Feces normally consist of food residues, various digestion products, sloughed epithelial cells and microorganisms such as bacteria and yeast.Given the heterogeneity of this material, it is not surprising that some fecal objects are mistaken, by inexperienced microscopists, as protozoan trophozoites or their cysts and as helminthic larvae or eggs.

One should never identify an object found in fresh, unfixed feces as an amoebic trophozoite even if it is seen moving, since macrophages exhibit motility similar to that of amoebae. Moreover, when active trophozoites are found in a preparation, one should make certain that they were not introduced into the sample via the saline solution (contaminated with free-living amoebae) in which the feces were diluted. To guard against this possibility, saline solution should be frequently renewed and sterilized by boiling. Neglect of this precaution is a frequent source of error.

Watery or dysenteric stools present the greatest difficulties, on account of the numerous cellular elements from intestinal tissues, especially because these cellular elements are usually in an advanced state of degradation. Dead, degraded and motionless specimens often cannot be identified with certainty even by an expert microscopist.

Polymorphonuclear (PMN) leucocytes are often found in bacterial dysentery and in cases of intestinal amebiasis and ulcerative colitis, owing to their small size (mean diameter, 14 Ám) can be mistaken for cysts of Entamoeba histolytica.As seen with temporary stains or, better, with permanent staining, PMN nuclei are irregular in shape and they are segmented and connected by chromatin strands, sometimes appearing as separate nuclei like those of Entamoeba histolytica cyst; focus carefully to reveal connecting chromatin strands.

When a microscopist thinks that certain objects in the specimen are protozoan trophozoites or their cysts, but these objects do not exactly match the morphological descriptions and images provided by manuals and atlases, then he is probably mistaken.These objects are more likely artifacts or pseudoparasites. During wet mount examination, the correct morphology (shape and size) of trophozoites and cysts can usually be seen after tapping the coverslip and causing the cell to turn edgewise.

The microscopist must always be able to accurately measure objects and other structures encountered during microscopic examination, using an ocular micrometer. The size of organisms is one of the major criteria in the differential diagnosis of parasite species.The simple procedure of determining size often helps avoid confusion between true and false parasites that may have similar morphology but different sizes.


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