Cryptosporidiosis laboratory tests
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]
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Overview
Laboratory Findings
Stool Test
Wet mounts
Wet mount examination (with iodine) is used mainly for screening, and is especially useful with specimens containing moderate to high numbers of oocysts. However, it should be combined with a more sensitive confirmatory stain or assay. Fresh or concentrated fecal specimens can be examined, using either conventional bright light, phase contrast or differential interference contrast (or Nomarsky) microscopy.
Shown below is Oocysts of Cryptosporidium parvum, in wet mount, seen with differential interference contrast (DIC) microscopy. The oocysts are rounded, 4.2 to 5.4 µm in diameter. Sporozoites are visible inside the oocysts, indicating that sporulation has occurred. (In comparison, oocysts of Cyclospora cayetanensis, another important coccidian parasite of humans, are twice larger and upon excretion are not sporulated, i.e., do not contain sporocysts.)
Stained smears
Traditional parasitology stains (e.g., Giemsa) are of limited value. They do not differentiate between oocysts and similarly-sized fecal yeasts (the main differential diagnosis of Cryptosporidium in microscopy) and other debris. Modified acid-fast staining technique is a simple and effective method: the oocysts stain bright red against a background of blue-green fecal debris and yeasts. The acid-fast staining technique has been modified and improved, including: hot and cold modified acid-fast stains; incorporation of dimethyl sulfoxide (DMSO); and incorporation of the detergent tergitol.
Image below shows cryptosporidium oocyst in modified acid fast stain.
Immunofluorescence microscopy for detection of oocysts
This method offers increased sensitivity and specificity compared to staining techniques. It has found widespread application in research and clinical laboratories as well as for monitoring oocyst presence in environmental samples. The assays generally work well with fresh or preserved stools (formalin, potassium dichromate), but some fixatives can cause problems (e.g. MIF). Several commercial IFA products are presently available, including MeriFluor™ Cryptosporidium/Giardia (Meridian Diagnostics Inc., Cincinnati, OH, 45244, USA); Detect IF Cryptosporidium (Shield Diagnostics, Ltd., Dundee DD1 1 SW, Scotland, UK); and Crypto IF Kit (TechLab, Blacksburg, VA, 24060, USA). These assays exhibit broad reactivity with C. parvum and other Cryptosporidium species, so they should be applicable to human and veterinary specimens.
Shown below is oocysts of C. parvum (upper left) and cysts of Giardia intestinalis (lower right) labeled with immunofluorescent antibodies.
Several additional methods for microscopic detection of oocysts
- Alternate bright-field stains (e.g., hot safranin-methylene blue stain, modified Kohn’s stain, modified Koster stain, aniline-carbol-methyl violet and tartrazine)
- Negative stains
- Fluorescent stains (including auramine O, auramine-rhodamine, auramine-carbol-fuchsin, acridine orange, mepacrine, and 4’,6-diamidino-2-phenylindole (DAPI) and Propidium iodide staining)
- These exhibit potentially higher sensitivities but, like all nonspecific chemical staining methods, yield false-positives and may leave some oocysts unstained; these methods may be useful for screening samples, but identification should be confirmed with more specific assays (IFA, EIA).
Shown below is oocysts of Cryptosporidium parvum stained with the fluorescent stain auramine-rhodamine.
PCR
Molecular methods (e.g., polymerase chain reaction – PCR) are increasingly used in reference diagnostic labs, since they can be used to identify Cryptosporidium spp. at the species level. Tests for Cryptosporidium are not routinely done in most laboratories; therefore, health care providers should specifically request testing for this parasite.