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Artifacts and Confounders
Introduction and Summary
Cysts and trophozoites must be examined carefully in different fields of view and measurement is often essential. Objects such as epithelial cells and macrophages are around the same size as amoebic trophozoites: the latter may also move and contain red blood cells.
White blood cells, plant and vegetable cells, fat globules, muscle fibers, pollen grains, yeasts cells and air bubbles may be confused with cysts or eggs. Air bubbles trapped under adhesive tape often resemble Enterobius eggs.
Plant hairs and fibers are easily confused with larvae; algae such as Psorospermium haeckelii may be found in the feces of patients who have eaten crayfish.
Earthworms may resemble roundworms.
A variety of non-pathogenic ova, cysts and parasites resemble pathogens in terms of size and morphology and careful examination is essential. Eggs of Heterodera, a parasitic nematode of root vegetables, may resemble hookworm eggs. Eggs originating from harmless mites in cereals or flour could be confused with hookworm ova but are usually larger. We recently encountered 160 micron "Schistosome ova" in the urine of a patient complaining of hematuria: we suspected Schistosoma haematobium but, on closer analysis, the eggs contained unidentified insects. This volume provides examples of artifacts that may be confused for parasitic life stages. Artifacts should be considered on the basis of size, shape, lack of organelles and defining feature, and variable reactivity with common stains.
Red and White Blood Cells
Red blood cells and a variety of white blood cells can be easily mistaken for parasitic cells or cysts when observed with microscopy.
White Blood Cells
Charcot-Leyden crystals are a product of eosinophil breakdown and are, therefore, occasionally found in feces of patients suffering from parasitic disease. They appear red when stained with in a trichrome fecal preparation.
Fat globules present in a fecal slide preparation may appear similar to parasitic cysts or cell bodies.
Emulsifying agents are a useful tool to eliminate potential confusion involving fat globules. The removal of such particles from slide preparations will undoubtedly reduce cases of misdiagnosis.
Yeast may resemble protozoan cysts because they are uniform in color, have few inclusions and no nucleus. Yeast could also be confused with small protozoans like E. nana or with Cryptosporidium or Cyclospora oocysts in wet preparation. In acid-fast stains, the oocysts of Cryptosporidium and Cyclospora species stain pink to red. Yeasts are not acid fast and stain green.
Plant cells and associated elements seen in feces may resemble eggs, cysts or cell bodies. Plant cells are often identified by a more irregular outer membrane.
Pollen grains are often misinterpreted as parasite eggs, but can often be discerned through size and the presence or absence of important structural elements.
Animal and plant hairs are most often and easily mistaken for parasitic nematode worms. Their size and shape may be comparable in many cases, but a lack of internal definition will identify the artifact when compared to the worm. Although nematodes are non-segmented and externally simple organisms, they will often show unique structural characteristics under close examination.
Image 13-21. Image illustrating Heterodera spp. in slide preparation. Such parasitic nematodes attack root vegetables such as beetroot, turnips and radishes. Their eggs are 80-120mm by 25-40 mm and can conceivably be confused with hookworm eggs. (SOURCE: Unknown)
Image 13-22. Image illustrating an Annelid earthworm in detritus. They belong to the Annelida (Lumbricus and Allolobophora) and are elongated, segmented and circular in section and are occasionally confused with Ascaris. They have a purple-brown dorsal surface and a paler ventral surface, swell out at around segment 12 and possess a marked thickening (the clitellum) a third of the way from the anterior. (SOURCE: Unknown)