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 Jun 2022 | Modified Sep 2022

Amebiasis is infection with Entamoeba histolytica. It is acquired by fecal-oral transmission. Infection is commonly asymptomatic, but symptoms ranging from mild diarrhea to severe dysentery may occur. Extraintestinal infections include liver abscesses. Diagnosis is by identifying E. histolytica in stool specimens and confirmed with immunoassays-detecting antigen in the stool, or by serologic tests if extraintestinal disease is suspected. Treatment for symptomatic disease is with metronidazole or tinidazole followed by paromomycin or another drug active against cysts in the lumen of the colon.

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, uncertain pathogenicity)

  • E. bangladeshi (less common, uncertain pathogenicity)

Amebiasis is caused by E. histolytica and tends to occur in regions with poor socioeconomic conditions and poor sanitation. The parasite is present worldwide, but most infections occur in Central America, western South America, western and southern Africa, and the Indian subcontinent. In countries with sanitary food and water supplies (eg, US), 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 100,000 die.

Pathophysiology of Amebiasis

Entamoeba species exist in 2 forms:

  • Trophozoite

  • Cyst

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, would be 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.

Cysts predominate in formed stools and resist destruction in the external environment. They may spread directly from person to person or indirectly via food or water. Amebiasis can also be sexually transmitted by oral-anal contact.

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, common in the tropics, 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

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).

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: Microscopic examination, enzyme immunoassay of stool, molecular tests for parasite DNA in stool, and/or serologic testing

  • Extraintestinal infection: Imaging and serologic testing or a therapeutic trial with an amebicide

Hepatic amebiasis and amebic abscess must be differentiated from other hepatic infections 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 patients. Also, symptoms of echinococcosis Echinococcosis Echinococcosis is infection with larvae of the tapewormEchinococcus granulosus (cystic echinococcosis, hydatid disease) or Echinococcus multilocularis (alveolar disease). Symptoms... read more Echinococcosis are unusual until the cyst grows to 10 cm in diameter, and hepatocellular carcinoma Hepatocellular Carcinoma Hepatocellular carcinoma (HCC) usually occurs in patients with cirrhosis and is common in areas where infection with hepatitis B and C viruses is prevalent. Symptoms and signs are usually nonspecific... read more usually has no symptoms other than those caused by chronic liver disease. However, imaging and laboratory tests and tissue biopsy are often needed. 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

  • 10% of asymptomatic carriers

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

Microscopic identification of intestinal amebas may require examination of 3 to 6 stool specimens and concentration methods (see table ). 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 amebas microscopically, including E. coli, E. hartmanni, E. polecki, Endolimax nana, and Iodamoeba bütschlii. Molecular analysis using polymerase chain reaction-based assays and enzyme immunoassay for fecal antigens are more sensitive and differentiate E. histolytica from other amebas.

In symptomatic patients, sigmoidoscopy or colonoscopy may show nonspecific inflammatory changes or characteristic flask-shaped mucosal lesions, which 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.

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 drug 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

  • Microscopic examination of stool is usually negative in patients with extraintestinal amebiasis.

Treatment of Amebiasis

  • Initially, metronidazole, tinidazole, or sometimes nitazoxanide

  • Iodoquinol, paromomycin, or diloxanide furoate subsequently for cyst eradication

For gastrointestinal symptoms and extraintestinal amebiasis, one of the following is used:

  • Oral metronidazole 500 to 750 mg 3 times a day in adults (12 to 17 mg/kg 3 times a day in children) for 7 to 10 days

  • Tinidazole 2 g orally once/day in adults (50 mg/kg [maximum 2 g] orally once/day in children > 3 years) for 3 days for mild to moderate gastrointestinal symptoms, 5 days for severe gastrointestinal symptoms, and 3 to 5 days for amebic liver abscess

Metronidazole and tinidazole should not be given to pregnant women. Alcohol must be avoided because these drugs have a disulfiram-like effect. In terms of gastrointestinal adverse effects, tinidazole is generally better tolerated than metronidazole.

Therapy for patients with significant gastrointestinal symptoms should include rehydration with fluid and electrolytes and other supportive measures.

Although metronidazole, tinidazole, and nitazoxanide have some activity against E. histolytica cysts, they are not sufficient to eradicate cysts. Consequently, a 2nd oral drug is used to eradicate residual cysts in the intestine.

Options for cyst eradication are

  • Iodoquinol 650 mg orally 3 times a day after meals in adults (10 to 13 mg/kg [maximum of 2 g/day] orally 3 times a day in children) for 20 days

  • Paromomycin 8 to 11 mg/kg orally 3 times a day with meals for 7 days

  • Diloxanide furoate 500 mg orally 3 times a day in adults (7 mg/kg orally 3 times a day in children) for 10 days

Diloxanide furoate is not available commercially in the US but may be obtained through some compounding pharmacies.

The pathogenicity of E. moshkovskii and E. bangladeshi is 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 drugs used for E. histolytica.

Asymptomatic people who pass E. histolytica cysts should be treated with paromomycin, iodoquinol, or diloxanide furoate (see above for doses) to prevent development of invasive disease and spreading elsewhere in the body and to others.

Treatment is not necessary for E. dispar or asymptomatic E. moshkovskii and E. bangladeshi infections until more is known about their pathogenicity.

Treatment references

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. 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 is often asymptomatic, but 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.

  • Treat with metronidazole or tinidazole to eliminate amebic trophozoites, followed by iodoquinol or paromomycin to kill cysts in the intestine.

Drugs Mentioned In This Article

Drug Name Select Trade
Flagyl, Flagyl ER, Flagyl RTU, LIKMEZ, MetroCream, MetroGel, MetroGel Vaginal, MetroLotion, Noritate, NUVESSA, Nydamax, Rosadan, Rozex, Vandazole, Vitazol
Diodoquin , Yodoxin
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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