caused by a parasitic infection or by other antigens such as pollen or food. Eosinophils are essentially the same size as PMNs and are characterized by the presence of large, purple-staining granules (trichrome) and usually a bilobed nucleus, which may be obscured by the granules (Fig. 10.19). Eosinophils can also be seen in other specimens such as blood specimens and sputum specimens from asthmatic patients (10, 11). When reviewing blood smears for parasites, it is helpful to recognize the typical white blood cells as seen in Fig. 10.20. Unfortunately, not all microbiologists have had training in hematology per blood cell recognition, although in fecal specimens the white blood cells do not tend to have the precise morphology as that seen in routine blood films.
Figure 10.20 White blood cells in a stained blood film. (1) Lymphocyte. (2) Basophil. (3) Eosinophil. (4) Monocyte. (5) PMN. doi:10.1128/9781555819002.ch10.f20
Macrophages (monocytes) are large, mononuclear, phagocytic cells that may resemble E. histolytica/E. dispar trophozoites (Fig. 10.19). These cells may be found in patients with intestinal amebiasis and ulcerative colitis and other inflammatory bowel diseases and can be differentiated from amebae as follows. One of the main differences is the ratio of nuclear material to cytoplasm, which is much larger in human cells.
Macrophages
1. Size: 30 to 60 µm, may be 5 to 10 µm less on permanent stained smear
2. Ratio of nuclear material to cytoplasm, 1:4–1:6
3. One large nucleus that may be irregular in shape (like monocyte nucleus)
4. Usually contains ingested debris, PMNs, and RBCs
5. May contain red-staining round bodies and nucleus may be absent
6. Trichrome staining characteristics similar to E. histolytica/E. dispar
E. histolytica/E. dispar (trophozoites)
1. Size: 12 to 60 µm; average, 20 µm (less on permanent stained smear)
2. Ratio of nuclear material to cytoplasm, 1:10 to 1:12
3. One nucleus, round, with central karyosome and peripheral chromatin
4. May contain RBCs and some debris; no PMNs
5. Nucleus always present
6. Trichrome: green cytoplasm, dark red nuclear material
Lymphocytes have a large, dense, dark-staining nucleus surrounded by very little cytoplasm. They are approximately two-thirds the size of PMNs (Fig. 10.20).
In a buffy coat preparation, some RBCs may be present. These cells measure ∼7.5 µm in a wet preparation and may be present in the stool as an indication of ulceration (parasitic or nonspecific) or other vascular or hemorrhagic problems. In the trichrome-stained slide, they appear as round or elongate (distortion may occur during smear preparation) red-purple bodies with no granules or inclusions and may be somewhat smaller than 7.5 µm (Fig. 10.19 and 10.20).
Charcot-Leyden (CL) crystals are formed from the breakdown products of eosinophils and basophils and may be present in the stool or sputum with eosinophils or alone. They are slender crystals with pointed ends, and they stain red-purple with trichrome stain. Many different crystal sizes can be seen in the same specimen (Fig. 10.21). They indicate that an immune response has taken place, but the cause may or may not be parasitic. The presence of eosinophils and/or CL crystals in the stool may not correlate with an increased eosinophilia on the peripheral blood smear.
Figure 10.21 Charcot-Leyden (CL) crystals. These crystals are formed from the breakdown products of eosinophils and basophils and may be present in the stool, sputum, or other specimens with or without eosinophils. They tend to stain red to red-purple on the permanent stained fecal smears, often darker than nuclear material, and although the shape is consistent, there is a large size range in a single fecal smear or sputum mount. (Upper, left and right) CL crystals in trichrome-stained fecal smear. (Lower, left) CL crystals in a sputum specimen. (Lower, right) Pineapple crystals in stool. These may be confused with CL crystals; however, the pineapple crystals are much more slender. doi:10.1128/9781555819002.ch10.f21
Identification of CL crystals in body fluids and secretions is considered an indicator of eosinophil-associated allergic inflammation. The overall structural fold of CL crystal protein is similar to that of galectins, and this is the first structure of an eosinophil protein to be determined (5). The protein exhibits weak carbohydrate binding activity for simple saccharides. There may be a potential intracellular and/or extracellular role(s) for the galectin-associated activities of CL crystal protein in eosinophil and basophil function in allergic diseases and inflammation (10).
Parasitic (primarily helminthic) infections push the immune system towards TH2 cytokine production and eosinophilia. Since eosinophil infiltration in infected organs and skin is a common finding, eosinophils are thought to have a specific role in parasite killing. Eosinophils have been considered as effectors of adaptive immune responses during parasitic infections and inflammatory processes. Their role in allergic and mucosal responses is mediated by membrane receptors that allow them to interact with IgE and IgA antibodies (10).
It has been reported that pineapple crystals sometimes mimic CL crystals in stool specimens. Apparently, these crystals can be found in fresh and canned pineapple and pineapple juice; also, they are not digested in the alimentary canal of humans. They range from 30 to 130 µm in length and 1 to 2 µm in width (Fig. 10.21). They appear to have parallel edges and are pointed at both ends. In wet preparations, they can resemble CL crystals. Those who routinely examine stool specimens for parasites should probably examine some pineapple juice under the microscope to become familiar
