Amebiasis is an infection caused by intestinal protozoa (amoeba) called Entamoeba histolyticaand is spread by cysts. Approximately 90% of the infections with Entamoeba histolytica are asymptomatic and the remaining about 10% infections produce symptoms dysentery to abscess of liver and other organs of the body.

How Amebiasis is transmitted?

Infection of E. histolytica is acquired by ingestion of viable cysts from fecal contamination of water, food, or hands (hence it is common in developing countries where safe drinking water not available).

Food borne infection is common, especially when food handlers are shedding cysts or food is being grown with contaminated (with feces) water, soil or sometimes fertilizer. Other than common food or water borne infection amebiasis can also occasionally be transmitted by oral and anal sexual practices and rarely by direct rectal inoculation through colonic irrigation devices. From cysts motile trophozoites are released in the small intestine and generally (in 90% of the cases) remain as harmless commensals in the large intestine. The trophozoites invade the bowel mucosa and cause colitis, or they invade bloodstream, which causes distantabscesses, especially in the liver, lungs, or brain.

In patients with active dysentery trophozoites may not encyst (only cysts can cause amebiasis) and they are passed in the stool, which are rapidly killed when exposed to air and therefore cannot transmit infection.

How common is amebiasis?

Approximately 10% of the world population (majority with noninvasive Entamoeba dispar) harbor entamoeba organisms. But amebiasis occurs only if infected by Entamoeba histolytica(in normal human host Entamoeba dispar can not cause invasive disease, it only cause a self limiting illness). Amebiasis is the third common cause of death due to parasite infection, the first two being malaria and schistosomiasis.

The areas of highest incidence of amebiasis are developing countries in tropics like India, Bangladesh, Mexico, Central and South America, tropical Asia, and Africa. In developed countries the people at risk are travelers, recent immigrants, homosexual men, and inmates of institutions like mental asylums.

What are the symptoms of amebiasis?

The most common is asymptomatic cyst passage, even in highly endemic areas of amebiasis. The symptoms of Intestinal Amebiasis (amebic colitis) develops 2–6 weeks after the ingestion of infectious cysts (through contamination of food, water etc.). Initially there is lower abdominal pain and mild diarrhea, followed by malaise and lower abdominal pain or back pain. Patients with amebiasis (dysentery) may pass 10–12 stools (the stools consist mainly of blood and mucus and little fecal material) per day. Fever is present in less than 40% of patients (in bacterial diarrhea fever is commoner). Sometimes it may mimic appendicitis due to involvement of cecum.

In severe amebiasis there is severe abdominal pain, high fever, and profuse diarrhea (this is rare and may be seen in children, but patients receiving glucocorticoids are at risk for severe amebiasis). Rarely patients develop a chronic amebic colitis, which may be difficult to distinguish from inflammatory bowel disease. If inflammatory bowel disease is suspected in a patient receiving glucocorticoid therapy amebiasis should be excluded.

How amebiasis is diagnosed?

Stool examination is the most important diagnostic test for intestinal amebiasis, although sometimes serological tests are done to confirm diagnosis of amebiasis. In the stool presence of cyst of ameba or trophozoites, presence of heme, lesser nutrophils etc. are suggestive of colitis. The definitive diagnosis of amebic colitis (intestinal amebiasis) is done by demonstration of trophozoites of E. histolytica. Demonstration of trophozoites may be difficult because they are readily killed by exposure to water, drying and barium that is why at least three fresh stool specimens should be examined. To get better result of stool examinations a combination of wet mounts, iodine-stained concentrates, concentrates for cysts or trophozoites should be done. Culture of amebas can give better results but are not routinely available. Sigmoidoscopy with biopsy of the edge of ulcers (this procedure is dangerous during fulminant colitis due to the risk of perforation) can be helpful in giving higher positive results if stool examinations are negative.

Serologic tests are useful addition in diagnosis of amebiasis. ELISA (enzyme-linked immunosorbent assay) and agar gel diffusion assays are positive in more than 90% of patients with colitis. Positive serological test with clinical findings can be suggestive of active disease because serological findings revert to negative within 6–12 months. Serologic tests are helpful in assessing the risk of invasive amebiasis in asymptomatic, cyst-passing individuals, especially in nonendemic areas.

What is the differential diagnosis of amebiasis?

There are several other diseases that can be confused with intestinal amebiasis and these are bacterial diarrheas due to enteroinvasive Escherichia coli, Campylobacter, Shigella,Salmonella, and Vibrio species. To diagnosed correctly bacterial cultures, microscopic examination of stools (to identify cysts) and serological test for ameba are required.

How to treat intestinal amebiasis?

The treatment of amebiasis depends on the type of disease it is producing. If patient is suffering from acute colitis, it should be treated with metronidazole 400-800 mg 8 hourly for 5-10 days followed by luminal agent Furamide (diloxanide furoate) 500 mg 8 hourly for 10 days to eliminate luminal cysts. Asymptomatic carriers (cyst passers) should be treated with diloxanide furoate 500 mg 8 hourly for 10 days.

In United States only luminal agents available are iodoquinol and paromomycin, so either of these is used instead of Furamide. Iodoquinol (available as 650 mg tablets) 650 mg three times a day for 20 days or paromomycin (available as 250 mg tablets) 500 mg three times a day for 10 days. For treatment of asymptomatic carriers of intestinal amebiasis either of these 2 luminal agents (iodoquinol or paromomycin) is used.

How to prevent intestinal amebiasis?

Amebic infection (E. histolytica or Entamoeba dispar) is spread by ingestion of contaminated food or water by cysts of these organisms. Asymptomatic carrier of amebiasis may excrete up to 15 million cysts per day. Most important part of prevention of amebic infection requires eradication of cyst carriage and adequate sanitation. In high-risk areas (endemic areas), amebic infection can be reduced by the avoiding eating of unpeeled fruits and vegetables and the use of safe bottled water. Because cysts are resistant to readily attainable levels of chlorine, disinfection by iodination (tetraglycine hydroperiodide) is recommended. Filtration of water can also remove the cysts of ameba. There is no effective prophylaxis for amebiasis.

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