Amebiasis is an infection of the large intestine and sometimes the liver and other organs that is caused by the single-celled protozoan parasite Entamoeba histolytica, an ameba. Amebiasis may cause diarrhea, cramping abdominal pain, pain over the liver, and fever.
Amebiasis is relatively common in areas of Africa, the Indian subcontinent, and Latin America where sanitation is poor. In the United States, it is most likely to occur in immigrants and, less commonly, in people who have traveled to developing countries.
Entamoeba histolytica exists in two forms: as an active parasite (trophozoite) and as a dormant parasite (cyst). Infection begins when cysts are swallowed. The cysts hatch, releasing trophozoites that multiply and can cause ulcers in the lining of the intestine. Occasionally, they spread to the liver or other parts of the body. Some trophozoites become cysts, which are excreted in stool (feces) along with trophozoites. Outside the body, the fragile trophozoites die, but the hardy cysts survive.
Cysts can be spread directly from person to person or indirectly through food or water.
In places with poor sanitation, amebiasis is acquired by ingesting food or water that is contaminated with feces. Fruits and vegetables may be contaminated when grown in soil fertilized by human feces, washed in polluted water, or prepared by someone who is infected. Amebiasis may occur and spread in places with adequate sanitation if infected people are incontinent or hygiene is poor (for example, in day care centers or mental institutions). Amebiasis can also be spread through certain sexual practices (such as oral-anal sex).
Many infected people have few or no symptoms. Symptoms that may occur include increased gas (flatulence), cramping abdominal pain, and intermittent diarrhea, constipation, or both. In severe cases, the abdomen is tender when touched, and the stool contains mucus and blood. The person may also have a fever. Diarrhea may lead to dehydration. Wasting of the body (emaciation) and anemia can occur in people with chronic infection. Sometimes a large lump (ameboma) forms and blocks the intestine. Occasionally, trophozoites cause tearing (perforation) of the intestinal wall, resulting in severe abdominal pain and an abdominal infection (peritonitis) that requires immediate medical attention.
In some people, the amebas spread to the liver where they can cause an abscess. Symptoms include fever, sweats, chills, weakness, nausea, vomiting, weight loss, and pain or discomfort in the right upper abdomen over the liver.
Rarely, amebas spread to other organs (including the lungs or brain). The skin may also become infected, especially around the buttocks, genitals, or wounds caused by abdominal surgery or injury.
To diagnose amebiasis, a doctor collects stool samples for analysis. The best approach is to test the stool for a protein released by the amebas (antigen testing). Microscopic examination is often inconclusive. Three to six stool samples may be needed to find the amebas, and even when they are seen, they cannot be distinguished from other amebas such as Entamoeba dispar, which look the same but are genetically different and do not cause disease. A flexible viewing tube (colonoscope) may be used to look inside the large intestine and to obtain a tissue sample if ulcers or other signs of infection are found there.
When amebas spread to sites outside the intestine (such as the liver), they may no longer be present in the stool. Ultrasonography, computed tomography (CT), or magnetic resonance imaging (MRI) can be done to confirm an abscess in the liver, but these tests do not indicate the cause. Blood tests are then done to check for antibodies to the amebas. Or, if doctors suspect that a liver abscess is due to amebas, they often simply start a drug that kills amebas (an amebicide). If the person improves, the diagnosis is probably amebiasis.
If amebiasis is suspected and the person has symptoms, an amebicide—either metronidazole or tinidazole—is used. Tinidazole, given in a single dose, has fewer side effects than metronidazole, which requires several doses. Drinking alcohol within a few days of taking metronidazole or tinidazole may result in nausea, vomiting, flushing, and headaches.
Neither metronidazole nor tinidazole always kills cysts that are in the large intestine. A second drug (such as paromomycin, iodoquinol, or diloxanide) is used to kill these cysts and thus prevent a relapse. One of these drugs can be used alone to treat people who do not have symptoms but have the amebas in their stool.
People who are dehydrated are given fluids.
Last full review/revision March 2007 by Richard D. Pearson, MD