Non-pathogenic intestinal protozoa are single-celled parasites commonly found in the intestinal tract but never associated with illness. They do not harm the body, even in people with weak immune systems. There are two stages in the lifecycle, a motile vegetative form (trophozoite) which reside in the small intestine and is responsible for disease manifestations and an infective resistant form (cyst) responsible for transmission. Cysts are oval-shaped thin-walled cyst that is 10 to 20 μm in length, 7-10 μm in width and 0.3-0.5 μm thickness. Trophozoites resemble a pear or teardrop and measure 9-21 μm in length, 5-15 μm in width and 1-2 μm thickness. The trophozoite is an aerotolerant anaerobe that requires glucose as a source of carbohydrate energy and divides by longitudinal binary fission every 9 to 12 hours.
Symptoms take from a few days to a few weeks to develop and manifest themselves, but usually it is about two to four weeks. Symptoms can range from mild diarrhea to severe dysentery with blood and mucus. In about 10% of invasive cases the amoebae enter the bloodstream and may travel to other organs in the body. Most commonly this means the liver, as this is where blood from the intestine reaches first, but they can end up almost anywhere in the body. Infection with single or multiple species of intestinal protozoa is common in humans and can result in either asymptomatic colonization or symptoms of intestinal disease. The key to diagnosis and treatment of amebiasis is knowledge of the epidemiologic risk factors and clinical manifestations, a rational approach to diagnosis, and an understanding of the sites of action and uses of anti-amebic drugs