Manual of basic techniques for a health laboratory - libdoc.who.int

118 Manual of basic techniques for a health laboratoryThe eosin stain provides a pink background against which unstained trophozoitesand cysts are clearly visible.Note: If 1% eosin solution is not available, use a drop of Field stain B (see above).4.3.2 Identification of cystsCysts are the resistant forms of certain intestinal amoebae, flagellates and ciliates.They are small, round and non-motile and may have one or several nuclei.Measurement of cysts is useful for the correct identification of species.Importance of cystsThe clinical importance of cysts varies from country to country. The cyst is theinfective form of the organism. Healthy persons may be asymptomatic carriers ofcysts and are, therefore, a public health hazard.The most important problem in the laboratory is the precise identification of cystsof Entamoeba histolytica, Giardia intestinalis and Balantidium coli. Some of the featuresused in the identification of these cysts and those of other intestinal protozoa areillustrated in Fig. 4.24.Identification of cysts of amoebaeEntamoeba histolytica (Fig. 4.25)Size: 12–15mm (1–2 erythrocytes).Shape: round.Nuclei: 1–4 nuclei:membrane — thin, regular, circularkaryosome — small, compact, central (like a black dot).Cytoplasm: yellowish-grey after staining with iodine solution, granular; “dirty”appearance.Chromatoid bodies: oblong, rounded at ends (sausage-shaped); not found in all cysts.Vacuole: sometimes a large glycogen vacuole (stained reddish-brown by iodine solution)in young cysts with one or two nuclei.E. histolytica may cause dysentery. Identification of cysts of other amoebae that donot cause disease may be difficult. The main thing is to differentiate between themand the cysts of E. histolytica.Entamoeba coli (Fig. 4.26)Size: 12–20mm (1–2 erythrocytes; slightly larger than the cyst of E. histolytica).Shape: round or slightly oval, sometimes irregular.Nuclei: 1–8 nuclei:membrane — irregular, thick in parts, not a perfect circlekaryosome — large, diffuse, often eccentric.Cytoplasm: pale yellow after staining with iodine solution, bright (as compared withE. histolytica).Chromatoid bodies: sharp or jagged ends (dagger-shaped or needle-shaped); not foundin all cysts.Vacuole: sometimes a very large vacuole (stained reddish-brown by iodine solution)compressing two nuclei, one at either pole.