image Library, Division of Parasitic Diseases at the National Center for Infectious Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, Georgia.
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