The coccidian parasite Cystoisospora belli (=Isospora belli) (phylum Apicomplexa) infects the epithelial cells of the small intestine of humans and causes diarrheal disease and a suite of symptoms known as cystoisosporiasis. It is the least common of the intestinal coccidia that infect humans. Cystoisospora belli occurs worldwide, especially in tropical and subtropical areas. Infection and more serious symptoms occur especially in immunodepressed individuals (e.g., AIDS patients) and outbreaks have been reported in institutionalized groups in the United States. (Source: Centers for Disease Control Parasites and Health Website) Contaminated food and water are generally believed to be the primary mode of transmission.
This parasite appears not to require a non-human host in its life cycle, completing both asexual and sexual phases within a single human host. However, it is not yet clear whether other animals may nevertheless function as reservoirs or paratenic hosts (i.e., hosts not required for the life cycle, but which may sustain it until it reaches an appropriate host). The oocysts of C. belli usually require less than one day to a few days after passage from a human intestine to complete sporogonic development and become infective. (Lindsay et al. 1997; Jongwutiwes et al. 2007) Lindsay et al. (1997) reviewed the life cycle of C. belli and related species (Lindsay et al. 1997). The biology of related parasites infecting domesticated mammals and non-human primates was reviewed by Lindsay and Blagburn (1994).
Isospora belli is a rare gastrointestinal pathogen in Human Immunodeficiency Virus (HIV) infected patients in North America, whereas it is endemic in some developing countries such as Haiti. 
Cryptosporidium, Isospora, Cyclospora and Microsporidia are increasingly becoming prevalent in Acquired Immunodeficiency Syndrome (AIDS) patients.  Isosporiasis is a chronic diarrheal illness in AIDS patients, caused by the protozoan Isospora belli. The parasitic infection has been commonly reported from different centers of India. 
1. Wuhib T, Silva TM, Newman RD, Garcia LS, Pereira ML, Chaves CS, et al. Cryptosporidial and Microsporidial infections in human immunodeficiency virus infected patients in northernestern. Brazil J Infect Dis 1994;170:494-7.
2. Prasad KN, Nag VL, Dhole TN, Ayyagari A. Identifection of enteric pathogens in HIV positive patients with diarrhoea in northern India. J Health Popul Nutr 2000;81:23-6.
From (Mudholkar and Namey 2010)
Cystoisospora belli from Wikipedia
Cystoisospora belli, previously known as Isospora belli, is a coccidian parasite that causes an intestinal disease known as cystoisosporiasis. This parasite is transmitted by ingesting food or water that has been contaminated with feces from someone who is infected. The infective stage found in stool is the mature oocyst. The distribution of this parasite is worldwide but it is most commonly found in tropical and subtropical areas of the world.
Isospora belli was discovered by Rudolf Virchow in 1860 and was named by Charles Morley Wenyon in 1923. The parasite is now known as Cystoisospora belli.
The oocysts of Cystoisospora belli are long and oval shaped. The oocysts are usually between 20 and 33 micrometers in length and between 10 and 19 micrometers wide.
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, mereozoites are released and continue to invade more epithelial cells
Trophozoites develop into schizonts, containing many mereozoites
After about one week, development of male and female gametocytes begin in the mereozoites
Fertilization results in the development of oocysts, which are released in the stool .
Symptoms of cystoisosporiasis include abdominal pain, cramps, loss of appetite, nausea, vomiting, and fever. The most common symptom of this infection is watery diarrhea. These symptoms can last from weeks to months. Immunocompromised people are more severely affected by Cystoisospora belli and can experience extreme diarrhea that can lead to weakness, anorexia, and weight loss.
Diagnosis and Treatment
Cystoisospora belli is diagnosed by identification of the oocyst through examining a stool sample under a microscope. The diagnostic stage is the immature oocyst that contains a spherical mass of protoplasm. This infection is easily treated with antibiotics. The most common antibiotic that is prescribed is trimethoprimsulfamethoxazole, more commonly known as Bactrim, Septra, or Cotrim.
1. Centers For Disease Control: http://www.cdc.gov/parasites/cystoisospora/index.html
2. Garcia, L. (2006). Isospora belli. Waterborne Pathogens (217-219). Denver: American Water Works Association.
3. Cystoisospora belli. In Encyclopedia of Life (http://eol.org/pages/5006037/overview)
4. Velasquez, J., Osvaldo, G., Risio, C. D., Etchart, C., Chertcoff, A., Perisse, G., et al. Molecular characterization of Cystoisospora belli and unizoite tissue cyst in patients with Acquired Immunodeficiency Syndrome. Parasitology, 138, 279-286. Retrieved May 5, 2014, from http://www.ncbi.nlm.nih.gov/pubmed/20825690]
From Wikipedia (2014)
Cystoisospora belli occurs worldwide, especially in tropical and subtropical areas (Centers for Disease Control Parasites and Health Website).
Relevance to Humans and Ecosystems
I. belli, a relative of Toxoplasma, Cryptosporidium and Sarcocystis species, is a protozoan parasite of the phylum apicomplexa, class sporozoea, subclass coccidian and family eimeriidae. 
The diagnosis of isosporiasis is made following detection of oocysts (10 × 20 μm) of I. belli, which are passed in stool. Usually, the oocyst contains only one immature sporont but two may also be present. Continued development occurs outside the body with the development of two mature sporocysts, each containing four sporozoites. The sporulated oocyst is the infective stage, which upon ingestion, excyst in the small intestine releasing the sporozoites that penetrate the mucosal cells and initiate the life cycle in the infected host. Intestinal infection due to I. belli is distributed worldwide and the highest incidence has been reported from underdeveloped countries. The prevalence of I. belli infection was estimated to be 15% of AIDS patients in Haiti, but is <0.2% in US AIDS population. ,
In India,  the prevalence of I. belli infection is about 12% in AIDS patients. Very rarely, disseminated extraintestinal infection may occur.  Kumar et al. have found the incidence of I. belli infection 18.6% of chronic diarrhea and 7.3% of acute diarrhea in HIV positive patients. Lanjewar et al., in their study of 77 cases of HIV patients, found four cases of isolated isosporiasis. 
Diarrhea is often the presenting symptom of full blown AIDS and is characterized by large volume, presence of blood and abdominal pain, as seen in this patient.
Isosporiasis of the gastrointestinal tract responds readily to therapy with trimethoprim-sulfmethoxazole. I. belli should be considered a part of spectrum of potentially treatable infectious agents in patients with AIDS. It is therefore recommended that for patients of AIDS with diarrhea, an apprehensive examination for pathogenic bacteria, protozoa helminthes, etc. should be done.
3. Trier JS, Moxey PC, Shimmel EM, Robles E. Chronic intestinal coccidiosis in man. Intestinal morphology and response to treatment. Gastroenterology 1974;66:923-5.
4. Soave R, Johnson WD JR, Cryptosporidium and Isospora belli infections. J Infect Dis 1998;157:225-9.
5. DeHoitz JA, Pape JW, Boncy M, Johnson WD. Clinical manifestations and therapy of Isospora belli infection in patients with acquired immunodeficiency syndrome. N Engl J Med 1986;315:87-90.
6. Lanjewar DN. Immunopathology of HIV. Proceedings of international symposium on AIDS. Pune, India, May 1995.
7. Bernard E, Delgiudice P, Carles M. Bossy C, Saintpaul MC, Fichouxy L, et al. Dissminated Isosporiasis in an AIDS patients. Eur J Clin microbio Infec Dis 1997;16:699-701.
8. Kumar SS, Ananthan S, Lakshmi P. Intestinal Parasitic infection in HIV infected patients with diarrhoea in Chennai. Ind J Med Microbiol 2002;20:88-91.
9. Lanjewar DN, Rodrigues C, Saple DG, Hira SK, DuPont HL. Cryptosporidium Isospora and Strongyloides in AIDS. Natl Med J India 1996;9:17-9. [PUBMED]
(From Mudholkar and Namey 2010)
A 35-year-old male patient, positive for HIV antibodies and residing in a slum area, presented with episodes of diarrhea since 2 months, and fever and vomiting since 8 days. He also had history of weight loss. The patient was moderately dehydrated and cachexic. He had episodes of watery diarrhea with blood and mucus. The stool sample received was examined for saline mount, iodine mount and modified Zeihl-Neilson (Z-N) stain. The reports of routine biochemical investigations were within normal limits. The peripheral smear showed leukopenia with TLC 2700/cmm. The CD4 count was 85/cmm. The wet mount of stool revealed numerous cysts of I. belli measuring about 10 × 20 μm in size, with round granular center. Some of the cysts contained two sporocysts. Stool smear stained by modified Z-N stain showed many acid fast (pink) oocysts. The patient was put on antimicrobial agent trimethoprim-sulfmethoxazole and was followed up. However, he died after 1 month.
(Mudholkar and Namey 2010)
Isosporiasis is a human intestinal disease caused by the parasite Isospora belli. It is found worldwide, especially in tropical and subtropical areas. Infection often occurs in immuno-compromised individuals, notably AIDS patients, and outbreaks have been reported in institutionalized groups in the United States. The first documented case was in 1915.
The coccidian parasite Isospora belli infects the epithelial cells of the small intestine, and is the least common of the three intestinal coccidia that infect humans (Toxoplasma, Cryptosporidium, and Isospora).
At time of excretion, the immature oocyst contains usually one sporoblast (more rarely two). In further maturation after excretion, the sporoblast divides in two, so the oocyst now contains two sporoblasts. The sporoblasts secrete a cyst wall, thus becoming sporocysts; and the sporocysts divide twice to produce four sporozoites each. Infection occurs by ingestion of sporocyst-containing oocysts: the sporocysts excyst in the small intestine and release their sporozoites, which invade the epithelial cells and initiate schizogony. Upon rupture of the schizonts, the merozoites are released, invade new epithelial cells, and continue the cycle of asexual multiplication. Trophozoites develop into schizonts which contain multiple merozoites. After a minimum of one week, the sexual stage begins with the development of male and female gametocytes. Fertilization results in the development of oocysts that are excreted in the stool. Isospora belli infects both humans and animals.
Infection causes acute, non-bloody diarrhea with crampy abdominal pain, which can last for weeks and result in malabsorption and weight loss. In immunodepressed patients, and in infants and children, the diarrhea can be severe. Eosinophilia may be present (differently from other protozoan infections). (CDC 2013)
Microscopic demonstration of the large typically shaped oocysts is the basis for diagnosis. Because the oocysts may be passed in small amounts and intermittently, repeated stool examinations and concentration procedures are recommended. If stool examinations are negative, examination of duodenal specimens by biopsy or string test (Enterotest) may be needed. The oocysts can be visualized on wet mounts by microscopy with bright-field, differential interference contrast (DIC), and epifluorescence. They can also be stained by modified acid-fast stain.
Typical laboratory analyses include:
Morphologic comparison with other intestinal parasites
Bench aids for Isospora
Trimethoprim-sulfamethoxazole is the usual treatment choice (Lagrange- Xélot et al. 2008). See recommendations in The Medical Letter (Drugs for Parasitic Infections) for complete information.
Centers for Disease Control and Prevention, November 29, 2013. Cystoisosporiasis [Cystoisospora belli (syn. Isospora belli)]. Retrieved October 10 2014 from http://www.cdc.gov/dpdx/cystoisosporiasis/
Lagrange-Xélot M, Porcher R, Sarfati C, et al. (February 2008). "Isosporiasis in patients with HIV infection in the highly active antiretroviral therapy era in France". HIV Med. 9 (2): 126–30. doi:10.1111/j.1468-1293.2007.00530.x. PMID 18257775.
From Wikipedia 2014
Cystoisospora belli causes intestinal disease in several mammalian hosts, with infections believed to arise from the ingestion of sporulated oocysts in contaminated food or water. Infection, which is difficult to distinguish from cryptosporidiosis, is usually self-limiting and characterized by watery diarrhea, abdominal cramps, anorexia, and weight loss. Cystoisospora belli is often the agent responsible for traveler's diarrhea in travelers to developing countries where it is widespread. It is more common in AIDS patients, other immunocompromised patients, and indigenous populations in the United States.
(Fletcher et al. 2012 and references therein)
The coccidian parasite Cystoisospora belli (=Isospora belli) (phylum Apicomplexa) infects the epithelial cells of the small intestine of humans and causes diarrheal disease and a suite of symptoms known as cystoisosporiasis. It is the least common of the intestinal coccidia that infect humans. Infection and more serious symptoms occur especially in immunodepressed individuals (e.g., AIDS patients) and outbreaks have been reported in institutionalized groups in the United States.
- Bialek, R; Binder, N; Dietz, K; Knobloch, J; Zelck, Ue (Sep 2002), "Comparison of autofluorescence and iodine staining for detection of Isospora belli in feces" (Free full text), The American journal of tropical medicine and hygiene 67 (3): 304–5, ISSN 0002-9637, PMID 12408672
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