Dysentery Amoeba

(Entamoeba histolytica), a protozoan of the order Amoebina; the causative agent of amoebic dysentery.

The dysentery amoeba was first described in 1875 by the Russian scientist F. A. Lesh. Upon entering the human intestine the dysentery amoeba usually multiplies within the contents of the large intestine, without taking root in the tissues or disrupting intestinal function. At this point the person is a healthy carrier of the parasite. This stage is called the luminal form of the amoeba, or forma minuta (dimensions, about 20 microns). It moves by means of pseudopodia. The nucleus is spherical, 3-5 microns (μm) in diameter; chromatin is found under the nuclear membrane in small clumps, and there is a small karyosome in the center of the nucleus. There may be a few phagocytized bacteria in the endoplasm. When the fecal matter in the large intestine thickens in consistency, the luminal form becomes surrounded by a membrane and is transformed into a spherical cyst (about 12 μm) with four nuclei that are structurally identical to the nucleus of the vegetative form. Immature cysts contain up to three nuclei. There is a vacuole with glycogen. Some of the cysts also contain short barlike formations, or chromatoid bodies. The cysts are discharged with the feces and may again enter the gastrointestinal tract where, after a stage in which each divides into eight daughter cells, they give rise to luminal forms.

Sometimes the luminal form of the dysentery amoeba embeds itself and multiplies in the wall of the large intestine, forming ulcers (amoebic dysentery). This is called the tissue form (20-25 μm) of the amoeba; in contrast to the luminal form, it has no cytoplasmic inclusions. Ulcerative lesion of the large intestine is accompanied by the discharge of mucus, pus, and blood. Under these conditions, both the luminal and the tissue forms, having entered the lumen of the intestine from the ulcers, grow to 30 μm or more and acquire the ability to phagocytize erythrocytes. This is called the large vegetative form of the dysentery amoeba, or the erythrophage. Discharged during defecation, it quickly dies. With remission of the acute phase of the disease the large vegetative form diminishes in size, turns into the luminal form, and becomes encysted in the intestine. The cysts, discharged in defecation, may become a source of infection. The vegetative form dies when outside the body in 15-20 min. The cysts may remain viable in water or damp soil for more than a month.

In addition to man, the dysentery amoeba parasitizes in rats, monkeys, dogs, and cats (although rarely; these animals have no substantial significance as sources of infection).