Intestinal amoebiasis, diagnosis

When amoebic dysentery is suspected, a fresh faecal sample or a swab from a rectal ulcer should be examined under a microscope. If examined quickly (a fresh stool, still warm) the colourless motile trophozoites can be seen. Motility disappears when cooled, and the parasites are then difficult to recognise. They should be differentiated from actively motile macrophages. The trophozoite (motile form) has one nucleus. When colourless this nucleus is scarcely if at all visible. Once stained the nucleus is moderately visible. Lugol staining kills the parasite almost immediately (motility disappears). Stained Entamoeba histolytica trophozoites have a transparent outer border (ectoplasm) and an opaque inner border (endoplasm). The border between endoplasm and ectoplasm is not distinct in Entamoeba coli. The trophozoite measures 20 to 40 m and may contain red blood cells (unlike other amoebae). The last detail is pathognomonic for pathogenic Entamoeba histolytica, but is not always present. Ribosomes can be arranged in characteristically shaped elongate bars with rounded ends (=chromatoid bodies).

The cysts have 1, 2 or 4 nuclei and measure 8 to 15-20 m. The nuclei are best revealed by means of an iodine stain. They have a dark circumference and a dark central point (karyosome). The karyosome of Entamoeba coli is not centrally located, but eccentric. Iodine staining can also detect glycogen (brown) in young cysts. Fresh cysts of Entamoeba histolytica also contain what are called chromatoid bodies. These are squat, oval inclusions which can easily be detected (black) with an iron-haematoxylin stain (not with iodine stain). They are not present in Entamoeba coli or Endolimax nana cysts. In active dysentery, often no cysts are found in the faeces, but if there is little diarrhoea, the parasites have time to encyst. Since excretion of the parasites is intermittent, it is best to carry out 3 different stool analyses before deciding upon a negative result. Sometimes it is easier to reveal the parasites in a stool obtained by means of a purgative.

Tests for tracing Entamoeba histolytica antigen in the faeces have been developed, but need to be further evaluated. They may permit swift differentiation between Entamoeba histolytica and Entamoeba dispar.

Category: Medicine Notes