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 spread directly from person to person through fecal-oral transmission, including through oral-anal contact, or indirectly 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 medication that kills the amebas, followed by a medication that kills the dormant form (cysts) of the amebas in the large intestine.
(See also Overview of Parasitic Infections.)
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 countries with sanitary food and water supplies, most cases occur among recent immigrants and travelers returning from countries with poor sanitation.
Worldwide each year, about 50 million people develop amebiasis, and as many as 73,000 of them die.
Entamoeba species exist in 2 forms:
A dormant parasite (cyst)
An active parasite (trophozoite)
Other species of ameba do not infect people through the intestine and can directly infect the brain (amebic brain infection) or the eye (amebic keratitis).
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 for weeks to months.
Cysts can be spread directly from person to person through fecal-oral transmission, including through oral-anal contact.
In places with poor sanitation, amebiasis can be acquired indirectly 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 1 to 3 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, ulcers on the penis 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 3 to 6 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 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 medication that kills amebas (an amebicide). If the person improves, the diagnosis is probably amebiasis.
Treatment of Amebiasis
Entamoeba histolytica
People who are dehydrated are given fluids.
Prevention of Amebiasis
To reduce the risk for amebiasis, people should follow food and water precautions, practice good hand hygiene, and avoid fecal exposure during sexual activity. (See Centers for Disease Control and Prevention: Yellow Book: Amebiasis and Food & Water Precautions.)
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.
Research continues on the development of a vaccine, but none is available yet.
More Information
The following English-language resources may be useful. Please note that THE MANUAL is not responsible for the content of these resources.
Centers for Disease Control and Prevention (CDC): Yellow Book: Amebiasis
CDC: Yellow Book: Food & Water Precautions
CDC: Parasites - Amebiasis: General Information
