Entamoeba histolytica is a pathogen found throughout the world, especially in areas where sanitation is poor. Cysts infect the person through the fecal-oral route; trophozoites are rarely infective since they die quickly in the environment. A morphologically identical species, E. dispar, which is not a pathogen, cannot be differentiated based solely on a microscopic exam.
After a 1-4 week incubation period, E. histolytica will invade the mucosa of the large intestine, adhere to the mucosa and destroy host cells to produce the characteristic flask-shaped ulcers. It can also produce cysteine proteinases that degrade mucin, and digest hemoglobin and villi.
Symptoms of the intestinal phase include diarrhea, dysentery (which is diarrhea with blood), colitis, and amoebic liver abscesses. It gains entrance to the liver via the portal liver vein. Patient symptoms can vary from mild to severe and be acute or may last for months.
In a small proportion of patients, the organism can become disseminated and travel to other organs. Thoracic amoebiasis is the most common extraintestinal site, followed by pericardial infection. Other organs, including the skin and brain, have been known to be affected.
