1. Mitra Ranjbar M.D. Associate Professor of Medicine
Amebiasis
Mitra Ranjbar M.D.
Associate Professor of Medicine

2. Intestinal amebiasis is caused by the protozoan Entamoeba histolytica
Intestinal amebiasis is caused by the protozoan Entamoeba histolytica. Worldwide, approximately 40 to 50 million people develop colitis or extraintestinal disease annually with 40,000 deaths.

3. Most infection is asymptomatic, but amebic dysentery, amebic liver abscess, and rarely other manifestations such as pulmonary, cardiac or brain involvement can occur.

4. The parasite exists in two forms, a cyst stage which is the infective form, and a trophozoite stage which is the form that causes invasive disease.

5. Infection occurs following ingestion of amebic cysts; this is usually via contaminated food or water but can be associated with venereal transmission through fecal-oral contact, particularly in sexually active homosexuals.

6. Cysts can remain viable in the environment for weeks to months, and ingestion of a single cyst is sufficient to cause disease.

7. The cysts pass through the stomach to the small intestine where they excyst to form trophozoites. The trophozoites can invade and penetrate the mucous barrier of the colon causing tissue destruction and increased intestinal secretion, and can thereby ultimately lead to bloody diarrhea.

8. It has become progressively apparent over the last 15 to 20 years that intestinal amebae with identical morphologic appearances can in fact be one of three species, E. histolytica or E. dispar or E. moshkovskii.

9. The clinical importance is that E. dispar and E
The clinical importance is that E. dispar and E. moshkovskii are non-pathogenic and do not cause clinical disease; thus all symptomatic disease is caused by E. histolytica.

10. PATHOGENESIS
Investigators have shown that the pathogenicity of amebic trophozoites is associated with their adherence to colonic epithelial cells via a specific lectin (the galactose/N-acetylgalactosamine lectin).

11. Mammalian cells without N-terminal galactose or N-acetylgalactosamine residues are resistant to adherence by amebic trophozoites, which is consistent with an important role for the lectin in adhesion.

12. This lectin also plays a role in immunity, since mucosal immunity against the lectin seems to mediate some degree of protection from invasive disease following colonization.

13. One study from Bangladesh showed that children with a mucosal IgA response against the lectin had 86 percent fewer new infections during a one-year period than children without this response and while reinfected had a lower incidence of infection and disease over a four year follow-up period.

14. Colitis results after penetration of the trophozoite through the intestinal mucous layer, which would otherwise act as a barrier to invasion.

15. the trophozoite is able to kill both epithelial cells and inflammatory cells. This is thought to occur through a number of different mechanisms including:

16. Secretion of proteinases by the trophozoites
Lysis of target cells via a contact-dependent mechanism
Killing of mammalian cells by apoptosis (programmed cell death)
Formation of amebepores, a family of small peptides that can form pores in lipid bilayers, resulting in cytolysis of infected cells
Changes in intestinal permeability, probably via disruption of tight-junction proteins

17. EPIDEMIOLOGY
Amebiasis is a worldwide disease, however developing countries have significantly higher prevalence rates because of poorer socioeconomic conditions and sanitation levels.

18. Areas that have high rates of amebic infection include India, Africa, Mexico and parts of Central and South America. E. dispar and E. moshkovskii infections occurs approximately ten times more commonly than infection with E. histolytica, and the overall prevalence of amebic infection may be as high as 50 percent in certain developing areas.

19. Although many epidemiologic studies of amebiasis were performed prior to the recognition of the three distinct species, more recent antigen detection methods and serology have been used to estimate the prevalence of E. histolytica infection.

20. As an example, the seroprevalence of E
As an example, the seroprevalence of E. histolytica in one Mexican study was 8.4 percent. In another series from urban Bangladesh, children had a 4.2 percent prevalence rate of E. histolytica infection.

21. Infection with pathogenic E
Infection with pathogenic E. histolytica is not a common cause of travelers' diarrhea, and gastrointestinal infection is uncommon in travelers who have spent less than one month in endemic areas. In one prospective study of German travelers to the tropics, only 0.3 percent had pathogenic E. histolytica infection.

22. In Japan and Taiwan, however, E
In Japan and Taiwan, however, E. histolytica is much more prevalent amongst male homosexuals. Invasive, extraintestinal amebiasis (eg, hepatic abscesses) are more frequent in HIV-infected patients in these countries, whereas the same is not true elsewhere, including Mexico.

23. CLINICAL MANIFESTATIONS
E. dispar infection and E. moshkovskii infections and 90 percent of E. histolytica infections are asymptomatic.

24. The strain of E. histolytica as well as host factors such as genetic susceptibility, age and immunocompetence ultimately determine whether infection leads to asymptomatic or invasive disease.

25. Young age, pregnancy, corticosteroid treatment, malignancy, malnutrition, and alcoholism are risk factors for severe disease and increased mortality following E. histolytica infection.

26. Clinical amebiasis generally has a subacute onset, usually over one to three weeks. Symptoms range from mild diarrhea to severe dysentery producing abdominal pain (12 to 80 percent), diarrhea (94 to 100 percent), and bloody stools (94 to 100 percent).

27. Weight loss is present in just under 50 percent of patients
Weight loss is present in just under 50 percent of patients. Fever occurs in 8 to 38 percent.

28. Fulminant colitis with bowel necrosis leading to perforation and peritonitis occurs in approximately 0.5 percent of cases but is associated with a mortality rate of more than 40 percent. Toxic megacolon can also develop.

29. Intestinal amebiasis can probably also present as a chronic, nondysenteric syndrome of diarrhea, weight loss, and abdominal pain which can last for years and can mimic inflammatory bowel disease, although one study has called into question whether nondysenteric intestinal amebiasis truly exists.

30. No differences were noted in the occurrence of E
No differences were noted in the occurrence of E. histolytica infection, assessed by stool examination and serology, among 144 Indian patients with intestinal symptoms and 100 controls without symptoms.

31. Amebic colitis has been recognized even in asymptomatic patients
Amebic colitis has been recognized even in asymptomatic patients. In Japan, for example, amongst 5193 asymptomatic subjects undergoing colonoscopy solely for positive fecal occult blood tests, four were found to have amebic ulcerative lesions in the cecum or ascending colon.

32. Localized colonic infection resulting in a mass of granulation tissue forming an ameboma is another uncommon presentation, which can mimic colon cancer Patients with amebomas usually are found to have a tender palpable mass. Perianal cutaneous amebiasis and rectovaginal fistulae are other rare complications of amebic intestinal disease.

33. DIAGNOSIS
The demonstration of cysts or trophozoites in the stool suggests an intestinal amebic infection, but microscopy cannot differentiate between E. histolytica and E. dispar or E. moshkovskii strains.

34. Antigen testing
Fecal and serum antigen detection assays that use monoclonal antibodies to bind to epitopes present on pathogenic E. histolytica strains but not on nonpathogenic E. dispar or E. moshkovskii strains are now being used commercially to detect E. histolytica infection.

35. Antigen detection kits using either enzyme linked immunosorbent assay (ELISA), radioimmunoassay or immunofluorescence have been developed.

36. Antigen detection has many advantages including:
Ease and rapidity of the tests
Ability to differentiate between
Greater sensitivity than microscopy
Potential for diagnosis in early infection and in endemic areas where serology is less useful

37. Serology
Serology is also useful for diagnosing amebiasis. Infection with E. histolytica results in the development of antibodies, while E. dispar infection does not. Antibodies will usually be detectable within five to seven days of acute infection and may persist for years.

38. Between 10 to 35 percent of uninfected individuals in endemic areas have anti-amebic antibodies due to previous, often undiagnosed infection with E. histolytica.

39. Consequently, negative serology in these populations helps to exclude disease, but a positive serology is not particularly helpful since it does not distinguish between acute infection and past exposure to the parasite.

40. Indirect hemagglutination (IHA) is the most sensitive assay and is positive in approximately 90 percent of patients with symptomatic intestinal infection.

41. Agar gel diffusion and counterimmunophoresis are less sensitive than IHA but usually only remain positive for 6 to 12 months which may make them more useful in endemic areas. A commercially available ELISA that has a sensitivity of 93 percent compared to IHA has also been developed.

42. Stool examination
Stool microscopy is a relatively poor method for making the diagnosis of intestinal amebiasis since it is less sensitive than antigen testing (misses one-half to two-thirds of E. histolytica colonic infections detected by culture ) and because it cannot differentiate between species.

43. Moreover, fecal leukocytes and nonpathogenic intestinal protozoans may be misidentified by inexpert diagnosticians.

44. Stool specimens will invariably be positive for microscopic (and often macroscopic) blood when invasive intestinal amebic disease is present. However, ingested erythrocytes have been observed with E. dispar organisms as well, so the presence of ingested erythrocytes is not pathognomonic for E. histolytica infection. Fecal leukocytes are not always present since white cells may be destroyed by the organisms.

45. A saline wet mount of stool should be performed and also a fresh smear stained with iron hematoxylin and/or Wheatley's trichrome to look for trophozoites; fixation with polyvinyl alcohol for delayed staining is often useful.

46. Specimens can also be concentrated and stained with iodine to detect cysts. Organism excretion can vary; thus, a minimum of three specimens on separate days should be sent to detect 85 to 95 percent of infections.

47. Other methods
Detection of parasitic DNA or RNA in feces via probes can also be used to diagnose amebic infection and to differentiate between the three different strains, but these methods currently are primarily reserved as research tools. Fecal culture is available but only in the research setting.

48. PCR techniques can detect E
PCR techniques can detect E. histolytica in stool specimens but are not currently widely available. One study showed that PCR was significantly more sensitive than either microscopy or cultures, and that it was 100 percent specific for E. histolytica.

49. Visual inspection of the colon
Sigmoidoscopy and/or colonoscopy can be performed either to make the diagnosis of amebiasis or to exclude other causes of the patients' symptoms.

50. Scrapings or biopsy specimens, best taken from the edge of ulcers, may be positive for cysts or trophozoites on microscopy, and antigen testing for E. histolytica may be positive.

51. Colonic lesions in amebic dysentery range from nonspecific mucosal thickening and inflammation to classic flask-shaped amebic ulcers. However, colonoscopy is not recommended as a routine diagnostic approach since intestinal amebic ulcerations increase the likelihood of perforation during instillation of air to expand the colon.

52. Recommendations
Antigen detection assays are the best current means for diagnosing intestinal amebiasis since they are sensitive, specific, rapid, easy to perform and distinguish E. histolytica from E. dispar infections.

53. Antibody measurements are also available but remain positive for years
Antibody measurements are also available but remain positive for years. Stool specimens for microscopy are far less sensitive than antigen detection and cannot differentiate between the species.

54. TREATMENT
E. dispar and E. moshkovskii infections does not require treatment, whereas even asymptomatic E. histolytica infection should be treated because of the potential risk of developing invasive disease and the risk of spread to family members.

55. In countries where amebic infections are endemic, asymptomatic patients incidentally found to have stools positive for amebae are often presumed to have infection with E. dispar/E. moshkovskii and are not further evaluated or treated.

56. When antigen tests that can differentiate between E. dispar/E
When antigen tests that can differentiate between E. dispar/E. moshkovskii and E. histolytica are more widely available in these countries, this practice may change.

57. Metronidazole
The goals of antibiotic therapy of intestinal amebiasis are both to eliminate the invading trophozoites and to eradicate intestinal carriage of the organism.

58. Metronidazole (500 to 750 mg PO three times daily for 7 to 10 days in adults and 35 to 50 mg/kg per day in three divided doses for 7 to 10 days in children) is the usual treatment for invasive colitis. The cure rate is approximately 90 percent.

59. Metronidazole is so well absorbed from the gastrointestinal tract that intravenous therapy offers no significant advantage as long as the patient can take oral medications and has no major defect in small bowel absorption.

60. A small number of patients have been treated with 2 g/day for three days, but cure rates were only 55 to 59 percent [39,40]. Metronidazole resistance in E. histolytica trophozoites has not been reported.

61. Alternative drugs
In countries where they are available, tinidazole and ornidazole are alternative therapies. Tinidazole (2 g PO daily for three days) has a cure rate of 90 to 93 percent [39,40]. It has been approved for the treatment of amebiasis by the United States Food and Drug Administration (FDA).

62. Agents for eliminating luminal cysts
Following therapy for invasive amebiasis with a nitroimidazole, treatment with a luminal agent may be required to eliminate intraluminal encysted organisms.

63. A ten day course of metronidazole will eliminate intraluminal infection in many cases, but a second agent is still recommended.

64. Intraluminal infection can be treated with one of the following regimens:
Paromomycin — mg/kg per day orally in three divided doses for 7 days

65. Diiodohydroxyquin (iodoquinol) — 650 mg orally three times daily for 20 days for adults and mg/kg per day in 3 divided doses for 20 days for children
Diloxanide furoate — 500 mg orally three times daily for 10 days for adults and 20 mg/kg per day in 3 divided doses for 10 days for children; this drug is not currently available in the United States.

66. Parasitologic cure rates are approximately 86 to 90 percent
Parasitologic cure rates are approximately 86 to 90 percent. Follow-up stool examinations are required after completion of the therapy since no regimen is completely effective.

67. Treatment of associated peritonitis
In patients who have suspected or proven peritonitis, broad spectrum antibacterial therapy should also be given.

68. Intestinal amebiasis can usually be treated medically with antibiotics, but surgical intervention is required if a significant bowel perforation occurs or if patients with abscesses following perforation fail to respond to antibiotic therapy.

69. It is unclear how beneficial surgical intervention is, even in cases of fulminant colitis with perforation, because it is often impossible to successfully suture the necrotic bowel. The one definitive indication for surgery is toxic megacolon which requires colectomy.

70. Recommendations
Metronidazole (500 to 750 mg PO three times daily for 7 to 10 days in adults and 35 to 50 mg/kg per day in three divided doses for 7 to 10 days in children) is the usual treatment for invasive colitis.

71. Patients with invasive colitis should generally be treated with a luminal agent following a nitroimidazole. Paromomycin and diiodohydroxyquin (iodoquinol) are available in the United States.
Asymptomatic patients with E. histolytica (and not E. dispar/E. moshkovskii) should be treated with an intraluminal agent alone.

72. PREVENTION
Prevention of amebic infection in travelers involves avoidance of drinking unboiled or unbottled water in endemic areas. Uncooked food such as fruit and vegetables that may have been washed in local water should also not be consumed.

73. Amebic cysts are resistant to chlorine at the levels used in water supplies, but disinfection with iodine may be effective. Avoiding sexual practices that may lead to fecal-oral contact is also advisable.

74. Vaccine development
There is some epidemiologic evidence in humans of partial acquired immunity to the organism. Protection from invasive disease has been associated with mucosal IgA antibodies to the amebic adherence lectin.

75. However, recurrent intestinal infection and persistent colonization does occur despite detectable antiamebic antibodies; even in the study in children from Bangladesh, the immunity that developed was short-lived.

76. Thus, it seems probable that acquired, but incomplete, immunity against infection occurs, and a vaccine that can reduce infection and/or invasive disease may therefore be feasible.

77. The relative importance of systemic and mucosal, cellular and humoral immunity is unclear. Several amebic proteins associated with virulence have been identified and are being studied as potential vaccine components. Development of both parenteral and oral vaccines for humans is in progress.

78. Extraintestinal Entamoeba histolytica amebiasis
AMEBIC LIVER ABSCESS — Amebic liver abscess is the most common extraintestinal manifestation of amebiasis. Amebae establish hepatic infection by ascending the portal venous system.

79. Epidemiology — There are some epidemiologic features of amebic liver abscess that are strikingly different from intestinal amebiasis.

80. Amebic liver abscess (and other extraintestinal disease) is 7 to 10 times more common in adult men despite an equal gender distribution in children and despite an approximately equal sex distribution of colonic amebic disease.

81. Although the reasons for the differences in gender distribution have not been fully explained, some possible mechanisms that have been suggested include: hormonal effects (postmenopausal women have increased rates) and a potential role of alcoholic hepatocellular damage in creating a nidus for portal seeding.

82. Intestinal amebiasis is not common in short term travelers, but amebic liver abscesses can occur after travel exposures as short as four days. In one study, 35 percent of travelers with amebic liver abscess had spent less than six weeks in an endemic area.

83. Most patients diagnosed with amebic liver abscess in the United States will either be from a country where amebiasis is endemic or will have a relevant travel history.

84. Conditions that affect cell mediated immunity such as extremes of age, pregnancy, corticosteroid therapy, malignancy, and malnutrition may also increase the chances that E. histolytica infection results in invasive disease with liver involvement.

85. Clinical manifestations
Patients with amebic liver abscess usually present acutely with one to two weeks of fever (38.5 to 39.5ºC) and right upper quadrant pain.

86. Concurrent diarrhea is present in less than one-third of patients, although some patients will give a history of having had dysentery within the previous few months.

87. Jaundice is uncommon. For travelers returning from an endemic area, presentation usually occurs within 8 to 20 weeks (median 12 weeks) and will be within five months of their return in 95 percent of patients, although a longer lag (sometimes years) has been reported.

88. Occasionally, patients have a more chronic presentation with months of fever, weight loss and abdominal pain. In these patients, hepatomegaly and anemia are often associated findings.

89. Patients with secondary cardiac or pulmonary involvement may present with symptoms primarily due to these complications. Examination of patients with uncomplicated liver abscess will reveal hepatomegaly and point tenderness over the liver in approximately 50 percent of cases. Clinical jaundice occurs in less than 10 percent of patients.

90. Occasionally, the abscess ruptures into the peritoneum causing clinical peritonitis (2 to 7 percent) .
Patients with amebic hepatic abscesses may also have ileocecal mass lesions, that are frequently amebomas

91. Diagnosis
Patients with amebic liver abscess will often have a leukocytosis (>10,000/mm3) without eosinophilia. Liver function testing will reveal an elevated alkaline phosphatase in 80 percent, and hepatic transaminases may also be raised.

92. Fecal microscopy will be positive for amebae in only 18 percent of cases, although culture, which is only available as a research tool, is positive in approximately 75 percent . Other common nonspecific findings include an abnormal chest radiograph and proteinuria.

93. Imaging
Radiologic imaging of the liver is the major method for diagnosing amebic liver abscess. This can be accomplished with ultrasound, computed tomography (CT) or magnetic resonance imaging (MRI).

94. At least one of these studies should be performed in all suspected cases. A single abscess in the right lobe of the liver will often be found (70 to 80 percent), although multiple lesions can be present.

95. Although none of these tests can definitively differentiate among a pyogenic abscess, an amebic abscess, or malignant disease, a patient with appropriate epidemiologic risk factors and a suspicious lesion on scanning can usually be presumed to have amebic infection, at least while awaiting other test results.

96. Even with appropriate therapy, the abscess may appear to increase in size initially and complete radiologic resolution may take up to two years; thus, repeated imaging is usually not helpful. Sometimes a gallium scan may be useful since amebic abscesses are "cold", possibly with a bright rim, whereas pyogenic abscesses are "hot".

97.   Serologic testing
Serum antibodies will be detectable in 92 to 97 percent of patients at the time of presentation with amebic liver abscess. Eventually 99 percent of patients will have positive antibody tests, but serologic testing may be negative in the first seven days.

98. Up to 25 percent of uninfected individuals in endemic areas have anti-amebic antibodies due to previous, often undiagnosed infection with E. histolytica.

99. Consequently, positive serology in these populations is not nearly as helpful although negative serology still assists in excluding this infection.

100. ndirect hemagglutination (IHA) is the most sensitive agar gel diffusion and counterimmunophoresis are less sensitive than IHA but usually only remain positive for 6 to 12 months, which may make them more useful in endemic areas.

101. Other new serologic tests based upon recombinant E
Other new serologic tests based upon recombinant E. histolytica antigens have also been developed. In one study performing serial tests with two different antigens in 20 patients with known amebic liver abscesses, more than 50 percent became negative in one or both tests within 180 days.

102. Other newer techniques for diagnosis include a rapid enzyme immunoassay with a sensitivity of 93 percent compared to IHA and a serum antigen detection test that is positive in 75 percent of patients with amebic liver abscess . These newer techniques may prove to be particularly useful in endemic areas.

103. Aspiration
Needle aspiration has not conventionally been recommended and is not usually required. However, if there has been no response to antibiotic therapy after three to five days, if a cyst appears to be at imminent risk of rupture, or if rapid exclusion of other diagnoses is necessary, aspiration under ultrasound or CT guidance should be strongly considered.

104. In cases refractory to medical therapy, percutaneous aspiration has enhanced clinical recovery .Amebic liver abscesses contain acellular, proteinaceous debris and an "anchovy paste", chocolate colored fluid, consisting predominantly of necrotic hepatocytes.

105. Trophozoites are seen on microscopy of the aspirate in fewer than 20 percent of cases and are often present only in the peripheral parts of the abscess, invading and destroying adjacent tissue.

106. This is in contrast to aspirates from pyogenic liver abscesses in which organisms (usually mixed morphologies) are usually readily apparent by Gram stain. Antigen tests and polymerase chain reaction (PCR) on aspirated material may also be helpful in establishing the diagnosis

107. Although highly uncommon, amebic hepatic abscesses may become secondarily infected with enteric bacteria, so aspirated fluid should be cultured for bacteria.

108. Suggested clinical approach
In cases where the clinician suspects the diagnosis of amoebic liver abscess because of a consistent clinical history (usually acute presentation with 1 to 2 weeks of fever and right upper quadrant pain), relevant epidemiology (resident in, migration from or travel to an endemic area), and suspicious imaging (most common in the right lobe and 70 to 80 percent are solitary lesions), it is reasonable to send off serology and begin empiric therapy.

109. Compared to patients with pyogenic liver abscesses, those with amebic abscesses are more also more likely to be <50 years old and male . In addition, those with pyogenic abscesses are more likely to be jaundiced and septic and have a history of abdominal surgery or biliary disease.

110. If the results of serology are already known to be positive, then a trial of empiric therapy is almost always justified, even in endemic countries.

111. In situations where the epidemiologic history, clinical profile and serologic studies (taken >7 days from symptom onset) are non-characteristic or non-diagnostic, or if the patient deteriorates on therapy, then aspiration may need to be performed to exclude alternative diagnoses, in particular a pyogenic abscess. In addition, therapeutic aspiration may need to be considered if there is a high risk of rupture (particularly lesions in the left lobe).

112. OTHER EXTRAINTESTINAL SITES
Although rare, amebic disease occasionally involves other sites. Brain abscess, lung disease, cardiac involvement, perinephric or splenic abscess, vaginal or uterine involvement, rectovaginal fistulae and cutaneous disease can all occur.

113. Pleuropulmonary infection
Pleuropulmonary involvement following E. histolytica infection often occurs secondary to a liver abscess. Mechanisms of lung and/or pleural infection include: development of a sympathetic serous effusion, rupture of a liver abscess into the chest cavity leading to empyema, or hematogenous spread resulting in parenchymal infection.

114. Factors that have been shown to contribute to the development of pulmonary amebiasis include: malnutrition, chronic alcoholism, and atrial septal defect with left to right shunt.

115. Pulmonary symptoms including cough, dyspnea and pleuritic chest pain may be present, and patients are often quite ill. If a bronchopleural fistula develops, patients may expectorate necrotic material which can include contents of the liver abscess.

116. An abnormality on chest radiograph will be seen an approximately 50 percent of patients with an amebic liver abscess, most commonly elevation of the right hemidiaphragm . This latter finding does not necessarily signal pulmonary involvement in the infection.

117. Cardiac infection
Cardiac involvement in amebic disease is much rarer than pleuropulmonary disease. When it occurs, it usually results from rupture of a liver abscess, particularly abscesses involving the left lobe of the liver, into the pericardium.

118. This can result in severe chest pain, congestive cardiac failure and pericardial tamponade. The associated mortality is high.

119. Brain abscess
Cerebral amebiasis is a rare cause of brain abscess which results from hematogenous dissemination. This form of amebiasis is characterized by an abrupt onset of symptoms and rapid progression to death if untreated.

120. Lesions may be seen on CT scan as irregular foci without a capsule or surrounding enhancement . CNS abscesses due to E. histolytica are distinct from CNS lesions caused by free-living amebae. Treatment with metronidazole should be started immediately, and surgical intervention for decompression and/or tissue biopsy may be required.