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Isospora belli is
another coccidian parasite of the phylum Apicomplexa that causes human
isosporiasis. I belli is worldwide in distribution
but is more common in tropical and subtropical regions, especially
Haiti, Mexico, Brazil, El Salvador, tropical Africa, the Middle
East, and Southeast Asia.1 The parasite causes
diarrhea in both immunocompetent and immunocompromised children
and adults. In the United States, sporadic outbreaks (mostly waterborne)
of isosporiasis have been reported among institutionalized patients
and daycare centers.2,3 It has been recognized
as a cause of diarrhea among travelers to endemic areas; immigrants;
and persons immunocompromised by hematologic malignancy, steroid use,
or HIV infection.1,4-7 In persons with AIDS, low
CD4 cell count (especially < 50 cells/μL),
multiple infections, and poor hygiene have been noted as risk factors.8,9 Transmission
occurs primarily by the fecal-oral route, but sexual transmission
has also been implicated among men who have sex with men.10
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After the host ingests the mature sporulated oocyst, excystation
releases sporozoites in the proximal small bowel.11 Occasionally,
sporozoites leave the intestinal tract to infect extraintestinal
sites.12,13 However the sporozoites typically invade
the enterocytes of duodenum and jejunum and mature into trophozoites.
The trophozoites then mature into merozoites, which undergo further
maturation by either asexual replication (schizogony or merogony)
or sexual replication (gametogony). Gametogony produces the immature unsporulated
oocyst that is passed in the stool.6 The oocyst
ripens in 24 to 48 hours into the infectious form or the mature
sporulated oocyst. The oocyst remains viable in a cool, moist environment
for months.
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Clinical Manifestations
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Clinically, isosporiasis is indistinguishable from other coccidian
infection such as cryptosporidiosis and cyclosporiasis. It usually
causes self-limiting diarrhea in the immunocompetent host and protracted,
severe diarrhea in the immunocompromised adult and child. Diarrhea
may be acute or chronic. Watery stool, crampy abdominal pain, and
weight loss are characteristic symptoms of isosporiasis. Other patients
may have flulike symptoms, nausea, and vomiting. Fecal blood and
leukocytes are absent. However, Charcot-Leyden crystals and, occasionally,
mucus may be present in the stool.1 Peripheral
eosinophilia is not uncommon.14,15 Severe wasting,
malabsorption, lactose intolerance, and steatorrhea have been reported,
particularly in the immunosuppressed patient. Extraintestinal presentations
such as acalculous cholecystitis and involvement of the lymph nodes,
liver, or spleen are rare but have been reported
in severely immunosuppressed individuals infected with HIV.12,13,16,17
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Diagnosis of I belli infection is established
by identifying the oocyst in feces or by visualizing the intracellular
stages of the parasite in biopsy specimens of the small bowel or
other sites. Isospora can be visualized in the
stool by direct fecal smears, but concentration techniques such
as flotation or sedimentation methods enhance detection. Modified
acid-fast or auramine-rhodamine stains are commonly used to identify
the oocysts of Isospora and other coccidian parasites found
in feces.1,4,18...