Entamoeba histolytica are amoebic parasites that are very harmful to humans and cause amoebic dissentery. They can be distinguished from other similar but harmless amoebic parasites, such as Entamoeba coli, Entamoeba dispar, and Entamoeba hartmanni, by the presence of four nuclei in the cyst stage, although the presence of four nuclei may also be an immature E. coli cell. They feed on the intestinal cells of infected humans.
Entamoeba histolytica have three stages of pathology. The first stage involves the parasite in the trophozoite stage feeding on host cells. There is minimal pathology to the host as the parasite benefits. The second stage involves flask legions on the intestinal wall. These legions cause severe pathology to the host as the host immune system launches an attack against the parasite. The parasite is harmed and has reduced reproductive success. The third stage is when the parasite enters circulation and travels to the brain and other organs. This is lethal to the host, and the parasite reaches a dead end and is not able to be transmitted further.
E. histolytica enters humans through the ingestion of infected fecies. Following ingestion, the parasite forms a trophozoite, travels to the intestine, and replicates by binary fission. Once inside the intestine, E. histolytica creates pockets of amoebas. The immune response of the human host leads to a pool of blood, allowing the parasite to enter the circulatory system and travel to the brain and other organs, causing death. If released in human feces while still a trophozoite, the parasite disintegrates after reaching the external environment. However, if the parasite encysts prior to its release into the external environment, the feces remain infected, and can infect future individuals by ingestion.