• Formerly known as Isospora belli.
    • More recently has been termed Cystoisospora belli.
  • Intestinal unicellular protozoan parasite, classified under Coccidia.
  • Isospora is the least commonly seen Coccidial infection compared to Toxoplasma or Cryptosporidia.
  • Epidemiology: worldwide, but especially seen in tropical/subtropical environs, especially in Caribbean, Central and S. America, India, Africa, & S.E. Asia.
    • In U.S., usually associated with HIV infection and institutional living.
    • Also travel-acquired, sporadic cases.
  • Transmission occurs by ingestion of fecally contaminated water and food. Life Cycle [Figure 1].
    • Sporulated oocysts are ingested, sporozoites penetrate epithelial cells of small intestine and develop into trophozoites.
    • Mature oocyst 23-36 x 12-15 microns contains two sporocysts, which contain 4 trophozoites each [Figures 2 and 3 of immature oocysts].


  • Isospora causes gastrointestinal infection that mostly afflicts patients with AIDS or profound immunosuppression; however, cases can occur in immunocompetent patients.
    • Incubation period ~7d.
      • May last for weeks without treatment.
    • More severe disease commonly seen in immunosuppressed patients (AIDS), infants and children.
      • Cases in the U.S. are relatively rare due to routine use of TMP/SMX for PCP prophylaxis in all HIV+ patients w/ low CD4.
  • Sx: typically profuse, watery, non-inflammatory diarrhea (although cases of hemorrhagic diarrhea reported).
    • Malaise and crampy abdominal pain may accompany.
      • Fever uncommon
    • May result in malabsorption syndrome.
    • Thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) both described in association with this infection.
  • Extra-intestinal isosporiasis (hepatic, splenic, cholangitic/gallbladder involvement): rare and usually only occurs in immunocompromised patients.
  • Dx: microscopic wet mount or iodine stains of fecal smears are adequate but also modified acid-fast stains may also identify (see CDC site for key points to differentiate among the human Coccidia).
    • Multiple samples improve sensitivity, as shedding is intermittent.
    • Epifluorescence microscopy more sensitive than iodine stain[15].
    • If stools negative, duodenal aspirates (string test [Enterotest®]) or intestinal biopsy may yield diagnosis.
  • Eosinophilia may be seen, which is different from other protozoal infections).


  • Small intestine: diarrhea, malabsorption
  • Gallbladder and biliary tree: acalculous cholecystitis, cholangitis
  • Liver: dissemination reported (rare).
  • Spleen: dissemination reported (rare).
  • Rheumatological: reactive arthritis may occur.


Acute-chronic diarrhea

  • Usually self-limiting infection in immunocompetent individuals, requiring only symptomatic support such hydration and nutrition.
  • For immunocompromised or those with more severe infection:
    • Adult: TMP/SMX (160/800) DS tab orally twice-daily x 10 d.
    • Pediatric: TMP/SMX (TMP 10mg/kd/d/SMX 50mg/kg/d) PO in two doses daily x 10d.
    • Sulfa allergic: pyrimethamine 50-75mg/d in divided doses x 14 days + folinic acid (leucovorin 10-25mg/kg/d).
    • Other alternatives:
      • Ciprofloxacin 500 mg PO twice-daily x 1 wk.
        • Regimen less effective than TMP/SMX12, consider sulfa desensitization.
      • Nitazoxanide: limited experience (case reports).
    • Pregnancy (HIV): despite theoretical risk in first trimester, if significantly symptomatic, some recommend TMP/SMX.
      • Consider holding therapy if possible to second trimester.
      • Others have advocated that fluoroquinolones in the first trimester offer a better safety profile than TMP/SMX.
        • Study of over 600 cases of quinolone use in pregnancy did not find an increased risk of birth defects or musculoskeletal abnormalities and a registry data base of over 1100 quinolone exposures during pregnancy found no increase in the rate of birth defects.
      • TMP/SMX, use in the first trimester should be avoided, if possible, because of an association with an increased risk of birth defects, specifically neural tube defects, cardiovascular and urinary tract defects[21][7].

Secondary prophylaxis

  • Consider in immunosuppressed populations as up to 50% relapse following primary therapy.
  • Preferred:
    • TMP/SMX DS 1 tab PO 3x/wk for secondary prophylaxis[12].
  • Alternatives:
  • Duration: indefinite unless immune recovery; no published recommendations on discontinuation of secondary prophylaxis with immune reconstitution, but presumably safe.

Selected Drug Comments




Less effective than TMP/SMX; appropriate when sulfa drug or pyrimethamine cannot be used.


Use only if pt allergic to TMP/SMX; add folinic acid. May be used as well for secondary prophylaxis, e.g., AIDS patients with CD4


First-line agent. Highly effective. Preferred choice for secondary prophylaxis.


  • Shedding may persist even after adequate therapy; follow patients symptomatically.
  • Continued prophylaxis the norm for patients with AIDS unless ART provides immune reconstitution.
    • No known cases of IRIS have been described in the treatment of C. belli.
    • For AIDS, discontinue secondary prophylaxis when CD4 >200cell/ul x 6mos post ART initiation.
  • Treatment failure is rare with TMP/SMX; ciprofloxacin considered less effective but a possible alternative regimen in this case.


  • Other agents with case report level data reporting successes/failures include nitazoxanide, albendazole, spiramycin, roxithromycin, diclazuril (veterinary compound).

Basis for recommendation

  1. The Medical Letter. Drugs for Parasitic Infections; Medical Letter; 2013.

    Comment: Usually self-limiting in non-immunosuppressed patients, higher doses may be needed in some immunosuppressed patients and longer duration. Pyrimethamine for sulfa intolerant patients.

  2. Guidelines for the Prevention and Treatment of Opportunistic Infections in HIV-Infected Adults and Adolescents. T-34Panel on Opportunistic Infections in HIV-Infected Adults and Adolescents. Guidelines for the prevention and treatment of opportunistic infections in HIV-infected adults and adolescents: recommendations from the Centers for Disease Control and Prevention, the National Institutes of Health, and the HIV Medicine Association of the Infectious Diseases Society of America. Available at http://aidsinfo.nih.gov/contentfiles/lvguidelines/adult_oi.pdf. Accessed (12/17/15)

    Comment: Guidance for treatment, but also prevention as stated in this module.


  1. Legua P, Seas C: Cystoisospora and cyclospora. Curr Opin Infect Dis 26:479, 2013  [PMID:23982239]

    Comment: In US sporadic cases seen as well as travel-acquired.

  2. Boyles TH et al: Failure to eradicate Isospora belli diarrhoea despite immune reconstitution in adults with HIV--a case series. PLoS One 7:, 2012  [PMID:22880120]

    Comment: Case series of 8 patients from S. Africa reporting that despite ART and rise in CD4, there remained persistent parasitic infection despite therapy. Authors postulate that host factors or TMP/SMX resistance may be at play.

  3. Walther Z, Topazian MD: Isospora cholangiopathy: case study with histologic characterization and molecular confirmation. Hum Pathol 40:1342, 2009  [PMID:19447468]

    Comment: Infection of the gallbladder described in AIDS patients, and such a case is presented here in a man from West Africa. The radiographic appearance resembled sclerosing cholangitis.

  4. Dillingham RA et al: High early mortality in patients with chronic acquired immunodeficiency syndrome diarrhea initiating antiretroviral therapy in Haiti: a case-control study. Am J Trop Med Hyg 80:1060, 2009  [PMID:19478276]

    Comment: Study performed in Haiti included patients with I. belli diarrhea. Mortality was 10% in group starting ART as opposed to 5% (p = 0.009) without diarrhea, suggesting that diarrhea is indeed linked to mortality risks when initiating antivirals.

  5. Cooper WO et al: Antibiotics potentially used in response to bioterrorism and the risk of major congenital malformations. Paediatr Perinat Epidemiol 23:18, 2009  [PMID:19228311]

    Comment: Information on tetragenicity.

  6. ten Hove RJ et al: Real-time polymerase chain reaction for detection of Isospora belli in stool samples. Diagn Microbiol Infect Dis 61:280, 2008  [PMID:18424043]

    Comment: A real-time polymerase chain reaction assay targeting the internal transcribed spacer 2 region of the ribosomal RNA gene was developed for the detection of Isospora belli DNA in fecal samples.

  7. Tatfeng YM et al: Mechanical transmission of pathogenic organisms: the role of cockroaches. J Vector Borne Dis 42:129, 2005  [PMID:16457381]

    Comment: Authors demonstrate that cockroaches represent important reservoir for infectious pathogens, including Isospora; they suggest that control of roach population might decrease disease transmission.

  8. Bialek R et al: Comparison of autofluorescence and iodine staining for detection of Isospora belli in feces. Am J Trop Med Hyg 67:304, 2002  [PMID:12408672]

    Comment: Examination by autofluorescence of 192 stool samples (95.7%; 95% CI, 85.2-99.5) significantly more sensitive than iodine staining (48.4%; 95% CI, 37.7-59.1). Authors suggest that autofluorescence is simple, highly sensitive, inexpensive, and easily applicable method to detect Isospora oocysts in feces.

  9. Bialek R et al: Case report: Nitazoxanide treatment failure in chronic isosporiasis. Am J Trop Med Hyg 65:94, 2001  [PMID:11508398]

    Comment: Though a broad spectrum antiparasitic, there is little published experience using this drug for Isospora infection.

  10. Verdier RI et al: Trimethoprim-sulfamethoxazole compared with ciprofloxacin for treatment and prophylaxis of Isospora belli and Cyclospora cayetanensis infection in HIV-infected patients. A randomized, controlled trial. Ann Intern Med 132:885, 2000  [PMID:10836915]

    Comment: This is the only randomized trial regarding this infection in HIV-infected individuals. This small study looked at 22 pts with with chronic diarrhea due to I. belli randomly assigned to receive PO TMP-SMX DS1 tab twice-daily or ciprofloxacin (500 mg) twice-daily x7d. Pts who responded received prophylaxis for 10 wks (1 tab 3x/wk). Diarrhea resolved more rapidly with TMP-SMX than with ciprofloxacin. All pts receiving secondary prophylaxis with TMP-SMX remained disease-free, and 15 of 16 receiving secondary prophylaxis with ciprofloxacin remained disease-free.
    Rating: Important

  11. Doumbo O et al: Nitazoxanide in the treatment of cryptosporidial diarrhea and other intestinal parasitic infections associated with acquired immunodeficiency syndrome in tropical Africa. Am J Trop Med Hyg 56:637, 1997  [PMID:9230795]

    Comment: Early study suggested activity against I. belli with nitazoxanide. Note failure with this drug also cited in the literature.

  12. Lindsay DS et al: Examination of extraintestinal tissue cysts of Isospora belli. J Parasitol 83:620, 1997  [PMID:9267401]

    Comment: Authors focus on the extraintestinal stages of I. belli in a pt with HIV infection. These stages are important because relapse of diarrhea is common in humans infected with I. belli and is believed to be associated with presence of extraintestinal stages.

  13. Franzen C et al: Uvitex 2B stain for the diagnosis of Isospora belli infections in patients with the acquired immunodeficiency syndrome. Arch Pathol Lab Med 120:1023, 1996  [PMID:12049103]

    Comment: Wet-mounts examined by phase-contrast and bright-field microscopy; smears stained with modified acid-fast stain compared to fluorescent stain with Uvitex 2B. Using fluorescent stain, the oocysts of I. belli stained bright white/blue fluorescent and showed a structure similar to that of oocysts in acid fast stains.

  14. French AL et al: Cholecystectomy in patients with AIDS: clinicopathologic correlations in 107 cases. Clin Infect Dis 21:852, 1995  [PMID:8645829]

    Comment: I. belli microsporidiosis cryptosporidiosis

  15. Pape JW, Verdier RI, Johnson WD: Treatment and prophylaxis of Isospora belli infection in patients with the acquired immunodeficiency syndrome. N Engl J Med 320:1044, 1989  [PMID:2927483]

    Comment: Authors investigation their experience in Haiti in a small cohort of 32 patients with AIDS and chronic diarrhea. In a subgroup, long-term prophylaxis for 16 months prevented relapse or reinfection.

  16. Weiss LM et al: Isospora belli infection: treatment with pyrimethamine. Ann Intern Med 109:474, 1988  [PMID:3261956]

    Comment: Two patients with AIDS, sulfonamide allergy, and I. belli infection are reported. They were treated successfully with pyrimethamine 75 mg/d alone; recurrence prevented with pyrimethamine 25 mg/d.
    Rating: Important

  17. DeHovitz JA et al: Clinical manifestations and therapy of Isospora belli infection in patients with the acquired immunodeficiency syndrome. N Engl J Med 315:87, 1986  [PMID:3487730]

    Comment: Study of 20 of 131 HIV+ pts in Haiti with diarrhea Dx'd with I. belli. Sx included chronic watery diarrhea & weight loss. In all pts with isosporiasis, diarrhea stopped within2 days of beginning oral TMP-SMX. Recurrent symptomatic isosporiasis developed in 47%, but responded promptly to re-initiation of therapy.
    Rating: Important

  18. de Oliveira-Silva MB et al: Seasonal profile and level of CD4+ lymphocytes in the occurrence of cryptosporidiosis and cystoisosporidiosis in HIV/AIDS patients in the Triângulo Mineiro region, Brazil. Rev Soc Bras Med Trop 40:512, 2007 Sep-Oct  [PMID:17992404]

    Comment: Brazilian cohort of patients with IDS who had coccidial diarrheal infections. Of the 389 patients seen between 1993-2003, 19.7% were positive by modified Ziehl-Neelsen staining for coccidian (8.6% with Cryptosporidium sp, 10.3% with Cystoisospora belli and 0.8% with both coccidian. Only 8.5% of this group received ART. Of note, there was no seasonality to C. belli infection.

  19. Czeizel AE et al: The teratogenic risk of trimethoprim-sulfonamides: a population based case-control study. Reprod Toxicol 15:637, 2001 Nov-Dec  [PMID:11738517]

    Comment: Information that led to recommendations to generally avoid in early trimester.


Figure 2

Immature oocysts. Unstained wet mount. Source: CDC

Figure 3

Immature oocyst w/ stain. Oocysts, safranin stain. Source: CDC

Figure 1

Cystoisoporiasis Life Cycle. Source: CDC

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Last updated: January 31, 2016