whether the object actually passed through the intestinal tract or entered the stool after passage (Fig. 10.14).
FIGURE 10.14 (Top) Slug recovered in stool; note the linear markings on the body. (Second) Underside of the slug. (Third) Fresh, living trematode; note the resemblance to the slug. (Bottom) Actual slug; note the markings on the body (this slug transmits Angiostrongylus). (Top and second, courtesy of Larry D. Gray, Clinical Microbiology Laboratory Consultants, LLC; bottom two images courtesy of the CDC Public Health Image Library.) doi:10.1128/9781555819002.ch10.f14
Plant cells tend to have thick, smooth walls and are not as symmetrical as helminth eggs (Fig. 10.15 and 10.16). Some of these plant cells range from 15 to 150 µm in diameter and may be confused with Ascaris eggs. Pollen grains are also thick-walled, symmetrical structures that stain very darkly with iodine, may be round or trilobed, and are 15 to 20 µm in diameter (Fig. 10.17). They may resemble Taenia eggs. Another example is the “Beaver body.” This structure is Psorospermium haeckelii, a stage of an alga that occurs in the tissues of crayfish. It is sometimes confused with helminth eggs when found in fecal specimens from individuals who have ingested crayfish (9) (Fig. 10.18). Accidental ingestion or contamination of food or water containing the eggs of plant nematodes, such as Heterodera species or mites or mite eggs, can also lead to confusing situations.
FIGURE 10.15 Various artifacts that may be seen in stool preparations (wet mounts or permanent stained smears). Many of these structures are pollen grains or egglike objects. Visually, they can be confused with some of the following helminth eggs: Hymenolepis nana, Ascaris lumbricoides, hookworm, and Enterobius vermicularis. doi:10.1128/9781555819002.ch10.f15
FIGURE 10.16 Various artifacts that may be seen in stool preparations (wet mounts). Note the egg-like structure in the fourth row (left). There is a small bubble (within the circle) that mimics the small knob found at the abopercular end of a Diphyllobothrium latum egg. doi:10.1128/9781555819002.ch10.f16
FIGURE 10.17 Various types of pollen grains and a root hair. These structures can mimic various helminth eggs (Ascaris lumbricoides, Trichuris trichiura), as well as nematode larvae. (Bottom row, left) Plant root hair (can mimic helminth larvae). (Row 4 from top: left, courtesy of Randy Oliver, photo by Gretchen D. Jones, United States Department of Agriculture; right, courtesy of http://cactiguide.com/forum/viewtopic.php?t=12480&view=next [accessed 7/23/13]). doi:10.1128/9781555819002.ch10.f17
Figure 10.18 (Upper) “Beaver bodies,” which are algae occasionally found in stool; (lower) leaf structure which resembles a trematode. (Courtesy of Joseph Dipersio.) doi:10.1128/9781555819002.ch10.f18
The human cells most likely to cause problems with identification are the polymorphonuclear leukocytes (PMNs) and the macrophages seen in stained fecal smears (Table 10.1; Fig. 10.19). These cells are frequently present in patients with nonspecific inflammatory bowel disease (ulcerative colitis). Therapy for this condition often includes immunosuppressive agents; this therapy would definitely be contraindicated in patients with amebiasis (2). Therefore, the correct identification between human cells and pathogenic protozoa is critical to good patient care. These cells should be reported and quantitated (rare, few, moderate, many). PMNs are frequently misidentified as a protozoan cysts, usually in the genus Entamoeba; as the PMNs begin to age, the lobed nucleus fragments into several parts, each resembling an amebic cyst nucleus. However, usually PMNs are seen in patients who tend to be symptomatic; patients with diarrhea do not tend to have protozoan cysts in the stool. The gut motility is so rapid that cyst formation does not occur; also, trophozoites do not encyst once they are outside of the body/gastrointestinal tract. The macrophage tends to have one large nucleus; the overall appearance can mimic the Entamoeba trophozoite.
Figure 10.19 (Left) PMNs in a fecal specimen stained with trichrome stain. Note the lobed nuclei; if these cells have been in the stool for some time (unpreserved), the nuclei may fragment into four or five pieces, thus resembling multiple nuclei seen in amebic cysts. Note the two RBCs in the upper left. (Middle) Macrophages. Although these cells often resemble amebic trophozoites, the ratio of nuclear material to cytoplasm is quite different from that seen in actual protozoa (more nuclear material per cytoplasm in human cells). (Right) PMNs and macrophages; also note the eosinophil (arrow). doi:10.1128/9781555819002.ch10.f19
Large numbers of PMNs are often found in patients with bacterial dysentery, and they may also be present in patients with intestinal amebiasis or ulcerative colitis (Fig. 10.19). These cells may be distinguished from Entamoeba histolytica as follows.
PMNs
1. Average size, 14 µm (10 to 12 µm on permanent stained smear)
2. Ratio of nuclear material to cytoplasm, 1:1
3. Nucleus: 2 to 4 segments connected by narrow, short chromatin bands. Segments may appear as separate nuclei like those of E. histolytica/E. dispar cysts—focus carefully to reveal connecting chromatin strands.
4. Granular cytoplasm
5. Trichrome staining characteristics similar to E. histolytica/E. dispar
E. histolytica/E. dispar (cysts)
1. Average size, 20 µm (less on permanent stained smear)
2. Ratio of nuclear material to cytoplasm, 1:10 to 1:12 (trophozoite), 1:2 to 1:3 (cyst)
3. Nucleus: round with central karyosome and peripheral chromatin
4. Uniform, agranular cytoplasm—may contain RBCs
5. Trichrome: green cytoplasm, dark red nuclear material
The identification of eosinophils in a fecal specimen usually indicates the presence of an immune response in the host. This allergic response
