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Infectious Diseases > MEDICAL TOPICS
Isosporiasis
Article Last Updated: May 12, 2006
AUTHOR AND EDITOR INFORMATION
Section 1 of 11  
Author: Venkat R Minnaganti, MD, Consulting Staff, Department of Medicine, Winthrop University Hospital; Clinical Instructor, Department of Internal Medicine, Division of Infectious Disease, State University of New York School of Medicine at Stony Brook
Venkat R Minnaganti is a member of the following medical societies: American College of Physicians, American Medical Association, American Society for Microbiology, and Infectious Diseases Society of America
Coauthor(s):
Burke A Cunha, MD, Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital
Editors: Klaus-Dieter Lessnau, MD, FCCP, Clinical Assistant 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; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; John W King, MD, Professor of Medicine, Section of Infectious Diseases, Louisiana State University Health Sciences Center; Director, Viral Therapeutics Clinics for Hepatitis; Consulting Staff, Department of Infectious Diseases, Overton Brook Veterans Affairs Medical Center; Eleftherios Mylonakis, MD, Clinical and Research Fellow, Department of Internal Medicine, Division of Infectious Diseases, Massachusetts General Hospital; Michael E Zevitz, MD, Assistant Professor of Medicine, Finch University of the Health Sciences, The Chicago Medical School; Consulting Staff, Private Practice
Author and Editor Disclosure
Synonyms and related keywords:
Isospora belli, Cryptosporidium, Cyclospora, Toxoplasma species
Background
Isosporiasis is an uncommon diarrheal illness caused by the protozoan Isospora belli. Humans are the only known hosts for I belli, and there is no known animal reservoir. Isospora has worldwide distribution, although it is more common in tropical and subtropical climates. In 1860, Virchow first described Isospora. In 1915, the first case of human infection with I belli was described. Isospora is related closely to Cryptosporidium, Cyclospora, and Toxoplasma.
Pathophysiology
The oocysts of Isospora are resistant and remain viable in the environment for months. Ingestion of mature oocysts of Isospora leads to invasion of the epithelial cells of the distal duodenum and proximal jejunum, with resulting cell damage. Symptoms of isosporiasis suggest a toxin-mediated mechanism, but no toxin has been identified. In humans, extraintestinal forms of the disease are rare but have been reported in patients with AIDS.
Frequency
United States
The exact incidence of isosporiasis in humans is not known, but Isospora has been reported as the cause of outbreaks of diarrheal illness in day care centers and mental institutions. I belli has been implicated in traveler's diarrhea in endemic areas.
The occurrence is increased in patients with AIDS, but it now is less common because of the widespread use of Pneumocystis carinii prophylaxis with trimethoprim-sulfamethoxazole (TMP-SMZ). Isosporiasis is an initial AIDS-defining illness in approximately 0.2% of patients with AIDS.
International
Endemic areas of infection are Africa, Australia, the Caribbean Islands, Latin America, and Southeast Asia.
Isosporiasis is the initial AIDS-defining illness in approximately 2-3% of patients with AIDS who are from Africa. In patients with AIDS who are from South America, 10% of patients with chronic diarrhea have isosporiasis. In patients with AIDS who are from Haiti and Africa, 7-20% of patients with chronic diarrhea have isosporiasis.
Mortality/Morbidity
In immunocompetent patients, isosporiasis usually is a transient illness, but it can result in a protracted diarrheal illness. Isosporiasis contributing to malabsorption syndrome in immunocompetent patients has been reported.
In patients with AIDS, isosporiasis can vary from a chronic and intermittent illness to a severe and life-threatening diarrheal illness.
Race
In patients with AIDS, isosporiasis is more prevalent in Hispanics than in blacks or whites.
Sex
Males and females are equally susceptible.
Age
Isospora can infect both adults and children and can cause severe diarrhea in infants.
History
The mode of transmission of isosporiasis is fecal-oral, ie, through food or water contaminated with human feces. Isospora usually causes a mild and protracted illness unless the patient is immunocompromised.
- The incubation period ranges from 3-14 days.
- Symptoms and signs
- Profuse, watery, nonbloody, offensive-smelling diarrhea, which may contain mucus
- Cramping abdominal pain, vomiting
- Malaise, anorexia, weight loss
- Low-grade fever
- Steatorrhea in protracted cases
Causes
- Isospora belli
- Increased incidence of infection in areas with poor sanitation
- Increased incidence of infection in patient with AIDS
Amebiasis
Crohn Disease
Cryptosporidiosis
Diverticulitis
Eosinophilic Gastroenteritis
Giardiasis
Inflammatory Bowel Disease
Intestinal Radiation Injury
Irritable Bowel Syndrome
Strongyloidiasis
Ulcerative Colitis
Other Problems to be Considered
Intestinal lymphoma
Lab Studies
- Diagnosis is based on a combination of clinical, epidemiological, and diagnostic tests.
- Routine laboratory tests are not diagnostic, but peripheral eosinophilia is an important clue.
- Stool specimen
- Large oocysts of I belli are observed on modified acid-fast stains of stool specimens (see Image 1).
- Multiple specimens or specimen concentration increases diagnostic yield.
- Charcot-Leyden crystals are observed in stool specimens.
- Polymorphonuclear leukocytes (PMNs) are not observed in fecal specimens.
- Serologic tests for isosporiasis are not available.
Imaging Studies
- Nonspecific radiographic findings (eg, prominent mucosal folds, thickening of intestinal wall) may be observed.
- If performed as a part of workup for malabsorption, I belli may be observed through electron microscopy of tissue specimens.
Other Tests
- Auramine-rhodamine fluorescent stain
- Modified Kinyoun acid-fast stain
- Ultraviolet autofluorescence microscopy - A simple, rapid test to identify I belli
- Zinc sulfate or sugar flotation - The most sensitive stool concentration technique
Procedures
- Examination of duodenal aspirate or string test may help.
- A small-bowel biopsy is not a routine test for diagnosis of isosporiasis.
Histologic Findings
Nonspecific findings observed in a small-bowel biopsy specimen are mucosal atrophy, shortened villi, hypertrophic crypts, and lamina propria infiltrated with eosinophils. I belli may be observed in the cytoplasm of enterocytes through electron microscopy.
Medical Care
In immunocompetent patients, isosporiasis is a mild, protracted illness. In patients with AIDS, patients with malignancy, or in patients undergoing chemotherapy, isosporiasis can be debilitating or life-threatening. Hemorrhagic colitis may occur in rare cases.
- Care is supportive and symptomatic.
- Antibiotics may be administered.
- Fluid losses may range from 2-20 L/d.
- Patients with severe diarrhea may require hospitalization.
Diet
- There is no specific diet recommended for patients with isosporiasis.
- A low protein, lactose-free diet is advised until the diarrhea is resolved.
Isosporiasis does not respond well to most antibiotics used to treat diarrhea. Oral TMP-SMZ is well absorbed even in patients with enteritis. A combination of 160 mg TMP and 800 mg of SMZ (1 double-strength tablet PO q6h for 2-4 wk) is the preferred drug of choice. In patients who are intolerant to sulfonamides, pyrimethamine (50-75 mg PO q24h) with folinic acid (5-10 mg PO q24h) may be given for 2-4 weeks.
In patients with AIDS, maintenance therapy with long-term suppressive treatment may be necessary using 1 TMP-SMZ double-strength tablet 3 times a week. Alternatively, 25 mg of pyrimethamine with 500 mg of sulfadoxine 3 times a week may be given. Patients with AIDS tend to have high relapse rates but respond well to retreatment. In addition, fluid replacement and correction of electrolyte imbalance is helpful.
Anecdotal case reports document improvement with albendazole, bismuth subsalicylate, diclazuril, furazolidone, metronidazole, and quinacrine; however, clinical trials are lacking.
One case report of isosporiasis refractory to TMP-SMZ has been described.
Drug Category: Antibiotics
Therapy must be comprehensive and cover all likely pathogens in the context of this clinical setting.
| Drug Name | Trimethoprim-sulfamethoxazole (Bactrim DS, Septra DS) |
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| Description | Inhibits bacterial growth by inhibiting synthesis of dihydrofolic acid. |
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| Adult Dose | 1 DS tab (160 mg TMP/800 mg SMZ) PO q6h for 2-4 wk |
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| Pediatric Dose | Not established |
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| Contraindications | Documented hypersensitivity; megaloblastic anemia due to folate deficiency |
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| Interactions | May increase PT when used with warfarin (perform coagulation tests and adjust dose accordingly); coadministration with dapsone may increase blood levels of both drugs; coadministration of diuretics increases incidence of thrombocytopenia purpura in elderly people; phenytoin levels may increase with coadministration; may potentiate effects of methotrexate in bone marrow depression; hypoglycemic response to sulfonylureas may increase with coadministration; may increase levels of zidovudine; increased nephrotoxicity in renal transplant patients taking cyclosporine; decreased efficacy of tricyclic antidepressants |
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| Pregnancy | C - Safety for use during pregnancy has not been established.
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| Precautions | Discontinue at first appearance of sore throat, skin rash, or sign of adverse reaction; obtain CBCs frequently; discontinue therapy if significant hematologic changes occur; goiter, diuresis, and hypoglycemia may occur with sulfonamides; prolonged IV infusions or high doses may cause bone marrow depression (if signs occur, give 5-15 mg/d leucovorin); caution in folate deficiency (eg, chronic alcoholics, elderly people, those receiving anticonvulsant therapy, or those with malabsorption syndrome); hemolysis may occur in G-6-PD–deficient individuals; patients with AIDS may not tolerate or respond to TMP-SMZ; caution in renal or hepatic impairment (perform urinalyses and renal function tests during therapy), hyperkalemia; give fluids to prevent crystalluria and stone formation; avoid use in pregnant patients and nursing mothers
Rare fatalities due to Stevens-Johnson syndrome, fulminant hepatic necrosis, agranulocytosis, and aplastic anemia have occurred |
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| Drug Name | Pyrimethamine (Daraprim) |
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| Description | Folic acid antagonist that selectively inhibits plasmodial dihydrofolate reductase. |
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| Adult Dose | 50-75 mg PO qd for 10 d, then 25 mg PO qd as maintenance dosage |
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| Pediatric Dose | Not established |
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| Contraindications | Documented hypersensitivity; megaloblastic anemia resulting from a folate deficiency |
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| Interactions | Concurrent use of antifolic acids (eg, methotrexate, pyrimethamine) may increase risk of bone marrow suppression; discontinue pyrimethamine therapy if signs of folate deficiency develop; mild hepatotoxicity may occur with concomitant administration of lorazepam and pyrimethamine |
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| Pregnancy | C - Safety for use during pregnancy has not been established.
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| Precautions | If signs of folate deficiency develop, reduce dose or discontinue drug depending on patient response; caution in hepatic or renal impairment; monitor for toxoplasmosis by performing semiweekly blood counts, including platelet counts; may precipitate hemolytic anemia in G-6-PD deficiency, generally in presence of other stressful events
Stop medication at onset of a rash, sore throat, pallor, purpura, or glossitis |
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Drug Category: Vitamins
Used to correct folic acid deficiency resulting from use of folic acid antagonists.
| Drug Name | Folinic acid; leucovorin (Wellcovorin) |
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| Description | A derivative of folic acid, which is used with folic acid antagonists such as sulfonamides and pyrimethamine. |
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| Adult Dose | 5-10 mg PO qd |
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| Pediatric Dose | Not established |
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| Contraindications | Documented hypersensitivity; pernicious anemia or vitamin-deficient megaloblastic anemias |
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| Interactions | May decrease efficacy of phenobarbital, phenytoin, primidone |
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| Pregnancy | C - Safety for use during pregnancy has not been established.
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| Precautions | Do not administer intrathecally or intraventricularly |
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In/Out Patient Meds
- Patients with AIDS may need lifelong suppressive treatment with a combination of TMP and SMZ.
Deterrence/Prevention
- Isosporiasis is a food-borne and water-borne illness; practice of hygienic measures may help in preventing the transmission.
Complications
- Dehydration is a complication of isosporiasis.
- Hemorrhagic colitis may occur.
- Disseminated extraintestinal disease may result from isosporiasis.
Medical/Legal Pitfalls
- Clinical presentation may mimic inflammatory bowel disease and irritable bowel syndrome.
- Isosporiasis can be an AIDS-defining illness in a few patients, and appropriate workup should be performed.
| Media file 1:
Oocyst of Isospora 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. |
 |  View Full Size Image | |
Media type: Photo
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Isosporiasis excerpt Article Last Updated: May 12, 2006
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