Blood and body fluids
Malaria parasites and Babesia spp.
Leishmaniae and trypanosomes
Microfilariae
Body fluids: ciliated epithelial cells
Helminths
Adult worms and larvae
Eggs
Human cells
Polymorphonuclear leukocytes
Eosinophils
Macrophages
Lymphocytes
Red blood cells
Charcot-Leyden crystals
Nonhuman elements seen in feces (yeast cells)
Insect larvae
Spurious infections
Delusory parasitosis (delusionary infestation)
Although many body sites and specimens can be examined for the presence of parasites, the most difficult specimen in which to differentiate parasites from artifacts is usually fecal material. Feces consist of a number of components, including (i) undigested food residue; (ii) digestive by-products; (iii) epithelial and other human cells, mucus, and other secretions from the digestive tract; and (iv) many types of microorganisms such as bacteria and yeasts. Considering the ratio between fecal debris and parasites, it is not surprising that many artifacts are responsible for incorrect identifications of protozoan trophozoites and cysts and of helminth eggs and larvae. Often, many yeast cells and other artifacts are confused with coccidian oocysts or microsporidial spores. Appropriate training, adherence to protocols, use of quality control measures, and availability of reference materials and consultants should help minimize identification errors.
A number of cells and other organisms can easily be confused with intestinal protozoa. These are listed in Tables 10.1 and 10.2 and illustrated in Fig. 10.1.
Occasionally, free-living amebae are found in feces or as contaminants in water. Morphologically, they differ from parasitic amebae in having one or more large contractile vacuoles in the trophozoite form and having very thick cyst walls. They can also be differentiated on the basis of cultivation; i.e., they are much easier to culture than the pathogenic protozoa. Amebae which have been recovered from stool material include Entamoeba moshkovskii, Naegleria gruberi, Hartmanella hyalina, Sappinia diploidea, Vahlkampfia punctata, and Vahlkampfia lobospinosa. Contamination of specimens can be avoided by using dry collection containers, saline, or formalin for the concentration rinses and dilution of the specimen and by rapid fixation or examination of the specimen immediately after passage (1).
Some protozoa, such as Entamoeba coli, may contain fungi. Sphaerita spp. can be found in the cytoplasm, and Nucleophaga spp. can be found in the nucleus. Sphaerita spp. (sometimes called Polyphaga spp.) measure approximately 0.5 to 1.0 µm and are found in tightly packed clusters (Fig. 10.2) (2).
FIGURE 10.1 Various structures that may be seen in stool preparations. (Top row) Macrophage (left) and epithelial cells (right) that can be confused with Entamoeba histolytica/E. dispar trophozoites. (Second row) PMN with a fragmented nucleus (left) and artifact (right) that can be confused with Entamoeba spp. cysts. (Third row) Two artifacts that can resemble protozoan cysts. (Fourth row) Yeast cells (left) and an artifact (right) that can be confused with Cryptosporidium spp. and Cyclospora cayetanensis, respectively, on positive acid-fast stains; it is important to measure the structures and organisms carefully before confirming organism identification. (Bottom row) Yeast cells (left) that can be confused with microsporidial spores (however, notice the budding cell within the circle), and artifacts (right) that can also be confused with microsporidial spores; these were thought to be small yeast cells. doi:10.1128/9781555819002.ch10.f1
Figure 10.2 Fungi parasitizing Entamoeba coli. (Upper) Entamoeba coli containing ingested Sphaerita. (Lower) (A) Sphaerita (or Polyphaga) sp. within the cytoplasm; (B) Nucleophaga sp. within the nucleus. doi:10.1128/9781555819002.ch10.f2
Flagellates can be difficult to differentiate, and free-living organisms are occasionally seen in a stool specimen that has been contaminated with water or saline containing Bodo caudatus and Cercomonas longicauda. These two organisms are classified in the same family as Retortamonas intestinalis (Fig. 10.3).
Figure 10.3 Free-living freshwater flagellates. (Upper) Bodo caudatus. (Lower) Cercomonas longicauda. These organisms can be confused with parasitic protozoa, particularly if motility is seen in a wet mount of fresh stool contaminated with freshwater. doi:10.1128/9781555819002.ch10.f3
Apparently free-living ciliates are found in stagnant water, sewage, and soil and may be seen in fecal specimens contaminated with water or saline. Organisms that have been reported are Litonotus and Paramecium (1–3) (Fig. 10.4).
