Taxonomy: f. Entamoebidae
Animal: Entamoeba histolytica 1 18.jpg
Entamoeba histolytica trophozoite containing ingested red cells which is diagnostic. Trophozoite is 12 - 60 microns (invasive forms greater than 20). Single nucleus with fine peripheral chromatin and small compact karyosome in faecal smear stained with Trichrome. Leucocytes can be mistaken for E. histolytica in a faecal stained smear. However, the nuclear:cytoplasmic ratio is 1:1 cf. 1:10-1:12 for trophozoites and 1:2 -1:3 for cysts, although the staining characteristics is the same. Leucocytes are 2-4 microns smaller in smears cf in thin blood films. In addition, the cytoplasm of leucocytes is granular cf the agranular, uniform appearance of amoeba which may contain red cells. Nuclei of macrophages if visible, do not have a karyosome but a fine network of chromatin and the nuclear:cytoplasmic ratio is 1:4 - 1:6.
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