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Amebiasis
is infection with Entamoeba histolytica.
It is commonly asymptomatic, but mild diarrhea to severe dysentery
may occur. Extraintestinal infections include liver abscesses. Diagnosis
is by identifying E. histolytica in
stool specimens or by serologic tests. Treatment for symptomatic
disease is with metronidazole or tinidazole followed by paromomycin
or other drugs active against cysts in the lumen.
Two species of Entamoeba are morphologically indistinguishable: E. histolytica is pathogenic, but E. dispar harmlessly colonizes the colon. They exist in 2 forms: the trophozoite and the cyst. The motile trophozoite feeds on bacteria and tissue, reproduces, colonizes the lumen and the mucosa of the large intestine, and sometimes invades tissues and organs. Trophozoites predominate in liquid stools but rapidly die outside the body. Some trophozoites in the colonic lumen become cysts that are excreted with stool. Cysts predominate in formed stools and are resistant to the external environment. They may spread either directly from person to person or indirectly via food or water. Amebiasis can be sexually transmitted by oral-anal contact.
E. histolytica trophozoites adhere to and kill colonic epithelial cells and PMNs and can cause dysentery with blood and mucus. They also secrete proteases that degrade the extracellular matrix and permit invasion into the intestine wall and beyond. Trophozoites can spread via the portal circulation and cause necrotic liver abscesses. Infection may spread by direct extension from the liver or through the bloodstream to the lungs, brain, and other organs.
Symptoms and Signs
Most infected people are asymptomatic but chronically pass cysts in stools. Symptoms that occur with tissue invasion include intermittent diarrhea and constipation, flatulence, and cramping abdominal pain. Tenderness over the liver or ascending colon may occur, and stools may contain mucus and blood.
Amebic dysentery common in the tropics, presents 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. Between relapses, symptoms diminish to recurrent cramps and loose or very soft stools, but emaciation and anemia may develop. Symptoms suggestive of appendicitis may occur. Surgery in such cases may result in peritoneal spread of amebas.
Chronic
amebic infection can mimic inflammatory bowel disease and presents as intermittent nondysenteric diarrhea with abdominal pain, mucus, flatulence, and weight loss. Chronic infection may also present as tender, palpable masses or annular lesions (amebomas) in the cecum and ascending colon that resemble carcinomas.
Extraintestinal
disease originates from infection in the colon and can involve any organ, but a liver abscess, usually single and in the right lobe, is the most common. It can present in patients without prior symptoms, is more common in men than in 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; 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. Skin lesions are occasionally observed, especially around the perineum and buttocks in chronic infection, and may also occur in traumatic or operative wounds.
Diagnosis
Nondysenteric amebiasis may be misdiagnosed as irritable bowel syndrome, regional enteritis, or diverticulitis. Amebic dysentery may be confused with shigellosis, salmonellosis, schistosomiasis, or ulcerative colitis. In amebic dysentery, stools are usually less frequent and 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 WBCs. Hepatic amebiasis and amebic abscess must be differentiated from other hepatic infections and tumors.
Diagnosis of amebiasis is supported by finding amebic trophozoites and/or cysts in the stool or tissues; however, pathogenic E. histolytica are morphologically indistinguishable from nonpathogenic E. dispar. Identification of intestinal amebas may require examination of 3 to 6 stool specimens and concentration methods (see Table 1: Approach to Parasitic Infections: Collecting and Handling Specimens for Microscopic Diagnosis of Parasitic Infections  ). Antibiotics, antacids, antidiarrheals, enemas, and intestinal radiocontrast agents interfere with recovery of the parasite and should not be given until the stool has been examined. E. histolytica also has to be distinguished from nonpathogenic amebas such as Entamoeba coli
, Endolimax nana, and others.
In symptomatic patients, proctoscopy often shows characteristic flask-shaped mucosal lesions, which should be aspirated and the material examined for trophozoites. Biopsy specimens from rectosigmoid lesions may also show trophozoites.
Extraintestinal amebiasis 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, ultrasound, CT, or MRI should be performed. They are of similar sensitivity; however, no technique can differentiate amebic from pyogenic abscess with certainty. Needle aspiration is usually reserved for lesions of uncertain etiology, those in which rupture seems imminent, or 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.
Serologic tests are positive in about 95% of patients with amebic liver abscess, > 70% of those with active intestinal infection, and 10% of asymptomatic carriers. The indirect hemagglutination and enzyme-linked immunosorbent assays are most sensitive. 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. E. histolytica and E. dispar are morphologically indistinguishable, so microscopic examination cannot be used to differentiate them. A sensitive and specific antigen detection assay for the E. histolytica adherence lectin has been developed. PCR-based assays are available in research settings.
Treatment
For mild to moderate GI symptoms, 7 to 10 days of oral metronidazole is recommended (500 to 750 mg tid for adults, 12 to 17 mg/kg tid for children). Metronidazole should not be given to pregnant women. Alcohol must be avoided because of the drug's disulfiram -like effect. Alternatively, tinidazole can be used (2 g po once/day can be given for 3 days in adults or 50 mg/kg [maximum 2 g] once/day for 3 days in children). Tinidazole is generally better tolerated than metronidazole .
For severe intestinal and extraintestinal amebiasis, metronidazole 750 mg tid for 7 to 10 days is used in adults or 12 to 17 mg/kg tid for 7 to 10 days in children. Alternatively, tinidazole 2 g once/day for 5 days can be used in adults and 50 mg/kg once/day (maximum 2 g) for 5 days in children.
A course of metronidazole or tinidazole should be followed by a 2nd oral drug to eradicate residual cysts in the lumen. The options are iodoquinol (650 mg po tid for 20 days for adults or 10 to 13 mg/kg tid for 20 days for children [maximum of 2 g/day]); paromomycin (8 to 11 mg/kg tid for 7 days); and diloxanide furoate (500 mg tid for 10 days for adults or 7 mg/kg tid for 10 days for children).
Therapy should include rehydration with fluid and electrolytes and other supportive measures.
Asymptomatic people who pass E. histolytica cysts should be treated with paromomycin , iodoquinol , or diloxanide furoate (see above for doses). Metronidazole and tinidazole are not sufficiently active against E. histolytica cysts to be used alone.
Treatment is not necessary for E. dispar infections.
Prevention
Contamination of food and water with human feces must be prevented, 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 developing areas. 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 has begun on the development of a vaccine, but none is available now.
Last full review/revision November 2005
Content last modified November 2005
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