Approach Considerations
The diagnosis of isosporiasis is based on a combination of clinical, epidemiologic, and diagnostic tests. Routine laboratory tests are not diagnostic, but peripheral eosinophilia is an important clue, seen in about one half of patients. Serologic tests for isosporiasis are not available.
Because isosporiasis is an AIDS-defining illness, an appropriate workup for HIV infection should be performed, if necessary.
Stool Examination
Stool examination for ova and parasites is the test of choice for isosporiasis. Multiple specimens or specimen concentration increases diagnostic yield. Zinc sulfate or sugar flotation is the most sensitive stool concentration technique.
Mature oocysts measure 30 × 12 µm and have a thin translucent wall and 2 round sporocysts, each of which has 4 crescentic sporozoites. Auramine-rhodamine fluorescent, modified Kinyoun acid-fast, hematoxylin/eosin, Giemsa, and/or carbol fuchsin staining may be helpful in identifying the translucent oocysts (see the image below). Oocysts autofluoresce under ultraviolet epifluorescence illumination using a 450- to 490-nm excitation filter.
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. Charcot-Leyden crystals and high fat content are often observed in stool specimens. Polymorphonuclear leukocytes (PMNs) are not observed in fecal specimens.
Studies suggest that real-time polymerase chain reaction (PCR) testing is promising for the detection of I belli in stool samples.[4]
Radiography
A double-contrast barium upper gastrointestinal (GI) series with small-bowel follow-through may be helpful. Nonspecific radiographic findings (eg, prominent mucosal folds, thickening of intestinal wall) may be observed.
The severity of radiographic findings seems to depend on duration of illness and to correlate with the degree of villous atrophy noted on biopsy findings (see Tissue Analysis and Histologic Findings). Short-term disease (< 1 y) seems to result in minimal or irregular thickening of mucosal folds. Long-term disease seems to correlate with markedly granular mucosal appearance with effacement of the folds.
Other Tests
String test
The Entero-Test (swallowed string test to provide a duodenal sample) may yield a positive specimen, if stool study results are negative.
Upper GI endoscopy
Upper GI endoscopy may provide useful specimens for examination, if the following test results are negative:
- Duodenal aspirate for ova and parasite examination
- Small bowel biopsy for histopathology
Electron microscopy
If performed as a part of workup for malabsorption, I belli may be observed with electron microscopy of tissue specimens.
Tissue Analysis and Histologic Findings
A small-bowel biopsy is not a routine test for diagnosis of isosporiasis. Nonspecific findings of isosporiasis observed in small-bowel biopsy specimens include mucosal atrophy, shortened villi, hypertrophic crypts, and lamina propria infiltrated with eosinophils. I belli may be observed in the cytoplasm of enterocytes with electron microscopy.
Histologic findings appear to correlate with the severity of the symptoms observed. Focal-to-widespread mucosal changes are associated with severe symptoms. In mild cases, the only findings may range from flattened villi to mild, nonspecific alterations and increased inflammatory cells in the lamina propria.
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