History
The incubation period for E histolytica infection is commonly 2-4 weeks but may range from a few days to years. The clinical spectrum of amebiasis ranges from asymptomatic infection to fulminant colitis and peritonitis to extraintestinal amebiasis, the most common form of which is amebic liver abscess.
Amebiasis is more severe in very young patients, in elderly patients, and in patients receiving corticosteroids. The clinical expression of amebiasis may be related to geography. For instance, amebic colitis is the predominant presentation in Egypt, whereas amebic liver abscesses predominate in South Africa.
Asymptomatic infections are common after ingestion of the parasite. E dispar does not cause invasive disease or antibody production. As many as 90% of E histolytica infections also are asymptomatic. The infection is self-limited but may be recurrent. It is not possible to distinguish between E histolytica and E dispar on clinical grounds; only antigen detection tests can make this distinction.
Amebic colitis
Amebic colitis is gradual in onset, with symptoms presenting over 1-2 weeks; this pattern distinguishes this condition from bacterial dysentery. Diarrhea is the most common symptom. Patients with amebic colitis typically present with cramping abdominal pain, watery or bloody diarrhea, and weight loss or anorexia. [48] Fever is noted in 10-30% of patients. Intestinal amebiasis may mimic acute appendicitis and rectal cancer [49] [50, 51] Rectal bleeding without diarrhea can occur, especially in children.
Fulminant amebic colitis is a rare complication of amebic dysentery (< 0.5% of cases). It presents with the rapid onset of severe bloody diarrhea, severe abdominal pain, and evidence of peritonitis and fever. Predisposing factors for fulminant colitis include poor nutrition, pregnancy, corticosteroid use, and very young age (< 2 years). Intestinal perforation is common. Corticosteroid therapy is a risk factor for the development of fulminant forms of amebic colitis [52] Mortality from fulminant amebic colitis may exceed 40%. Rarely diarrhea may be absent and this may make diagnosis more difficult. [44]
Chronic amebic colitis is clinically similar to inflammatory bowel disease (IBD). Recurrent episodes of bloody diarrhea and vague abdominal discomfort develop in 90% of patients with chronic amebic colitis who have antibodies to E histolytica. Amebic colitis should be ruled out before treatment of suspected IBD because corticosteroid therapy worsens amebiasis.
Amebic liver abscess
Amebic liver abscess is the most common form of extraintestinal amebiasis. [53] It occurs in as many as 5% of patients with symptomatic intestinal amebiasis and is 10 times as frequent in men as in women. Approximately 80% of patients with amebic liver abscess present within 2-4 weeks of infection. An estimated 95% of amebic liver abscesses related to travel develop within 5 months, though some may not manifest until years after travel to or residency in an endemic area.
The most typical presentation of amebic liver abscess is fever (in 85-90% of cases, in contrast to amebic colitis), right upper quadrant pain, and tenderness of less than 10 days’ duration. Involvement of the diaphragmatic surface of the liver may lead to right-side pleuritic pain or referred shoulder pain. [54, 55] Acute abdominal symptoms and signs should prompt rapid investigation for intraperitoneal rupture.
Associated gastrointestinal (GI) symptoms occur in 10-35% of patients and include nausea, vomiting, abdominal distention, diarrhea, and constipation. Approximately 40% of patients who have amebic liver abscess do not have a history of prior bowel symptoms. Although 60-70% of patients with amebic liver abscess do not have concomitant colitis, a history of dysentery within the previous year may be obtained. In a 2014 study of routine colonoscopy in patients with amebic liver abscess, colonic involvement was noted in two thirds of cases. [56] When colon was involved, right colonic lesion was universally present.
A small subset of patients with amebic liver abscess have a subacute presentation with vague abdominal discomfort, weight loss or anorexia, and anemia. Jaundice is unusual. Cough can occur. A history of alcohol abuse is common, but whether a causal relation exists is unclear.
Other manifestations of amebiasis
Ameboma
Ameboma, a less common form of intestinal disease, arises from the formation of annular colonic granulation in response to the infecting organisms, which results in a large local lesion of the bowel. It presents as a right lower quadrant abdominal mass, which may be mistaken for carcinoma, tuberculosis, Crohn disease, actinomycosis, or lymphoma. Biopsy findings assist in establishing the correct diagnosis. Rectal masses that resemble carcinoma on colonoscopy have also been noted. [57]
Pleuropulmonary amebiasis
Pleuropulmonary amebiasis is most commonly the result of contiguous spread from a liver abscess rupturing through the right hemidiaphragm. [58] However, a case of amebic lung abscess acquired through hematogenous spread has been reported. The typical age group is 20-40 years. The male-to-female ratio is 10:1.
Approximately 10% of patients with amebic liver abscess develop pleuropulmonary amebiasis, which presents with cough, pleuritic pain, and dyspnea. A hepatobronchial fistula is an unusual problem characterized by the expectoration of sputum resembling anchovy paste. The trophozoites of E histolytica may be found in the sputum sample. Primary amebic pneumonia as a result of hematogenous spread has been reported, though rarely. [59]
Cerebral amebiasis
Amebic abscesses resulting from hematogenous spread have occasionally been described in the brain. Cerebral amebiasis occurs in 0.6% of amebic liver abscess cases. Patients commonly present with the abrupt onset of nausea, vomiting, headache, and mental status changes. Computed tomography (CT) reveals irregular lesions without a surrounding capsule or enhancement. A tissue biopsy sample reveals the trophozoites. Progression can be very rapid, sometimes leading to death within 12-72 hours.
Amebic peritonitis
Amebic peritonitis is generally secondary to a ruptured liver abscess. Left-lobe liver abscesses are more likely to rupture. Patients present with fever and a rigid distended abdomen. Roughly 2-7% of liver abscesses rupture into the peritoneum.
Amebic pericarditis
Amebic pericarditis is rare but is the most serious complication of hepatic amebiasis. It usually is caused by a rupture of a left-liver lobe abscess and occurs in 3% of patients with hepatic amebiasis. It presents with chest pain and the features of congestive heart failure.
Genitourinary amebiasis
Genitourinary involvement may cause painful genital ulcers or fallopian tube amebiasis.
Amebic appendicitis
In countries of high prevalence, amebiasis occasionally presents as acute appendicitis. [60, 50]
Physical Examination
Patients with acute amebic colitis may have lower quadrant abdominal tenderness (12-85% of cases). Fever is noted in only a minority of patients (10-30%). Weight loss occurs in 40%. Dehydration is uncommon. Occult blood is nearly always present in stools (70-100%). Fulminant amebic colitis is commonly characterized by abdominal pain, distention, and rebound tenderness.
Amebic liver abscess may present with fever (85-90% of cases) and tender hepatomegaly (30-50%). Right lower intercostal tenderness may be elicited, particularly posteriorly (84-90%). Weight loss is noted in 33-50%. Breath sounds may be diminished at the right lung base, and rales may be heard. A small subset of patients has a subacute presentation with hepatomegaly, weight loss, and anemia. Jaundice is unusual (6-10%).
Other physical findings in amebiasis include the following:
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Pleuropulmonary amebiasis may produce right-side pleural effusions, empyema, basilar atelectasis, pneumonia, and lung abscess
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Patients with amebic peritonitis have fever and a tender, rigid, and distended abdomen
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Amebic pericarditis presents with features of congestive heart failure; a pericardial friction rub may be audible
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Cerebral amebiasis presents with altered consciousness and focal neurologic signs
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Genital ulcers due to amebiasis have a punched-out appearance and profuse discharge
Complications
Complications of amebic colitis include the following:
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Fulminant or necrotizing colitis
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Ameboma
Complications of amebic liver abscess include the following:
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Intraperitoneal, intrathoracic, or intrapericardial rupture, with or without secondary bacterial infection
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Direct extension to pleura or pericardium
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Dissemination and formation of brain abscess
Other complications due to amebiasis include the following [53, 61] :
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Bowel perforation
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GI bleeding
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Stricture formation
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Intussusception
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Peritonitis
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Empyema
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Trichrome stain of Entamoeba histolytica trophozoites in amebiasis. Two diagnostic characteristics are observed. Two trophozoites have ingested erythrocytes, and all 3 have nuclei with small, centrally located karyosomes.
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Trichrome stain of Entamoeba histolytica cyst in amebiasis. Each cyst has 4 nuclei with characteristically centrally located karyosomes. Cysts measure 12-15 mm.
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Entamoeba histolytica trophozoite. Image courtesy of Centers for Disease Control and Prevention.
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Entamoeba histolytica cyst. Image courtesy of Centers for Disease Control and Prevention.
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Gross pathology of intestinal ulcers due to amebiasis. Image courtesy of Centers for Disease Control and Prevention.
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Histopathology of typical flask-shaped ulcer of intestinal amebiasis. Image courtesy of Centers for Disease Control and Prevention.
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Entamoeba histolytica in liver aspirate, trichrome stain. Image courtesy of Centers for Disease Control and Prevention.
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Histopathology of amebiasis. Image courtesy of Centers for Disease Control and Prevention.
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Life cycle of amebiasis (Entamoeba histolytica). Courtesy of the Centers for Disease Control and Prevention (CDC).
