Amebiasis is infection primarily due to Entamoeba histolytica. It is acquired by fecal-oral transmission and can be sexually transmitted by oral-anal contact. Infection is commonly asymptomatic, but symptoms ranging from mild diarrhea to severe dysentery may occur. Extraintestinal infections include liver and rarely brain abscesses. Diagnosis is by identifying E. histolytica
(See also Overview of Intestinal Protozoan and Microsporidia Infections.)
Four species of Entamoeba are morphologically indistinguishable, but molecular techniques show that they are different species:
E. histolytica (pathogenic)
E. dispar (harmless colonizer, more common)
E. moshkovskii (less common, evidence of potential pathogenicity is emerging [1])
E. bangladeshi (less common, uncertain pathogenicity)
Amebiasis is primarily caused by E. histolytica and tends to occur in regions with poor sanitation. The parasite is present worldwide, but most infections occur in Central America, western South America, western and southern Africa, India, and parts of South Asia. In countries with sanitary food and water supplies, most cases occur among recent immigrants and travelers returning from endemic regions.
Worldwide each year, an estimated 50 million people develop amebic colitis or extraintestinal disease, and as many as 73,000 die (2).
References
1. Heredia RD, Fonseca JA, López MC: Entamoeba moshkovskii perspectives of a new agent to be considered in the diagnosis of amebiasis. Acta Trop. 2012;123(3):139-145. doi:10.1016/j.actatropica.2012.05.012
2. Lozano R, Naghavi M, Foreman K, et al: Global and regional mortality from 235 causes of death for 20 age groups in 1990 and 2010: a systematic analysis for the Global Burden of Disease Study 2010 [published correction appears in Lancet. 2013 Feb 23;381(9867):628. AlMazroa, Mohammad A [added]; Memish, Ziad A [added]]. Lancet. 2012;380(9859):2095-2128. doi:10.1016/S0140-6736(12)61728-0
Pathophysiology of Amebiasis
Entamoeba species exist in 2 forms:
Cyst (infective form)
Trophozoite (form that causes invasive disease)
Cysts predominate in formed stools and resist destruction in the external environment. Infection occurs following ingestion of amebic cysts. They may spread directly from person to person through fecal-oral transmission, including through oral-anal sexual contact, or indirectly via food or water. Cysts remain viable in the environment for weeks to months. After ingestion of a cyst, the ameba excysts to form a trophozoite in the colon.
The motile trophozoites feed on bacteria and tissue, reproduce, colonize the lumen and the mucosa of the large intestine, and sometimes invade tissues and organs. Trophozoites predominate in liquid stools but rapidly die outside the body and, if ingested, are killed by gastric acids. Some trophozoites in the colonic lumen become cysts that are excreted with stool.
E. histolytica trophozoites can adhere to and kill colonic epithelial cells and polymorphonuclear leukocytes (PMNs) and can cause dysentery with blood and mucus but with few PMNs in stool. Trophozoites also secrete proteases that degrade the extracellular matrix and permit invasion into the intestinal wall and beyond. Trophozoites can spread via the portal circulation and cause necrotic liver abscesses. Infection may spread by direct extension from the liver to the right pleural space, lung, or skin, or rarely through the bloodstream to the brain and other organs.
Symptoms and Signs of Amebiasis
Most people with amebiasis are asymptomatic but chronically pass cysts in stools.
Symptoms that occur with tissue invasion in the colon usually develop 1 to 3 weeks after ingestion of cysts and include
Intermittent diarrhea and constipation
Flatulence
Cramping abdominal pain
Tenderness over the liver or ascending colon and fever may occur, and stools may contain mucus and blood.
Amebic dysentery
Amebic dysentery manifests with episodes of frequent semiliquid stools that often contain blood, mucus, and live trophozoites. Abdominal findings range from mild tenderness to frank abdominal pain, with high fevers and toxic systemic symptoms. Abdominal tenderness frequently accompanies amebic colitis. Sometimes, fulminant colitis complicated by toxic megacolon or peritonitis may develop.
Between relapses, symptoms diminish to recurrent cramps and loose or very soft stools, but emaciation and anemia may develop. Symptoms suggesting appendicitis may occur. Surgery in such cases may result in peritoneal spread of amebas.
Chronic amebic infection of the colon
Chronic amebic infection of the colon can mimic inflammatory bowel disease and manifests as intermittent nondysenteric diarrhea with abdominal pain, mucus, flatulence, and weight loss. Chronic infection may also manifest as tender, palpable masses or annular lesions (amebomas) in the cecum and ascending colon. Ameboma may be mistaken for colonic carcinoma or pyogenic abscess.
Hepatic or other extraintestinal amebic disease
Extraintestinal amebic disease originates from infection in the colon and can involve any organ, but a liver abscess is the most common.
Liver abscess is usually single and in the right lobe. It can manifest in patients who have had no prior symptoms, is more common among men than among women (7:1 to 9:1), and may develop insidiously. Symptoms include pain or discomfort over the liver, which is occasionally referred to the right shoulder, as well as intermittent fever, sweats, chills, nausea, vomiting, weakness, and weight loss. Jaundice is unusual and low grade when present. The abscess may perforate into the subphrenic space, right pleural cavity, right lung, or other adjacent organs (eg, pericardium).
Brain abscess is rarely observed and almost only occur in patients who also have a liver abscess.
Skin lesions are occasionally observed, especially around the perineum and buttocks in chronic infection, and may also occur in traumatic or operative wounds.
Diagnosis of Amebiasis
Intestinal infection: Enzyme immunoassay of stool, molecular tests for parasite DNA in stool, microscopic examination, and/or serologic testing
Extraintestinal infection: Imaging and serologic testing or a therapeutic trial with an amebicide
Nondysenteric amebiasis may be misdiagnosed as irritable bowel syndrome, inflammatory bowel disease, or diverticulitis. A right-sided colonic mass may also be mistaken for cancer, tuberculosis, actinomycosis, or lymphoma.
Amebic dysentery may be confused with shigellosis, salmonellosis, schistosomiasis, or ulcerative colitis. In amebic dysentery, stools are usually less frequent and less watery than those in bacillary dysentery. They characteristically contain tenacious mucus and flecks of blood. Unlike stools in shigellosis, salmonellosis, and ulcerative colitis, amebic stools do not contain large numbers of white blood cells because trophozoites lyse them.
Hepatic amebiasis and amebic abscess must be differentiated from other hepatic infections (eg, echinococcal disease) and tumors. Patients with amebic liver abscess often present with right upper quadrant pain and fever. Amebic liver abscess is more common in men and younger adults exposed to endemic areas, whereas pyogenic liver abscess is more common in older adults. Also, symptoms of echinococcosis are unusual until the cyst grows to 10 cm in diameter, and hepatocellular carcinoma usually has no symptoms other than those caused by chronic liver disease. However, imaging and laboratory tests and tissue biopsy are often needed to diagnose amebiasis. Testing typically includes complete blood count (CBC), liver tests, and abdominal CT. Patients with pyogenic liver abscess often have left shift on white blood cell count, elevated serum bilirubin concentration, history of gallstones, and diabetes mellitus. Amebic liver abscess generally does not cause a left shift on white blood cell counts or elevated serum bilirubin concentration.
Diagnosis of amebiasis is supported by finding amebic trophozoites, cysts, or both in stool or tissues; however, pathogenic E. histolytica are morphologically indistinguishable from nonpathogenic E. dispar, as well as E. moshkovskii and E. bangladeshi, which are of uncertain pathogenicity. Immunoassays that detect E. histolytica antigens in stool are sensitive and specific and are done to confirm the diagnosis. Specific DNA detection assays for E. histolytica using polymerase chain reaction are available at diagnostic reference laboratories and have very high sensitivity and specificity.
Serologic tests are positive in
About 95% of patients with an amebic liver abscess
> 70% of those with active intestinal infection
Enzyme immunoassay (EIA) is the most widely used serologic test. Antibody titers can confirm E. histolytica infection but may persist for months or years, making it impossible to differentiate acute from past infection in residents from areas with a high prevalence of infection. Thus, serologic tests are helpful when previous infection is considered less likely (eg, in travelers to endemic areas).
Amebic intestinal infection
Molecular analysis using polymerase chain reaction (PCR) assays and EIA for fecal antigens are most sensitive and differentiate E. histolytica from other amebas. Microscopic identification of intestinal amebas may require examination of 3 to 6 stool specimens and concentration methods (see table Collecting and Handling Specimens for Microscopic Diagnosis of Parasitic Infections). Antibiotics, antacids, antidiarrheals, enemas, and intestinal radiocontrast agents can interfere with recovery of the parasite and should not be given until the stool has been examined. E. histolytica is indistinguishable morphologically from E. dispar, E. moshkovskii, and E. bangladeshi but can be distinguished from a number of nonpathogenic microorganisms microscopically, including E. coli, E. hartmanni, E. polecki, Endolimax nana, and Iodamoeba bütschlii.
In symptomatic patients, sigmoidoscopy or colonoscopy may show nonspecific inflammatory changes or characteristic flask-shaped mucosal lesions upon histologic exam. Lesions should be aspirated, and the aspirate should be examined for trophozoites and tested for specific E. histolytica antigen or DNA. Biopsy specimens from rectosigmoid lesions may also show trophozoites.
Image courtesy of CDC/Dr. Mae Melvin via the Public Health Image Library of the Centers for Disease Control and Prevention.
Amebic liver infection
Amebic extraintestinal infection is more difficult to diagnose. Stool examination is usually negative, and recovery of trophozoites from aspirated pus is uncommon. If a liver abscess is suspected, ultrasonography, CT, or MRI should be done. They have similar sensitivity; however, no technique can differentiate amebic from pyogenic abscess with certainty.
Needle aspiration is reserved for the following:
Those likely to be due to fungi or pyogenic bacteria
Those in which rupture seems imminent
Those that respond poorly to pharmacologic therapy
Abscesses contain thick, semifluid material ranging from yellow to chocolate-brown. A needle biopsy may show necrotic tissue, but motile amebas are difficult to find in abscess material, and amebic cysts are not present.
A therapeutic trial of an amebicide is often the most helpful diagnostic tool for an amebic liver abscess.
Pearls & Pitfalls
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Treatment of Amebiasis
For gastrointestinal symptoms and extraintestinal amebiasis, one of the following taken orally is used:
Ornidazole for 5 days
Alcohol must be avoided because these medications may have a disulfiram-like effect.
Therapy for patients with significant gastrointestinal symptoms should include rehydration with fluid and electrolytes and other supportive measures.
E. histolytica cysts, they are not sufficient to eradicate cysts. Consequently, a second oral medication is used to eradicate residual cysts in the intestine.
Options for cyst eradication are
Diloxanide furoate for 10 days
Diloxanide furoate is not available commercially in the United States but may be obtained through some compounding pharmacies.
The pathogenicity of E. moshkovskii and E. bangladeshi has been uncertain. They have been identified in stools primarily in children with and without diarrhea in impoverished areas where fecal contamination of food and water is present. Molecular diagnostic tests to identify them are available only in research settings. The optimal treatment is unknown, but they are likely to respond to medications used for E. histolytica. Given emerging data on the pathogenicity of E. moshkovskii (3), treatment for symptomatic infection can be considered using the same approach as for E. histolytica.
Asymptomatic people who pass E. histolytica4).
Treatment is not necessary for E. dispar or asymptomatic E. moshkovskii and E. bangladeshi infections until more is known about their pathogenicity.
Treatment references
1. Rossignol JF, Kabil SM, El-Gohary Y, et alTrans R Soc Trop Med Hyg 101(10):1025-31, 2007. doi: 10.1016/j.trstmh.2007.04.001
2. Escobedo AA, Almirall P, Alfonso M, et al: Treatment of intestinal protozoan infections in children. Arch Dis Child 94(6):478-82, 2009. doi: 10.1136/adc.2008.151852
3. Heredia RD, Fonseca JA, López MC: Entamoeba moshkovskii perspectives of a new agent to be considered in the diagnosis of amebiasis. Acta Trop. 2012;123(3):139-145. doi:10.1016/j.actatropica.2012.05.012
4. Blessmann J, Tannich E: Treatment of asymptomatic intestinal Entamoeba histolytica infection. N Engl J Med. 2002;347(17):1384. doi:10.1056/NEJM200210243471722
Prevention of Amebiasis
To prevent amebiasis, contamination of food and water with human feces must be avoided—a problem complicated by the high incidence of asymptomatic carriers.
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.)
Uncooked foods, including salads and vegetables, and potentially contaminated water and ice should be avoided in areas with poor sanitation. Boiling water kills E. histolytica cysts. The effectiveness of chemical disinfection with iodine- or chlorine-containing compounds depends on the temperature of the water and amount of organic debris in it. Portable filters provide various degrees of protection.
Work continues on the development of a vaccine, but none is available yet.
Key Points
E. histolytica infection is often asymptomatic, but it can cause intestinal symptoms, dysentery, or liver abscesses.
Diagnose amebic intestinal infection using stool antigen tests, molecular tests for DNA, or microscopy.
Diagnose amebic liver abscess using ultrasonography, CT, or MRI, or serologic tests, which are most helpful when previous infection is considered unlikely (eg, in travelers to endemic areas), or a therapeutic trial of an amebicide.
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: Yellow Book: Amebiasis
CDC: Yellow Book: Food & Water Precautions
CDC: Parasites - Amebiasis: General Information
