Amoebic infections

Article about amoebic infections (amoebiasis).

Topics covered:

  • Essentials
  • Introduction
  • Entamoeba histolytica infection
  • Other parasitic gut amoebae including Dientamoeba fragilis
  • Free-living amoebae
  • Further reading


Two very different groups of amoebic species infect humans. (1) Obligate anaerobic gut parasites—including the major pathogen Entamoeba histolytica, Dientamoeba fragilis (which causes relatively mild colonic involvement with diarrhoea), and eight non-pathogenic species including Entamoeba dispar. (2) Aerobic free-living, water and soil amoebae—these can become facultative tissue parasites in humans after cysts or trophozoites are inhaled, ingested, or enter damaged skin or mucosae.

Entamoeba histolytica infection

The term amoebiasis (when unqualified) generally refers to E. histolytica infection, which is common in Mexico, South America, Natal, the west coast of Africa, and South-East Asia; nearly all amoebic disease seen in temperate countries is acquired elsewhere. Transmission is by the faecal–oral route; following ingestion of infective cysts, a population of trophozoites becomes established in the caecum and proximal colon.

Clinical features—clinical features range from minimal changes in bowel habit to severe dysentery. Onset is usually gradual or intermittent, with initially mild constitutional upset, colicky abdominal pain, and foul-smelling stools that always contain visible or occult blood. Less typical presentations of amoebic colitis include (1) fulminant; (2) amoebic colitis without dysentery; (3) amoeboma—presenting as an abdominal mass, most frequently in the right iliac fossa; (4) localized perforation and amoebic appendicitis; (5) rectal bleeding. The most significant complication is hepatic amoebiasis.

Diagnosis, treatment, and prognosis—examination of dysenteric stool, bowel-wall scrapings, liver abscess aspirate, or other samples in temporary wet mounts is critical, with identification of live erythrocytophagous trophozoites confirming the diagnosis of invasive amoebic disease. Other diagnostic methods include (1) demonstration of amoebal DNA in faeces/tissues by PCR; (2) serology—but seropositivity does not distinguish current and past tissue invasion. Aside from supportive care, metronidazole for 5 days is usually the first-choice treatment, with the addition of diloxanide to eliminate infection from the bowel and so prevent recurrence of tissue invasion or transmission to others. Uncomplicated invasive intestinal disease (and uncomplicated hepatic amoebiasis) should have mortality less than 1%, but this may reach 40% for amoebic peritonitis with multiple gut perforation.

Hepatic amoebiasis—less than 50% of patients give any convincing history of dysentery and few have concurrent dysentery. Presentation is typically with fever, sweating, liver or diaphragmatic pain, weight loss, and tender hepatomegaly. Diagnosis is usually achieved by demonstration of a (most often solitary) liver abscess on ultrasonography or CT and positive serological testing, with a therapeutic amoebicide trial generally being preferable to diagnostic needling of the liver.

Prevention—simple hygienic measures and health education provide considerable protection: boiling water for 5 min kills cysts. Travellers to endemic areas may need a medical check on their return; but chemoprophylaxis is not appropriate.

Free-living amoebae

Three genera of free-living amoebae cause human disease: (1) Naegleria—causes a primary meningoencephalitis after bathing or diving in fresh water; Amphotericin B an effective drug, but most cases are fatal, partly because of diagnostic delays. (2) Acanthamoeba—causes a painful keratitis, mainly in contact lens users, which usually responds to intensive local amoebicides, although corneal grafting may be needed. (3) Acanthamoeba and Balamuthia—can cause granulomatous encephalitis in both immunocompromised and immunocompetent people; presentation is with headache and meningism or with evidence of a focal brain lesion; survival is rare.


The amoebic species infecting humans belong to two very different groups. The obligate anaerobic gut parasites include the major pathogen Entamoeba histolytica, which ranks second to malaria as the most dangerous parasite in humans; Dientamoeba fragilis, a minor pathogen; and eight nonpathogenic species including the common and important Entamoeba dispar. The second group includes certain aerobic free-living, water and soil amoebae which produce cytopathic changes in cultured cell monolayers and cerebral invasion after intranasal inoculation into mice. They can become facultative tissue parasites in humans after or cysts or trophozoites are inhaled, ingested, or enter damaged shin or mucosae.

All motile feeding amoebae are called ‘trophozoites’; they move with pseudopodia and divide by binary fission. The hyaline external cytoplasm, the ‘ectoplasm’, is a contractile gel that surrounds the sol endoplasm containing numerous phagocytic and pinocytic vacuoles. Noninvasive trophozoites feed on bacteria. Most species can form environmentally resistant transmissive cysts by rounding up and secreting a chitinous cyst wall.

entamoeba histolytica trophozoite

Above: Entamoeba histolytica trophozoite

The definitive taxonomic separation of E. dispar as a nonpathogenic species separate from E. histolytica was made in 1993. This was based upon genomic and biochemical differences. This distinction is of fundamental importance because their cysts and noninvasive trophozoites are morphologically indistinguishable, but they are now separated by specific antigen and PCR assays. All strains of E. histolytica are now regarded as pathogenic, whereas the commoner E. dispar is never pathogenic.

Entamoeba histolytica infection 

Biology and pathogenicity

Following ingestion of infective cysts, a population of trophozoites becomes established in the caecum and proximal colon. Some degree of tissue invasion occurs in all subjects with at least low-titre seroconversion. Tissue invasion is frequently mild, self-limiting, and with minimal symptoms, but at the other end of the clinical spectrum it can lead to extensive destruction of the colonic mucosa. Parasite genotype may partly determine clinical outcome. Invasive trophozoites have a characteristic morphology; they may reach 30 to 40 μm in diameter and are very active with apparently purposeful, unidirectional movements during which they become considerably elongated. Their most important diagnostic characteristic is the presence of host erythrocytes within the endoplasm, which otherwise appears clear and contains no bacteria. Trophozoites containing red blood cells are described as erythrocytophagous. Progression through tissues is by active movement, facilitated by secreted collagenase; leucocytes are drawn chemotactically towards the amoebae but most are rapidly destroyed on contact.

Entamoeba histolytica trophozoite

Above: Entamoeba histolytica trophozoite

The transmissive cystic form of the parasite is derived entirely from a commensal population within the colonic lumen. Live commensal amoebae measure from 10 to 20 μm in diameter, the endoplasm is granular and contains bacteria, and the pseudopodia are blunt and movement is sluggish. Intestinal hurry from any cause, including the use of laxatives, can lead to the appearance of commensal trophozoites in the faeces. Cysts are spherical and measure from 11 to 14 μm in diameter; when mature, they contain four nuclei, several chromatoid bodies that are ribosome aggregates, and a glycogen vacuole.

entamoeba histolytica cyst

Above: Entamoeba histolytica cyst

Host factors may increase susceptibility to overt disease. Steroid therapy given systemically or locally into the rectum carries great risk, as may cytotoxic therapy. Severe amoebic bowel disease is particularly common in late pregnancy and the puerperium. Before puberty, both sexes are equally susceptible to hepatic amoebiasis, but in adults this condition is much more common in males. Local disease can also favour tissue invasion; thus amoebic ulceration may be superimposed upon colonic and rectal cancers, or those of the uterine cervix. Colonic disease is favoured by concurrent Trichuris infection or intestinal schistosomiasis. Infection with HIV appears to have little effect on colonic disease but may facilitate liver involvement.

Amoebiasis Life Cycle

Above: Amoebiasis Life Cycle


The incidence of disease is particularly high in Mexico, South America, Natal, the west coast of Africa, and South-East Asia. In most temperate countries, E. histolytica is now rare and nearly all amoebic disease seen in such countries will have been acquired elsewhere. Symptomless or convalescent carriers are the main source of infection; patients with dysentery normally pass only trophozoites in their stool and are therefore noninfectious. Cysts remain viable in the environment for up to 2 months. The infection is eventually self-limiting and rarely exceeds 4 years. Tissue invasion can occur at any time during an infection but is much more common during the first 4 months; the incubation period may be as short as 7 days. E. histolytica-associated diarrhoea can retard growth in preschool children.

amebiasis, Entamoeba histolytica prevalence world map

The incidence of amoebiasis in a population is best estimated from seropositivity surveys. Surveys for cysts are of no value as their differentiation from E. dispar is impossible. All modes of faeco-oral transmission occur in amoebiasis. Of special importance are the food handler and contaminated vegetables; transmission by flies and drinking-water is less common. Drinking-water can be contaminated in the home or at surface-water sources. Direct spread can produce outbreaks; it occurs within institutions for children and people with learning difficulties and with contaminated colonic irrigation equipment. Household clustering is common; hand-fed infants are frequently infected from the fingers of their mother. Contamination of piped water supplies can lead to serious disease outbreaks, as happened in the Chicago hotels epidemic in 1933. Interruption of piped water supplies probably caused the recent outbreak in Georgia. Entamoeba infections are common among male homosexuals, but most are due to E. dispar.


The basic lesion is cell lysis and tissue necrosis, which, by creating locally anoxic and acidic conditions, favours further penetration of the parasite; most amoebae are seen at the advancing edge of the lesion with little inflammatory cell response. In tissue sections, amoebae stain indistinctly with haematoxylin and eosin but appear bright red with periodic acid–Schiff stain; iron haematoxylin is necessary to show nuclear detail. Cysts of E. histolytica are never seen in tissue.

Amoebic lesions of the gut are most common in the rectosigmoid and caecum but can occur anywhere in the large bowel; involvement may be patchy or continuous. Less commonly, the appendix or terminal ileum are affected. The initial lesions are either small, discrete erosions of the mucosa or minute crypt lesions. Unrestrained, the lesions extend through the mucosa, across the muscularis mucosa and into the submucosa, where they expand laterally to produce lesions that are typically flask shaped in cross-section. Further lateral spread of the submucosal lesions leads to their coalescence and, later, to denudation of overlying mucosa. The bowel wall may become appreciably thickened. Blood vessels involved in the disease may thrombose, bleed into the gut lumen, or, in the case of portal-vein radicles, enable dissemination of amoebae to the liver. In very severe lesions, and usually in association with toxic megacolon, there is an irreversible coagulative necrosis of the bowel wall.

Amoebae in a colon biopsy from a case of amoebic dysentery

Above: Amoebae in a colon biopsy from a case of amoebic dysentery

Amoebomas are tumour-like lesions of the colonic wall measuring up to several centimetres in length; they are most common in the caecum and may be multiple. Histologically there is tissue oedema, with a mixed picture of healing and new areas of epithelial loss and tissue destruction; round-cell infiltration is patchy. Lesions may be annular and rarely an amoeboma initiates an intussusception; narrow, stricture-like amoebomas may occur in the anorectal region.

Amoebae reach the liver in the portal vein. Once initiated, the amoebic lesion extends progressively in all directions to produce the liver-cell necrosis and liquefaction that constitute an amoebic liver abscess. The lesions are well demarcated from surrounding liver tissue; untreated nearly all will eventually extend into adjacent structures. Secondary bacterial infection is rare and usually follows rupture or aspiration.

Amoebic liver abscess

Above: Amoebic liver abscess

Clinical manifestations

Invasive intestinal amoebiasis

The clinical features show a wide spectrum from minimal changes in bowel habit to severe dysentery. Lesions may be limited to a small part of the large bowel or extend throughout its length. A relapsing course is common.

Amoebic colitis with dysentery

Dysentery, the passage of loose or diarrhoeal stools containing fresh blood, occurs when there is generalized colonic ulceration or when more localized lesions occur in the rectum or rectosigmoid. Onset may be gradual, intermittent, or, much less commonly, acute. Typically, constitutional upset is initially mild and the patient remains ambulant; mild or moderate abdominal pain is common, often colicky and maximal over affected parts of the gut. Tenesmus can occur but is rarely severe. Stools vary in consistency from semiformed to watery. They are foul smelling and always contain visible or occult blood; even when they are watery, faecal matter is nearly always present. Symptoms frequently wax and wane over a period of weeks or even months and such patients can become debilitated and wasted. In a few patients the disease runs a fulminating course. The most frequent physical sign is abdominal tenderness in one or both iliac fossae, but tenderness may be generalized. The affected gut may be palpably thickened. A low fever is common, but dehydration is uncommon. Abdominal distension occurs in the more severely ill patients, who sometimes pass relatively small amounts of stool.

A careful proctoscopy or sigmoidoscopy should be done. The endoscopic appearances may be nonspecific in early, acute, or very severe colitis; the findings are hyperaemia, contact bleeding, or confluent ulceration. In more chronic cases, the presence of normal-looking intervening mucosa is highly suggestive of amoebiasis. Early lesions are often elevated, with a pouting opening only 1 to 2 mm in diameter; later, ulcers may reach 1 cm or more in diameter, with an irregular outline and often a loosely adherent, yellowish or grey exudate. Mucosal scrapings or superficial biopsies taken at endoscopy should be examined immediately by wet-preparation microscopy.

Special forms of amoebic colitis

Fulminant colitis

This may arise de novo, e.g. in pregnant women or during steroid therapy, or it may evolve during a dysenteric illness. Patients show progressive abdominal distension, vomiting, and watery diarrhoea. Bowel sounds are absent and there may be little or no abdominal tenderness, guarding, or rigidity. Plain radiographs may reveal free peritoneal gas, together with acute gaseous dilatation of the colon; affected segments of bowel may appear relatively narrow and show visible mucosal pathology. Barium enema and full sigmoidoscopy are contraindicated. Stools contain erythrocytophagous trophozoites.

Amoebic colitis without dysentery

When ulceration is limited to the caecum or ascending colon, or when early, mild, or localized lesions occur elsewhere in the colon, there may be no dysenteric symptoms. Patients complain of change in bowel habit, bloodstaining of the stool, flatulence, and colicky pain. Often the only physical sign is tenderness in the right iliac fossa or elsewhere along the course of the colon. Some patients eventually go into complete remission; others progress to a dysenteric illness. The most important diagnostic measure is repeated stool examination for erythrocytophagous amoebae; the finding of cysts or commensal trophozoites is of little diagnostic value, especially in endemic areas. Sigmoidoscopy is often normal when the distal bowel is not involved but colonoscopy may reveal typical lesions.


This presents as an abdominal mass, most frequently in the right iliac fossa. The lesion may be painful, tender, and associated with fever. Bowel habit is altered and some patients have intermittent dysentery, especially if lesions are multiple or distal. Evidence of partial or intermittent bowel obstruction may be present, particularly when lesions are distal and annular.

Localized perforation and amoebic appendicitis

Sudden perforation with peritonitis can occur from any deep amoebic ulcer; alternatively, leakage may lead to a pericolic abscess or retroperitoneal cellulitis. Amoebic appendicitis is an uncommon but important condition that occurs when amoebic lesions are confined to the appendix and caecum. The clinical presentation can resemble that of simple appendicitis, often with some clinical evidence of dysentery. If it is unrecognized at appendicectomy the outcome can be disastrous, with gut perforation; fresh smears should be made from the resected appendix and examined immediately.

Rectal bleeding

Some patients with amoebiasis present with rectal bleeding, with or without tenesmus; this occurs particularly in children. Massive bleeding into the gut lumen can occur in any form of amoebic colitis but is rare.

Differential diagnosis

Amoebic colitis must be differentiated from other causes of infective colitis. High-volume diarrhoea, copious mucus, and severe tenesmus are all uncommon in amoebiasis. In temperate countries, nonspecific ulcerative colitis, Clostridium difficile colitis, and colorectal carcinoma create the greatest diagnostic problems. Parasitic conditions to be considered are intestinal schistosomiasis, heavy Trichuris infection, and balantidiasis. More chronic amoebic pathology may clinically resemble Crohn’s disease, ileocaecal tuberculosis, diverticulitis, or anorectal lymphogranuloma venereum.

Hepatic amoebiasis

Less than half of all patients give any convincing history of dysentery and few have concurrent dysentery. In those with no dysenteric history, the interval between presumed infection and presentation may be as short as 3 weeks or as long as 15 years; for most, it is between 8 weeks and 1 year.

The dominant symptoms are fever and sweating, liver or diaphragmatic pain, and weight loss. Onset of constitutional symptoms is often insidious, but pain may begin abruptly. Most patients seek medical help between 1 and 4 weeks. Fever is typically remittent, with a prominent evening rise, brief rigors, and very profuse sweating. Liver pain may be poorly localized initially and later become pleuritic, referred to the right shoulder tip or localized to the abdominal wall. Within a few weeks, patients lose much weight and often become anaemic; a painful dry cough is common.

The most important clinical finding is liver enlargement with localized tenderness, which should be searched for in the right hypochondrium, the epigastrium, and along all the intercostal spaces overlying the liver. Liver pain, on compression or heavy digital percussion, is a less useful sign. Left-lobe lesions can present as an epigastric mass. Hepatomegaly may be difficult to detect by abdominal palpation when enlargement is mainly upwards, but bulging of the right chest wall may be noted, together with a raised upper level of liver dullness on percussion. Reduced breath sounds or crepitations may be heard at the right lung base.

A well demarcated swelling is seen in the epigastrium in a case of a left lobe amoebic liver abscess

Above: Patient with amoebic liver abscess - A well demarcated swelling is seen in the epigastrium in a case of a left lobe amoebic liver abscess.

Important radiological findings are a raised or locally upward-bulging right diaphragm with immobility on screening, areas of lung collapse or consolidation, and sometimes a pleural effusion. A neutrophil leukocytosis is almost invariable, the ESR is raised, and normochromic normocytic anaemia is common. Liver function tests are frequently completely normal or there may be a raised alkaline phosphatase; less commonly the serum transaminase or bilirubin is elevated. Liver scanning to demonstrate a filling defect is of great value; about 70% of lesions are solitary, but multiple lesions are common in children and those with concurrent dysentery. Ultrasonographic and CT scans are the most useful. Lesions appear round or oval and are usually between 4 and 10 cm in diameter at the time of presentation. On ultrasonography most are hypoechoic with well-defined walls without enhanced echoes. Even when concurrent dysentery is absent, the stools are frequently, but not always, positive for E. histolytica. Colonoscopy may reveal unsuspected lesions.

a) Thoracic X-ray of a patient with amoebic liver abscess showing the elevation of the right hemi-diaphragm. Ultrasound images of: b) Single large amoebic abscess and c) Three amoebic hepatic abscesses. d) Contrasted computed tomography (CT) scan of a single abscess and e) Three clear amoebic liver abscesses

Above: a) Thoracic X-ray of a patient with amoebic liver abscess showing the elevation of the right hemi-diaphragm. Ultrasound images of: b) Single large amoebic abscess and c) Three amoebic hepatic abscesses. d) Contrasted computed tomography (CT) scan of a single abscess and e) Three clear amoebic liver abscesses.


Most complications involve extension of hepatic lesions into adjacent structures: usually the right chest, the peritoneum, and the pericardium. Upward extension usually produces adhesions between the liver, the diaphragm, and the lung; in consequence, subphrenic rupture and amoebic empyema are rare, although a right serous pleural effusion is not uncommon. Untreated, the disease process advances upwards through lung tissue leading to hepatobronchial fistula and expectoration of brownish, necrotic liver tissue, the so-called ‘anchovy sauce’ sputum. Rupture into the peritoneum can occur at any time; it is sometimes the mode of presentation of an amoebic liver abscess, the cause of peritonitis being discovered only at laparotomy. Amoebic pericarditis usually results from upward extension of a left-lobe liver lesion. Initially patients have retrosternal pain and a pericardial friction rub; later rupture or large serous effusion produces cardiac tamponade. The diagnosis is most difficult when an underlying liver abscess was not suspected.

Less commonly the lesion extends through the skin, producing a sinus and cutaneous lesion. The gut, stomach, vena cava, spleen, and kidney are occasionally involved by direct spread. Blood-borne spread to the lung produces a lesion resembling an isolated pyogenic lung abscess. Amoebic brain abscesses due to E. histolytica are rare; most are discovered postmortem. Jaundice occurs when a large lesion compresses the common bile duct or when multiple lesions compress several intrahepatic bile ducts. Rupture into a major bile duct can cause haemobilia. Portal-vein compression occasionally produces portal hypertension and congestive splenomegaly.

Differential diagnosis

Amoebic serology and scanning have now greatly simplified diagnosis. However, a few patients, generally less than 5%, are initially seronegative; scanning patterns may be atypical before lesions have liquefied. Pyogenic abscess, especially when cryptogenic, may be clinically indistinguishable and this condition is quite common in some Asian countries. Other conditions to be distinguished are primary and secondary carcinoma of the liver, lesions of the right lung base and right pleura, subphrenic abscess, cholecystitis, septic cholangitis including that resulting from aberrant Ascaris worms, and liver hydatid cysts.

Needle aspiration of the liver may be necessary for diagnostic or therapeutic purposes (see below). Suspected pyogenic abscess is the main indication for the former; blood cultures should also be taken. Typically the aspirate in hepatic amoebiasis is pinkish-brown, odourless, and bacteriologically sterile; a thinner, malodorous, or frothy aspirate suggests bacterial infection. A therapeutic amoebicide trial is generally preferable to diagnostic needling of the liver. 

Anchovy sauce' pus drained from and amoebic liver abscess

Above: Anchovy sauce' pus drained from and amoebic liver abscess

Cutaneous and genital amoebiasis

Skin ulceration due to E. histolytica produces deep, painful, and foul-smelling lesions that spread rapidly. Secondary bacterial infection is common and may mask the amoebic pathology. Lesions are most frequent in the perianal area, but also occur at colostomy stomas, laparotomy scars, and at the site of skin rupture by a hepatic lesion. Female genital involvement results from faecal contamination, the extension of perianal lesions, or by the formation of internal fistulae from the gut, which can involve the bladder. Lesions of the vulva and uterine cervix may resemble carcinoma. Male genital lesions follow rectal coitus, the lesion beginning as a balanoposthitis and progressing rapidly.

Laboratory diagnosis

Microscopy and culture

The identification of live erythrocytophagous trophozoites in temporary wet mounts is of prime importance because it confirms the diagnosis of invasive amoebic disease. Amoebae should be sought in dysenteric bowel-wall scrapings, the last portion of aspirate from a liver abscess, sputum, and tissue scrapings from skin lesions. In nondysenteric stools, flecks of pus, blood, or mucus should be looked for and examined. The amoebae remain active for about 30 min at room temperature. Other microscopical features of faeces in amoebic colitis are scanty or absent leucocytes, clumped or degenerating red cells, and, sometimes, Charcot–Leyden crystals. If wet preparations are not made or are negative, a portion of the specimen should be preserved in polyvinyl alcohol or sodium acetate–acetic acid–formalin fixative for later smear preparation; alternatively, drying faecal smears should be fixed in Schaudinn’s solution. In either case, fixed smears should be stained with Gomori trichrome or Heidenhain’s iron haematoxylin. 

Cysts and commensal trophozoites of E. histolytica found in wet faecal mounts are indistinguishable from those of E. dispar. The cysts of both species are four-nucleated and can be differentiated from the smaller E. hartmanni using an eyepiece micrometer. Direct mounts are made by emulsifying a small portion of stool in 1% eosin and in Lugol’s iodine; however, the diagnostic sensitivity, per specimen, is only about 30%. Concentration methods for cysts such as formol-ether sedimentation give a 70% sensitivity per specimen. Cultivation of intestinal amoebae from faeces in Robinson’s medium is relatively easy. Species identification requires immunofluorescent staining. Amoebae are often difficult to find microscopically in liver aspirates. Positive cultures from extraintestinal sites do confirm invasive E. histolytica.

DNA and immunological tests

Polymerase chain reaction (PCR) methods can now be used for both E. histolytica and E. dispar using either faecal or tissue material. E. histolytica antigen can be detected in faecal specimens, and assays for antigen in serum have also been used in extraintestinal disease. These new methodologies have excellent sensitivity and specificity. Where they are available, they greatly simplify diagnosis in both amoebic disease and in carriers. They are already revolutionizing our ideas on epidemiology.

Many serodiagnostic methods have been applied to amoebiasis. The most detectable antibody is IgG, with some IgM in active disease. However, seropositivity does not distinguish current and past tissue invasion. The more sensitive methods are indirect haemagglutination, enzyme immunoassay, and indirect immunofluorescence. Latex agglutination and gel-diffusion precipitation are also used, the former being commercially available as a slide test, taking only minutes to perform. Using sensitive tests, over 95% of patients with liver abscess are seropositive, as are about 60% of those with invasive bowel disease; patients with amoeboma are nearly all seropositive. All patients with tissue invasion eventually become seropositive. Titres decline after therapy but may remain positive for 2 years or more with the most sensitive tests.

Patient management


Metronidazole for 5 days will be the first choice in most patients. The usual adult dose of metronidazole is 800 mg thrice daily for 5 or 8 days; the paediatric dose is 35 to 50 mg/kg in three divided doses. The alternative is tinidazole, which has the advantage of a single daily dose, 2 g in adults and 50 to 60 mg/kg in children. A 5- or even a 3-day course may be sufficient for tissue amoebae but rates of parasite elimination from the intestine are low. When nitroimidazoles are contraindicated, or not available, erythromycin is useful in nonsevere colitis.

The synthetic derivative dehydroemetine is a potent tissue amoebicide. It has less cumulative cardiotoxicity than the alkaloid emetine and is more rapidly excreted in the urine. Where appropriate nitroimidazoles are unavailable, as continues to be the case in many tropical contexts, this drug will continue to be life saving, especially when a parenteral drug is needed. A daily intramuscular dose of dehydroemetine of 1.25 mg/kg (maximum 90 mg) is given for 5 days.

Cutaneous and genital amoebiasis responds well to metronidazole, partly perhaps because the lesions often contain anaerobic bacteria. Amoebiasis at other sites is nearly always secondary to hepatic lesions and the chemotherapy will be the same. Metronidazole crosses the blood–brain barrier and should be used in the desperate situation of amoebic brain abscess due to E. histolytica.

All patients with E. histolytica infection treated with a tissue amoebicide should also be given diloxanide to eliminate infection from the bowel and so prevent recurrence of tissue invasion or transmission to others. The dosage of diloxanide for adults is 500 mg thrice daily for 10 days; the daily dose in children is 20 mg/kg daily in three divided doses. Alternatives to diloxanide when it is not available are paromomycin 30 mg/kg daily for 5 to 10 days or iodoquinol 650 mg thrice daily for 20 days, but iodoquinol may cause optic or peripheral neuropathy if the dose is exceeded.

Convalescent carriers, and also infected family contacts, should always be treated. Persons entering temperate countries from the tropics or new residents from such countries should be screened if there is a significant risk of infection; those with E. histolytica faecal antigen, or who are seropositive and have four-nucleated Entamoeba cysts in their stools, should be treated. In these contexts diloxanide is the drug of choice. Metronidazole is less effective even using an 8-day course and side-effects are troublesome. Unfortunately cure rates with tinidazole are very low when followed up at 1 month.

Supportive and surgical management

Intestinal amoebiasis

Supportive management plays a major role in patients with complicated amoebic colitis, with emphasis on fluid and electrolyte replacement, gastric suction, and blood transfusion as necessary. Gut perforation complicating extensive colitis carries a very poor prognosis; management may have to be medical. Parenteral metronidazole is invaluable in these situations because of its activity against anaerobic bacteria in the peritoneum and blood stream. Gentamicin plus a cephalosporin will normally be given as well.

Amoebomas respond well to metronidazole; a slow response should arouse suspicion that the amoebic lesion is superimposed upon other pathology, particularly a carcinoma. Surgical management is important in several situations. Acute colonic perforation in the absence of diffuse colitis or ruptured amoebic appendicitis may be amenable to local repair. In the case of diffuse colitis, local repair, or end-to-end anastomosis, may not be possible because of the poor condition of the gut wall: temporary exteriorization with an ileostomy may be necessary. In fulminant colitis with multiple perforation the viability of the gut wall is uncertain and the only definitive option is total colectomy.

Hepatic amoebiasis

A favourable response to medical treatment alone can be expected in about 85% of patients. Liver abscesses may rupture before, during, or after oral chemotherapy; this requires parenteral metronidazole or dehydroemetine. Intra-abdominal rupture will always require laparotomy. Extension into the pleural or pericardial cavities necessitates drainage of these structures, together with aspiration of the liver lesion; pericardial drainage is most urgent when tamponade is present. Hepatopulmonary lesions generally require drainage of the liver lesion but medical treatment alone has been successful in some cases. Antibiotics will always be needed when the abscess ruptures into the peritoneum or lung.

The most common management problem is slow response to the amoebicide. Patients whose pain and fever do not subside by 72 h are at significantly greater risk of rupture or therapeutic failure, and aspiration is generally to be recommended. A likely explanation of poor initial response is a tense lesion that restricts drug entry. Regular ultrasonographic monitoring is of great value as it will indicate the risk of rupture and guide the aspiration procedure. No change in lesion size on ultrasound can be expected during the first 2 weeks, although its outline may become clearer. Percutaneous aspiration with a wide-bore needle will be possible in most patients; if unsuccessful or anatomically contraindicated, then surgical help should be sought. Catheter drainage is a possible alternative to repeated needle aspiration with very large abscesses. Resolution times for small or moderate lesions are unaffected by aspiration. All patients with hepatic amoebiasis should be give diloxanide to eliminate bowel infection.


Uncomplicated invasive intestinal disease and uncomplicated hepatic amoebiasis should normally have a mortality rate of less than 1%. In complicated disease, the mortality is much greater and may reach 40% for amoebic peritonitis with multiple gut perforation. Prognosis is usually better in centres where the disease is common and more likely to be recognized early. Late diagnosis increases the probability of complicated disease and mortality rises accordingly.

Unless parasitological cure is achieved and the gut completely freed of E. histolytica, clinical relapse is quite common, although probably limited by immunological responses. There is, so far, no evidence of naturally occurring strains of E. histolytica being resistant to normally used drugs. Hepatic scans show that nearly all liver abscesses completely disappear within 2 years; the median resolution time is 8 months. In secondarily infected lesions, bizarre hepatic calcification may be seen years afterwards. Healing of the bowel is remarkably rapid and complete; occasionally fibrous strictures persist after severe dysentery.


Chlorination of water supplies does not destroy amoebic cysts, but adequate filtration will remove them. Regular stool screening of food handlers and domestic staff is of no value, but health education is important with encouragement to have a medical check if diarrhoea occurs.

Visitors to the tropics should not attempt chemoprophylaxis; in particular, long-term unsupervised use of hydroxyquinoline drugs must be strongly deprecated. Simple hygienic measures provide considerable protection. Boiling water for 5 min kills cysts. Routine examinations in temperate countries for returning visitors from the tropics or for new residents coming from such countries is of no value unless E. histolytica can be differentiated from E. dispar. Amoebic serology is particularly useful in those with gut symptoms or a history of dysentery.

Other parasitic gut amoebae including Dientamoeba fragilis 

The nuclei of Entamoeba species have a fine ring of peripheral chromatin and a small central endosome. E. gingivalis has no cystic stage and lives in the mouth within gingival pockets and tonsillar crypts. It is spread by kissing or more indirect oral contact. Its possible role in periodontal disease was formerly dismissed but there is now renewed interest following recognition of its high prevalence in individual lesions in people with this condition; it may act as a bacterial vector within the lesions. It has been found on intrauterine devices that have been removed because of symptoms. Both in the uterus and in the mouth, this amoeba occurs in association with the bacterium Actinomyces israelii.

Five other Entamoeba species are nonpathogenic colonic commensals. Entamoeba coli has eight-nucleated cysts and is the commonest species in most surveys. E. dispar and E. hartmanni both have cysts with four nuclei; the former was previously known as ‘nonpathogenic E. histolytica’ and the latter as ‘small race E. histolytica’. Size is the only simple diagnostic criterion for E. hartmanni; its cysts are less than 10 μm in diameter. The relative prevalence of E. dispar and E. histolytica varies greatly, but the former is usually much more common, especially where sanitation and water supplies are better. E. chattoni is primarily a pig and primate parasite; the cyst has one nucleus and an ‘inclusion body’. Human infections are common in highland Papua New Guinea where humans and pigs may share a peridomestic environment; elsewhere it is rare. Lastly there is E. moshkovskii, which normally lives in soil and sewage; it infects and can be transmitted between humans. It was previously incorrectly referred to a low-temperature variant of E. histolytica.

Endolimax nana and Iodamoeba bütchlii both have nuclei with large endosomes and no visible peripheral chromatin. Cysts of the former are oval in shape with four nuclei; those of the latter are somewhat irregular in shape with a single nucleus and a large glycogen vacuole that stains prominently with iodine. Neither species is pathogenic.

Dientamoeba fragilis is overlooked in most parasitological laboratories and most reports are from developed countries. There is good evidence that it can cause colonic inflammation; however, this is not severe and there is no ulceration or systemic spread. It has no cystic stage and, unless this organism is specifically looked for, it will be missed. In fixed stained smears, about 60% of trophozoites have two nuclei; the endosome is large and lobulated and there is no peripheral chromatin. Alternatively it may be identified in faeces or cultures using immunofluorescence with specific antibody or by PCR; some patients are seropositive. Transmission is direct but possibly within eggs of the threadworm Enterobius. It causes a relatively mild diarrhoeal illness that may persist for several weeks and sometimes there is a superficial eosinophilic colitis. Irritable bowel syndrome may be suspected. Protein-losing enteropathy is reported and blood eosinophilia is quite common. This infection is frequent in some institutional contexts. It is found within some resected appendices but a causal role is unlikely. Electron micrographs and genetic studies indicate that D. fragilis is a trichomonad rather than a true amoeba. The infection responds to metronidazole, but a single dose of ornidazole is also effective.

Free-living amoebae 

A shared feature of these species is the very large central nuclear endosome, quite different from that of E. histolytica, from which differentiation may be necessary in tissue sections. Under dry conditions, trophozoites form resistant cysts that permit survival and also airborne dispersal; cysts can resist chlorination.

Histopathology of amebic meningoencephalitis due to Naegleria fowleri. Direct fluorescent antibody stain

Above: Histopathology of amebic meningoencephalitis due to Naegleria fowleri. Direct fluorescent antibody stain.

Many species are thermophilic and they are one of the causes of ‘humidifier fever’, a form of extrinsic allergic alveolitis presenting with fever, cough, and dyspnoea. Some bacteria including Legionella and Parachlamydia acanthamoebae may live symbiotically within these amoebae persisting within the phagosome, being resistant to lysosomal enzymes. Surprisingly, Legionella can survive encystment: the amoebae provide a refuge for these bacteria when chlorination or other antibacterial measures are applied. Three genera of free-living amoebae cause human infections:

  1. Naegleria is an amoeboflagellate with two trophozoite forms. The amoeba moves rapidly with a single pseudopodium, it can transform into a nonfeeding flagellate in hypotonic media, and these free-swimming forms facilitate dispersal. Cysts are thin walled and spherical.

brain-eating amoeba Naegleria fowleri

Above: Naegleria fowleri

  1. Acanthamoeba has no flagellate form. The small pseudopodia are multiple, thin, and spike-like; they are called acanthopodia. Cysts are thick walled, angulated, and buoyant; their dispersal may be wind borne. Several species are pathogenic but morphological classification is unsatisfactory; rRNA sequences differentiate 15 genotypes. Acanthamoeba is sometimes isolated from throat or nasal swabs or from stool specimens.
  2. Balamuthia is closely related to Acanthamoeba and not a leptomyxid amoeba; it shows little directional movement and has an irregular or branched shape. Cysts are thick walled and spherical. Human infections formerly attributed to Hartmanella are now all thought to be due to Balamuthia mandrillaris, a species described in 1993 from a mandrill baboon that died of meningoencephalitis in San Diego zoo. Balamuthia can only be cultured on tissue culture monolayers. About 100 cases have been reported worldwide, but many are from Latin America. 

Primary amoebic meningoencephalitis due to Naegleria fowleri

Epidemiology and pathology

Nearly all patients give a history of swimming or diving in warm fresh water or spa water between 2 and 14 days before the illness began. Common-source outbreaks occur during warm summer months in temperate countries.

State and local health officials have been notified of a probable case of primary amoebic meningoencephalitis ...

Above: Sign in Kansas, USA - State and local health officials have been notified of a probable case of primary amoebic meningoencephalitis.

Amoebic trophozoites cross the cribriform plate from the nasal mucosa to the olfactory bulbs and subarachnoid space. At autopsy the brain shows cerebral softening and damage to the olfactory bulbs; cysts are never formed in the tissues.

Hemorrhage in the frontal cortex due to Primary Amebic Meningoencephalitis

Above: Hemorrhage in the frontal cortex due to Primary Amebic Meningoencephalitis

Only about 200 cases have been documented since the first human case was reported in 1965. However, some are missed clinically and are discovered at autopsy or in preserved pathological material. Specific antisera enable amoebae to be recognized by immunofluorescence staining.

Clinical features and diagnosis

Patients are immunocompetant; most are young adults and children. Initial nasal symptoms and headache are soon followed by fever, neck rigidity, coma, and, later, convulsions; most die within a few days. Cerebrospinal fluid is often turbid and bloodstained with high protein, low glucose and neutrophils. Amoebae must be urgently looked for in wet specimens using phase-contrast microscopy. Unless amoebae are seen, bacterial meningitis will be suspected; on Gram staining amoebae appear as indistinct smudges. Fixed preparations stained with iron haematoxylin will show full details of nuclear structure. Confirmation is by culture at 37°C using a bacterial lawn on non-nutrient agar. Amphotericin B is an effective drug, it should be given by daily intravenous infusion, and intrathecally; other additional drugs that have been used are miconazole or fluconazole, and rifampicin; in mouse models, azithromycin is effective. So far, very few patients have survived but this may partly be due to diagnostic delays.

Amoeba kills teen girl

Above: "These are rare infections, but super tragic for families," said Jonathan Yoder, the waterborne disease and outbreak surveillance coordinator at the Centers for Disease Control and Prevention. "We don't want to minimize how hard it is for families."The amoeba, called Naegleria fowleri, is the only type that infects humans and is more than 95% lethal. The first death in 2011 occurred in June in Louisiana, according to the CDC.A 16-year-old died Saturday after becoming infected by an amoeba in Brevard County, Florida, according to CNN's affiliate WFTV. The amoeba could have entered the teen's body as the teen swam in a nearby river.Her mother, P.J. Nash-Ryder, said her daughter complained of a headache, threw up 20 times and ran a fever as high as 104 degrees."She would sit up in bed and just look at me, and I would ask her what was wrong," Nash-Ryder said. "She would say, 'I don't know.' And I'd tell her to lay back down. Her eyes were rolling ... and she wouldn't shut them all the way."A spinal tap showed that Naegleria fowleri was present in her spinal fluids

Amoebic keratitis due to Acanthamoeba

Most patients, but not all, are contact lens users. Among the latter, annual incidence rates of 1.49 and 0.33 per 10 000 are reported from Scotland and Hong Kong, respectively, but most figures are lower. Risk factors include poor hygiene when handling lenses and their cases, use of chlorine-based disinfectants, swimming or washing eyes while wearing lenses, handling lenses after gardening, and too prolonged use of plastic or unwashed lenses. The most appropriate disinfectants are chlorhexidine and hydrogen peroxide.

Corneal lesions are painful and present as indolent and progressive ulcers leading eventually to perforation. Recognition may be in the context of lesions unresponsive to antibiotics or corticosteroids. Differentiation must be made from commoner causes of microbial keratitis, including Pseudomonas, Staphylococcus, and herpes simplex. Inflammatory cells are mainly neutrophils. Infection may be by wind-borne cysts upon a damaged epithelium or from contact lenses. Solutions used to store or wash lenses can be contaminated by these amoebae, many of which are resistant to some antiseptics, especially as cysts. Amoebae are found in corneal scrapings or histologically in corneal tissue, but can be missed unless stained with iron haematoxylin or immunofluorescence. PCR methods are now available. Cysts may be seen in tissue. Cultures from fresh material, using a bacterial lawn on non-nutrient agar, should be at 30°C. The majority (90%) of cases are due to genotype T4.

Above: Acanthamoeba. Both one-step hydrogen peroxide and two-step hydrogen peroxide solutions, which involve a second neutralizing solution.

Early aggressive topical treatment using a biguanide together with a diamidine is usually successful, however only the former is cysticidal. Initially, hourly application is needed, and courses may last a month. Additional topical neomycin or chloramphenicol may be necessary. Corneal grafting may be needed.

Granulomatous amoebic encephalitis due to Acanthamoeba and Balamuthia

The main route of infection is the lower respiratory tract followed by haematogenous spread to the brain. Other routes of entry are the skin, the nasopharynx, the lungs and the stomach. Primary lesions have been described at all these sites.

Almost all patients infected by Acanthamoeba are immunocompromised, as are about 75% of those with B.mandrillaris. Soil contamination of skin and craniofacial wounds is an important risk factor. Causes of immunocompromise include malignancy, collagen disorder, alcoholism, diabetes mellitus, AIDS, and steroid or immunosuppressant therapy. Recently two patients with B.mandillaris infection have been described both of whom had received a kidney graft from the same donor. However, in Peru, most of the patients infected with B. mandrillaris have no obvious cause for immunosuppression.

Pathologically lesions resemble chronic bacterial brain abscesses or localized subacute haemorrhagic necrosis; involvement of the meninges is common. Some patients present with headache and meningism, others with evidence of a focal brain lesion.

Above: Granulomatous Amoebic Encephalitis Histology

Unless these amoebae are found in wet tissue preparations or cerebrospinal fluid, the diagnosis will be usually based on histology, often at autopsy. Cysts may be seen in tissue but trophozoites may be missed unless stained with iron haematoxylin or immunofluorescence using specific antisera. Cultural diagnosis at 37°C from fresh biopsies or cerebrospinal fluid is sometimes possible. PCR methods are becoming available.

Granulomatous Amebic Encephalitis in a Child with Acute Lymphoblastic Leukemia Successfully Treated with Multimodal Antimicrobial Therapy and Hyperbaric Oxygen

Above: Granulomatous Amoebic Encephalitis in a Child with Acute Lymphoblastic Leukemia who was Successfully Treated with Multimodal Antimicrobial Therapy and Hyperbaric Oxygen

Survival of patients with this condition is still only rarely reported. Total excision of cerebral lesions is occasionally possible. Drug treatment with combinations of fluconazole with pentamidine, 5-fluorocytosine, sulphadiazine, and azithromycin has been successful in a few patients.

Further reading


Gut amoebae (Entamoeba)

Ali IKM, et al. (2007). Evidence for a link between parasite genotype and outcome with Entamoeba histolytica. J Clin Microbiol, 45, 285–89.

Barwick RS, et al. (2002). Outbreak of amebiasis in Tbilisi, Republic of Georgia, 1998. Am J Trop Med Hyg, 67, 623–31.

Calderaro A, et al. (2006). Entamoeba histolytica and Entamoeba dispar: comparison of two PCR assays for the diagnosis in a non-endemic setting. Trans R Soc Trop Med Hyg, 100, 450–7.

Diamond LS, Clark CG (1993). A redescription of Entamoeba histolytica Schaudinn, 1903 (emended Walker 1911) separating it from Entamoeba dispar Brumpt, 1925. J Eukaryot Microbiol, 40, 340–4.

Fotedar R, et al. (2007). Laboratory techniques for Entamoeba species. Clin Microbiol Rev, 20, 511–32.

Mondal D, et al. (2006). Entamoeba histolytica-associated diarrheal illness is negatively associated with the growth of preschool children: evidence from a prospective study. Trans R Soc Trop Med Hyg, 100, 1032–8.

Pritt BS, Clark CG (2008). Amebiasis. Mayo Clin Proc, 83, 1154–60.[Abstract/Full Text] Ravdin JI, ed. (2000). Amebiasis (tropical medicine: science and practice). Imperial College Press, London.

Singh O, et al. (2009). Comparative study of catheter drainage and needle aspiration in management of large liver abscess. Ind J Gastroenterol, 28, 88–92.

Stanley SL Jr. (2003). Amoebiasis. Lancet, 361, 1025–34.

Dientamoeba fragilis

Ginginkardesler KO, et al. (2008). A comparison of metronidazole and single dose ornidazole for the treatment of dientamoebiasis. Clin Microbiol Infect, 14, 601–4.

Johnson EH, et al. (2004). Emerging from obscurity: biological, clinical, and diagnostic aspects of Dientamoeba fragilis. Clin Microbiol Rev, 17, 553–70.

Stark D, et al. (2005). Detection of Dientamoeba fragilis in fresh stool specimens using PCR. Int J Parasitol, 35, 57–62.

Stark D, et al. (2006). Dientamoebiasis: clinical importance and recent advances. Trends Parasitol, 22, 92–6.

Windsor JJ, Macfarlane L (2005). Irritable bowel syndrome: the need to exclude Dientamoeba fragilis. Am J Trop Med Hyg, 72, 501–2.

Free-living amoebae

Carter R.F. (1972). Primary amoebic meningo-encephalitis. Trans R Soc Trop Med Hyg, 66, 193–208.

Dart JKG, Saw VPJ, Kilvington, S (2009). Acanthamoeba keratitis: diagnosis and treatment update 2009. Am J Ophthalmol, 148, 487–99.

Greub GD, Raoult D (2004). Microorganisms resistant to free-living amoebae. Clin Microbiol Rev, 17, 413–33.

Jung SRL, et al. (2004). Balamuthia mandrillaris meningoencephalitis in an immunocompetent patient: an unusual clinical course and a favorable outcome. Arch Pathol Lab Med, 128, 466–8.

Khan NA. (2008). Acanthamoeba and the blood brain-barrier: the breakthrough. J Med Microbiol, 57, 1051–57.

Khan NA, ed. (2009). Acanthamoeba: biology and pathogenesis. Caister Academic Press, Norwich, UK.

Matin A, et al. (2008). Increasing importance of Balamuthia mandillaris. Clin Microbiol Rev, 21, 435–48

Paltiel ME, et al. (2004). Disseminated cutaneous acanthamebiasis: a case report and review of the literature. Cutis, 73, 241–8.

Visvesvara GS, Moura H, Schuster FL (2007). Pathogenic and opportunistic free-living amoebae: Acanthamoeba spp., Balamuthia mandrillaris, Naegleria fowleri, and Sappinia diploidea. FEMS Imm Med Microbiol, 50, 1–26.

Visvesvara GS, Schuster FL, Martinez AJ. (1993). Balamuthia mandrillaris, N.G., N. Sp., agent of amebic meningoencephalitis in humans and other animals. J Eukaryot Microbiol, 40, 504–14.