Quick Facts
Amebiasis is a parasitic disease caused by a protozoan parasite Entamoeba histolytica. The most common manifestations of E. histolytica infection are dysentery and liver abscesses. E. histolytica usually resides in the large intestine, and can persist there for months or years and cause an asymptomatic intestinal infection. However, clinical amebiasis occurs when the parasite penetrates the colon wall, causing ulcerative colitis, or disseminates into other organs, most commonly the liver, leading to abscess formation. Current therapy for amebiasis relies on metronidazole, which was first introduced in 1959. Potential resistance of E. histolytica to metronidazole is an increasing concern and clinical failures have been linked to other metronidazole-resistant protozoa. Standard treatment with metronidazole requires three doses per day for 10 days and is accompanied by frequent side effects. In addition, metronidazole has no effect on intraluminal carriage, so follow-up treatment with a second, luminal active drug such as paromomycin is recommended to prevent prolonged carriage and excretion. Therefore, a shorter, less toxic, and more effective therapy is needed.
What is it?
E. histolytica has a simple life cycle that usually consists of an infective cyst stage and a multiplying trophozoite stage. Transmission of the infection occurs via ingestion of cysts in faecally contaminated food, water, or hands. Once ingested, excystment occurs in the small bowel; the resulting trophozoites migrate to the large intestine where they multiply by binary fission, and form new cysts, thus completing the replication cycle. In many cases, the trophozoites remain confined to the intestinal lumen of individuals who are asymptomatic carriers, passing cysts in their stool. In some patients the trophozoites invade the intestinal mucosa, or through the bloodstream, extraintestinal sites such as the liver, brain, and lungs, with resultant pathologic manifestations.
Amebiasis Life Cycle
Entamoeba histolytica
There is no insect vector: the parasite passes from person to person through food, water or hands contaminated with feces. Stages 1 and 2 happen outside the body and stages 3–5 in the intestine. After stage 5, the cycle begins again at stage 1.
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Stage 1: Cysts and trophozoites are passed in feces
Infective stage Diagnostic stage
Leaving the body
Cysts can survive for days to weeks outside the body, protected by their cell walls, and remain infectious. Trophozoites are destroyed quickly once outside.
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Stage 2: Mature cysts are swallowed
Infective stage
Contaminated food, water or hands
A new host becomes infected by swallowing mature cysts in food, water or on hands contaminated with feces.
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Stage 3: Cysts release trophozoites
In the small intestine
The cysts break open (excystation), releasing trophozoites.
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Stage 4: Trophozoites multiply
Diagnostic stage
In the large intestine
Trophozoites move to the large intestine and multiply by binary fission. Here the infection can take one of three courses (A–C below).
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Stage 5: Trophozoites form new cysts
Diagnostic stage
In the large intestine
Some trophozoites turn into cysts (encystation), which leave the body in feces together with trophozoites.
Cycle continues at stage 1
What trophozoites can do in the large intestine
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Outcome A: Noninvasive colonization
Trophozoites stay inside the intestine. The person has no symptoms but passes cysts in their stool (an asymptomatic carrier).
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Outcome B: Intestinal disease
Trophozoites invade the lining of the intestine, causing ulcers and dysentery.
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Outcome C: Extraintestinal disease
Trophozoites travel through the bloodstream to other organs, most often the liver (liver abscess), and also the lungs or brain.
All three are diagnostic: the parasite can be found in stool, tissue or other samples at each.
Key
- Blue headers and arrows: stages inside the human intestine
- Green headers and arrows: stages outside the body, when the parasite passes to a new person
Adapted from the CDC DPDx amebiasis life cycle (public domain).
Where does it occur?
Amebiasis occurs worldwide, but it is most common in tropical areas with crowded living conditions and poor sanitation. Africa, Mexico, parts of South America, and India have significant health problems associated with this disease. The World Health Organization estimates up to 50 million invasive infections by this parasite worldwide annually, resulting in about 70,000 deaths each year. Infection with E. histolytica is the fourth leading cause of death and the third leading cause of morbidity due to protozoan infections worldwide. E. histolytica has been listed by the NIH as a category B priority biodefense pathogen in the United States due to its low infectious dose and potential for dissemination through compromised food and water supplies.
