Merck Manual

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Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Oct 2022 | Modified Nov 2022

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.

  • The amebas may be spread from person to person or through food or water.

  • People may have no symptoms or may have diarrhea, constipation, cramping abdominal pain, tenderness in the upper abdomen, and fever.

  • Doctors base the diagnosis on analysis of a stool sample and, if needed, other tests, such as colonoscopy or ultrasonography and blood tests.

  • People are given a drug that kills the amebas, followed by a drug that kills the dormant form (cysts) of the amebas in the large intestine.

Amebiasis tends to occur in areas where sanitation is inadequate. The parasite is present worldwide, but most infections occur in areas of Africa, the Indian subcontinent, and parts of Central and South America. In the United States, it is most likely to occur in immigrants and, less commonly, in people who have traveled to countries with poor sanitation.

Worldwide each year, about 50 million people develop amebiasis, and as many as 100,000 of them die.

Entamoeba species exist in two forms:

  • An active parasite (trophozoite)

  • A dormant parasite (cyst)

Transmission of amebiasis

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. However, the hardy cysts can survive.

Cysts can be spread directly from person to person or indirectly through food or water. Amebiasis can also be spread through oral-anal sex.

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 psychiatric institutions).

Symptoms of Amebiasis

The majority of infected people have few or no symptoms. However, they excrete cysts in stool and can thus spread the infection.

Amebiasis symptoms typically develop over one to three weeks and may include

  • Diarrhea, sometimes with blood visible in the stool

  • Cramping abdominal pain

  • Weight loss and fever

In severe cases, the abdomen is tender when touched, and people may develop severe diarrhea with stools that contain mucus and blood (called dysentery). Some people have severe, crampy abdominal pain and a high fever. Diarrhea may lead to dehydration. Wasting of the body (emaciation) and anemia can occur in people with chronic infection.

Sometimes large lumps (amebomas) may form inside the large intestine (colon).

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 upper right part of the 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 (infection that has spread from contaminated stool), genitals (for example, penis ulcers from anal intercourse with an infected person), or wounds caused by abdominal surgery or injury.

Diagnosis of Amebiasis

  • Stool tests

  • Sometimes blood tests to detect antibodies to the amebas

  • Sometimes examination of a tissue sample from the large intestine

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) or to use the polymerase chain reaction (PCR) technique to check the stool for the ameba's genetic material. The PCR technique produces many copies of the ameba's genetic material and thus makes the ameba easier to identify. Antigen or PCR tests are more useful than microscopic examination of stool samples, which is often inconclusive. Also, microscopic examination may require three to six stool samples to find the amebas, and even when they are seen, Entamoeba histolytica cannot be distinguished from some other related amebas. For example, Entamoeba dispar, which looks the same but is genetically different, does not cause disease.

A flexible viewing tube (endoscope) may be used to look inside the large intestine. If ulcers or other signs of infection are found there, the endoscope is used to obtain a sample of fluid or tissue from the abnormal area.

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. (Antibodies Antibodies One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more Antibodies are proteins produced by the immune system to help defend the body against a particular attack, including that by parasites.) Or, if doctors suspect that a liver abscess is due to amebas, they may start a drug that kills amebas (an amebicide). If the person improves, the diagnosis is probably amebiasis.

Prevention of Amebiasis

Preventing food and water from being contaminated with human feces is key to preventing amebiasis. Improving sanitation systems in areas where the infection is common can help.

When traveling to areas where the infection is common, people should avoid eating uncooked foods, including salads and vegetables, and should avoid consuming potentially contaminated water and ice. Boiling water kills cysts. Hand washing with soap and water is important. Filtering water through a 0.1 or 0.4 micron filter can remove Entamoeba histolytica and other parasites, as well as bacteria that cause diseases. Dissolving iodine or chlorine in the water may help. However, the effectiveness of iodine or chlorine against Entamoeba histolytica depends on many factors, such as on how cloudy or muddy the water is (turbidity) and what its temperature is.

Treatment of Amebiasis

  • An amebicide and/or a drug to kill cysts

If amebiasis is suspected and the person has symptoms, an amebicide (a drug that kills amebas)—either metronidazole or tinidazole—is used. Metronidazole must be taken for 7 to 10 days. Tinidazole must be taken for 3 to 5 days. Tinidazole has fewer side effects than metronidazole. People should not drink alcohol while taking either of these drugs or for several days after stopping them because doing so may result in nausea, vomiting, flushing, and headaches. Nitazoxanide has been proposed as an alternative for treating amebiasis. Metronidazole, tinidazole, or nitazoxanide is given to pregnant women only if the benefits outweigh the risks.

Neither metronidazole, tinidazole, nor nitazoxanide kills all cysts that are in the large intestine. A second drug (such as paromomycin, diiodohydroxyquin, or diloxanide furoate) is used to kill these cysts and thus prevent a relapse. One of these drugs can be used alone to treat people who are not sick but have Entamoeba histolytica in their stool.

People who are dehydrated are given fluids.

Select Medical Literature

Drugs Mentioned In This Article

Generic Name Select Brand Names
Flagyl, Flagyl ER, Flagyl RTU, LIKMEZ, MetroCream, MetroGel, MetroGel Vaginal, MetroLotion, Noritate, NUVESSA, Nydamax, Rosadan, Rozex, Vandazole, Vitazol
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