Amebiasis

Amebiasis is caused by Entamoeba histolytica, a protozoan that is found worldwide (see Etiology).[1, 2, 3, 4]  Another species of ameba, ​Entamoeba dispar,  has been reported as a potential cause of amebic liver abscess.[5]  The highest prevalence of amebiasis is in developing countries where barriers between human feces and food and water supplies are inadequate (see Epidemiology).

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.

Although most cases of amebiasis are asymptomatic, dysentery and invasive extraintestinal disease can occur. Amebic liver abscess is the most common manifestation of invasive amebiasis, but other organs can also be involved, including pleuropulmonary, cardiac, cerebral, renal, genitourinary, peritoneal, and cutaneous sites. In developed countries, amebiasis primarily affects migrants from and travelers to endemic regions, men who have sex with men, and immunosuppressed or institutionalized individuals.

The National Institute of Allergy and Infectious Diseases (NIAID) has classified E histolytica as a category B biodefense pathogen because of its low infectious dose, environmental stability, resistance to chlorine, and ease of dissemination through contamination of food and water supplies.[6]

E histolytica is transmitted via ingestion of the cystic form (infective stage) of the protozoa. Viable in the environment for weeks to months, cysts can be found in fecally contaminated soil, fertilizer, or water or on the contaminated hands of food handlers. Fecal-oral transmission can also occur in the setting of anal sexual practices or direct rectal inoculation through colonic irrigation devices.

Excystation then occurs in the terminal ileum or colon, resulting in trophozoites (invasive form). The trophozoites can penetrate and invade the colonic mucosal barrier, leading to tissue destruction, secretory bloody diarrhea, and colitis resembling inflammatory bowel disease. In addition, the trophozoites can spread hematogenously via the portal circulation to the liver or even to more distant organs (see Pathophysiology).

Life cycle of amebiasis (Entamoeba histolytica). Courtesy of the Centers for Disease Control and Prevention (CDC).

E histolytica is capable of causing a spectrum of illnesses (see Presentation). Intestinal conditions resulting from E histolytica infection include the following:

• Asymptomatic infection

• Symptomatic noninvasive infection

• Acute proctocolitis (dysentery)

• Fulminant colitis with perforation

• Toxic megacolon

• Chronic nondysenteric colitis

• Ameboma

• Perianal ulceration

• Rectovaginal fistulae

Extraintestinal conditions resulting from E histolytica infection include the following:

• Liver abscess

• Pleuropulmonary disease

• Peritonitis

• Pericarditis

• Brain abscess

• Genitourinary disease

Laboratory diagnosis of amebiasis is made by demonstrating the organism or by employing immunologic techniques. In addition to standard blood tests, other laboratory studies employed for diagnosis include microscopy, culture, serologic testing, and polymerase chain reaction (PCR) assay (see Workup).

Treatment of amebiasis includes pharmacologic therapy, surgical intervention, and preventive measures, as appropriate (see Treatment). Most individuals with amebiasis may be treated on an outpatient basis, though several clinical scenarios may favor inpatient care.

See Common Intestinal Parasites, a Critical Images slideshow, to help make an accurate diagnosis.

E histolytica is a pseudopod-forming, nonflagellated protozoal parasite that causes proteolysis and tissue lysis (hence the species name) and can induce host-cell apoptosis. Humans and perhaps nonhuman primates are the only natural hosts.

Ingestion of E histolytica cysts (see the first image below) from the environment is followed by excystation in the terminal ileum or colon to form highly motile trophozoites (see the second image below). Upon colonization of the colonic mucosa, the trophozoite may encyst and is then excreted in the feces, or it may invade the intestinal mucosal barrier and gain access to the bloodstream, whereby it is disseminated to the liver, lung, and other sites. Excreted cysts reach the environment to complete the cycle.

Entamoeba histolytica cyst. Image courtesy of Centers for Disease Control and Prevention.

Entamoeba histolytica trophozoite. Image courtesy of Centers for Disease Control and Prevention.

Disease may be caused by only a small number of cysts, but the processes of encystation and excystation are poorly understood. The adherence of trophozoites to colonic epithelial cells seems to be mediated by a galactose/N -acetylgalactosamine (GAL/GalNAc)–specific lectin,[4, 7, 8] a 260-kd surface protein containing a 170-kd subunit and a 35-kd subunit. A mucosal immunoglobulin A (IgA) response against this lectin can result in fewer recurrent infections.[9]

Both lytic and apoptotic pathways have been described. Cytolysis can be undertaken by amebapores, a family of peptides capable of forming pores in lipid bilayers.[4] Furthermore, in animal models of liver abscess, trophozoites induced apoptosis via a non-Fas and non–tumor necrosis factor (TNF)-α1 receptor pathway.[10] The amebapores, at sublytic concentrations, can also induce apoptosis.

Cysteine proteinases have been directly implicated in invasion and inflammation of the gut and may amplify interleukin (IL)-1–mediated inflammation by mimicking the action of human IL-1–converting enzyme, cleaving IL-1 precursor to its active form.[4, 11] The cysteine proteinases can also cleave and inactivate the anaphylatoxins C3a and C5a, as well as IgA and immunoglobulin G (IgG).[12, 13]

E histolytica possesses about 100 putative transmembrane kinases (TMKs), which are commonly divided into 9 subgroups. Of these, EhTMKB1-9 is expressed in proliferating trophozoites and induced by serum. In an animal model, it was found to be involved in phagocytosis and to play a role as a virulence factor in amebic colitis.[14] These findings suggest that TMKs such as EhTMKB1-9 may be attractive targets for future drug development.

Epithelial cells also produce various inflammatory mediators, including IL-1β, IL-8, and cyclooxygenase (COX)-2, leading to the attraction of neutrophils and macrophages.[15, 16] Corticosteroid therapy is known to worsen the clinical outcome, possibly because of its blunting effect on this innate immune response.

Additional host defenses, including the complement system, could be inhibited directly by the trophozoites, as is suggested by the finding that a region of the GAL/GalNAc–specific lectin showed antigenic crossreactivity with CD59, a membrane inhibitor of the C5b-9 attack complex in human red blood cells.[17]

Spread of amebiasis to the liver occurs via the portal blood. The pathogenic strains evade the complement-mediated lysis in the bloodstream. Trophozoites that reach the liver create unique abscesses with well-circumscribed regions of dead hepatocytes surrounded by few inflammatory cells and trophozoites and unaffected hepatocytes. These findings suggest that E histolytica organisms are able to kill hepatocytes without direct contact.[4]

Serum antibodies in patients with amebic liver abscess develop in 7 days and persist for as long as 10 years. A mucosal IgA response to E histolytica occurs during invasive amebiasis; however, no evidence suggests that invasive amebiasis is increased in incidence or severity in patients with IgA deficiency.

Cell-mediated immunity is important in limiting the disease and preventing recurrences. Antigen-specific blastogenic responses occur, leading to production of lymphokines, including interferon-delta, which activates the killing of E histolytica trophozoites by the macrophages. This killing depends on contact, oxidative pathways, nonoxidative pathways, and nitric oxide (NO).

Lymphokines, such as TNF-α, are capable of activating the amebicidal activity of neutrophils. Incubation of CD8+ lymphocytes with E histolytica antigens in vitro elicits cytotoxic T-cell activity against the trophozoites. During acute invasive amebiasis, T-cell response to E histolytica antigens is depressed by a parasite-induced serum factor.

Amebiasis is a parasitic infection caused by the protozoal organism E histolytica, which can give rise both to intestinal disease (eg, colitis) and to various extraintestinal manifestations, including liver abscess (most common) and pleuropulmonary, cardiac, and cerebral dissemination.

The genus Entamoeba contains many species, some of which (ie, E histolytica, Entamoeba dispar, Entamoeba moshkovskii, Entamoeba bangladeshi, Entamoeba polecki, Entamoeba coli, and Entamoeba hartmanni) can reside in the human interstitial lumen. Of these, E histolytica is the only one definitely associated with disease, although a 2012 report suggested E moshkovskii to be the causative agent of diarrhea in infants.[18, 19] Although E dispar and E histolytica cannot be differentiated by means of direct examination, molecular techniques have demonstrated that they are indeed two different species, with E dispar being commensal (as in patients with HIV infection) and E histolytica pathogenic.[18]

It is currently believed that many individuals with Entamoeba infections are actually colonized with E dispar, which appears to be 10 times more common than E histolytica[18] ; however, in certain regions (eg, Brazil and Egypt), asymptomatic E dispar and E histolytica infections are equally prevalent.[4] In Western countries, approximately 20%-30% of men who have sex with men are colonized with E dispar.[18]

E histolytica is transmitted primarily through the fecal-oral route. Infective cysts can be found in fecally contaminated food and water supplies and contaminated hands of food handlers. Sexual transmission is possible, especially in the setting of oral-anal practices (anilingus).[20] Poor nutrition, through its effect on immunity, has been found to be a risk factor for amebiasis.[21]

United States statistics

The overall prevalence of amebiasis in the United States is approximately 4%. E dispar infection, which is always asymptomatic, is 10 times more common than E histolytica infection. Moreover, only 10% of E histolytica infections cause invasive disease. Therefore, only 1% of persons with stool microscopy findings that reveal Entamoeba develop symptomatic amebiasis.

Certain groups are predisposed to amebic colitis, including very young patients, pregnant women, recipients of corticosteroids, and malnourished individuals.[4] In 1993, a total of 2970 cases of amebiasis were reported to the Centers for Disease Control and Prevention (CDC); 33% of cases were reported in Hispanic immigrants and 17% in immigrants from Asia or the Pacific Islands.

An increased prevalence of amebiasis is noted in institutionalized persons (especially those with mental retardation), male homosexuals, and persons living in communal settings.

Travelers to endemic areas are at risk for infection and represent most US cases. It is estimated that 14 of every 1,000 returning travelers with diarrhea have amebiasis, which accounts for 12.5% of all microbiologically confirmed cases.[22] Travel to South Asia, the Middle East, and South America poses the highest risk. Amebic liver abscess has been reported after a median duration of 3 months but may occur years later.

The US national death certificate data from 1990 to 2007 identified 134 deaths from amebiasis. A declining trend of deaths has been noted, with a current rate of 5 deaths per year.[23] More than 40% of reported deaths occurred in residents of California and Texas, with US-born persons accounting for most amebiasis deaths.[24]

International statistics

Worldwide, approximately 50 million cases of invasive E histolytica disease occur each year, resulting in as many as 100,000 deaths. This represents the tip of the iceberg because only 10%-20% of infected individuals become symptomatic.[25, 26, 27] The incidence of amebiasis is higher in developing countries.[28] Amebiasis is the second leading cause of death due to parasitic diseases, killing about 40,000-100,000 people per year globally.[29]

Earlier estimates of E histolytica infection, based on examination of stool for ova and parasites, are inaccurate, because this test cannot differentiate E histolytica from E dispar and E moshkovskii. In developing countries, the prevalence of E histolytica, as determined by enzyme-linked immunosorbent assay (ELISA) or polymerase chain reaction (PCR) assay of stool from asymptomatic persons, ranges from 1% to 21%. On the basis of current techniques, it is estimated that 500 million people with Entamoeba infection are colonized by E dispar.[18]

The prevalence of Entamoeba infection is as high as 50% in areas of Central and South America, Africa, and Asia.[30]  E histolytica seroprevalence studies in Mexico revealed that more than 8% of the population were positive.[31] In endemic areas, as many as 25% of patients may be carrying antibodies to E histolytica as a result of prior infections, which may be largely asymptomatic. The prevalence of asymptomatic E histolytica infections seems to be region-dependent; in Brazil, for example, it may be as high as 11%.

In Egypt, 38% of individuals presenting with acute diarrhea to an outpatient clinic were found to have amebic colitis.[4] A study in Bangladesh indicated that preschool children experienced 0.09 episodes of E histolytica -associated diarrhea and 0.03 episodes of amebic dysentery each year. In Hue City, Vietnam, the annual incidence of amebic liver abscess was reported to be 21 cases per 100,000 inhabitants.[32]

An epidemiologic study in Mexico City reported that 9% of the population was infected with E histolytica in the 5-year to 10-year period preceding the study. Various factors, such as poor education, poverty, overcrowding, contaminated water supply, and unsanitary conditions, contributed to fecal-oral transmission.

Several studies have evaluated the association of amebiasis with AIDS.[33, 34, 35, 36, 37, 38] The impact of the AIDS pandemic on the prevalence of invasive amebiasis remains controversial. Some reports suggest that invasive amebiasis is not increased among patients with HIV infection[39] ; however, others suggest that amebic liver abscess is an emerging parasite infection in individuals with HIV infection in disease-endemic areas, as well as in non–disease-endemic areas.[40]

Of 31 patients with amebic liver abscess at Seoul National University Hospital from 1990 to 2005, 10 (32%) were HIV-positive.[41] In a case-control study of persons seeking voluntary counseling and testing for HIV infection, homosexual activity, fecal-oral contamination, lower educational achievement, and older age were associated with increased risk for amebiasis.[42]

Age-related demographics

Symptomatic intestinal amebiasis occurs in all age groups. Liver abscesses due to amebiasis are 10 times more frequent in adults than in children. Very young children seem to be predisposed to fulminant colitis.[43]

Sex-related demographics

Amebic colitis affects both sexes equally.[4] However, invasive amebiasis is much more common in adult males than in females. In particular, amebic liver abscess is 7-12 times more common in men than in women, with a predominance among men aged 18-50 years. The reason for this disparity is unknown, though hormonal effects may be implicated, as the prevalence of amebic liver abscess is also increased among postmenopausal women. Alcohol also may be an important risk factor.[44]

Among prepubertal children, amebic liver abscess is equally common in both sexes.[4] Acuna-Soto et al noted that asymptomatic E histolytica infection is distributed equally between sexes.[45] Therefore, the higher proportion of adult males with invasive amebiasis may be due to a male susceptibility to invasive disease.

Race-related demographics

In Japan and Taiwan, HIV seropositivity is a risk factor for invasive extraintestinal amebiasis.[40] This association has not been observed elsewhere. Among HIV-positive patients, homosexual intercourse, and not immunosuppressed status, seems to be a risk factor for amebic colitis.[46]

Amebic infections can lead to significant morbidity while causing variable mortality. In terms of protozoan-associated mortality, amebiasis is second only to malaria. The severity of amebiasis is increased in the following groups:

• Children, especially neonates

• Pregnant and postpartum women

• Those using corticosteroids

• Those with malignancies

• Malnourished individuals

Intestinal infections due to amebiasis generally respond well to appropriate therapy, though it should be kept in mind that previous infection and treatment will not protect against future colonization or recurrent invasive amebiasis.

Asymptomatic intestinal amebiasis occurs in 90% of infected individuals. However, only 4%-10% of individuals with asymptomatic amebiasis who were monitored for 1 year eventually developed colitis or extraintestinal disease.[18]

With the introduction of effective medical treatment, mortality has fallen below 1% for patients with uncomplicated amebic liver abscess. However, amebic liver abscess can be complicated by sudden intraperitoneal rupture in 2-7% of patients, and this complication leads to a higher mortality.[4]

Case-fatality rates associated with amebic colitis range from 1.9% to 9.1%. Amebic colitis evolves to fulminant necrotizing colitis or rupture in approximately 0.5% of cases; in such cases, mortality may exceeds 40%[47] or even, according to some reports, 50%.

Pleuropulmonary amebiasis has a 15%-20% mortality rate. Amebic pericarditis has a case-fatality rate of 40%. Cerebral amebiasis carries a very high mortality (90%).

A study of 134 deaths in the United States from 1990 to 2007 found that mortality was highest in men, Hispanics, Asian/Pacific Islanders, and people aged 75 years or older.[24] An association with HIV infection was also observed. Although deaths declined during the course of the study, more than 40% occurred in California and Texas. US-born persons accounted for the majority of amebiasis deaths; however, all of the fatalities in Asian/Pacific Islanders and 60% of the deaths in Hispanics were in foreign-born individuals.

Individuals traveling to endemic areas should be advised on practices that minimize the risk of amebiasis, such as the following:

• Avoid drinking contaminated water; use bottled water while traveling if possible

• If local water is to be drunk, purify it by (a) boiling it for more than 1 minute, (b) using 0.22 µm filtration, or (c) iodinating it with tetraglycine hydroperiodide

• Avoid eating raw fruits and salads, which are difficult to sterilize; eat only cooked food or self-peeled fruits if possible

• Wash uncooked vegetables and soak them in acetic acid or vinegar for 10-15 minutes

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]

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:

• Pleuropulmonary amebiasis may produce right-side pleural effusions, empyema, basilar atelectasis, pneumonia, and lung abscess

• Patients with amebic peritonitis have fever and a tender, rigid, and distended abdomen

• Amebic pericarditis presents with features of congestive heart failure; a pericardial friction rub may be audible

• Cerebral amebiasis presents with altered consciousness and focal neurologic signs

• Genital ulcers due to amebiasis have a punched-out appearance and profuse discharge

Complications of amebic colitis include the following:

• Fulminant or necrotizing colitis

• Toxic megacolon

• Ameboma

• Rectovaginal fistula

Complications of amebic liver abscess include the following:

• Intraperitoneal, intrathoracic, or intrapericardial rupture, with or without secondary bacterial infection

• Direct extension to pleura or pericardium

• Dissemination and formation of brain abscess

Other complications due to amebiasis include the following[53, 61] :

• Bowel perforation

• GI bleeding

• Stricture formation

• Intussusception

• Peritonitis

• Empyema

Laboratory diagnosis of amebiasis is made by demonstrating the organism or by employing immunologic techniques.[62, 63, 64, 65, 66]

Findings from basic blood tests may include the following:

• Leukocytosis without eosinophilia (80% of patients)

• Elevated alkaline phosphatase level (80%)

• Elevated transaminase levels

• Mildly elevated bilirubin level

• Reduced albumin level

• Mild anemia

• Elevated erythrocyte sedimentation rate (ESR)

Other laboratory studies employed for diagnosis include microscopy, culture, serologic testing, and polymerase chain reaction (PCR) assay.

Microscopy

Laboratory diagnosis of intestinal amoebiasis in developing countries still relies on labor-intensive and insensitive methods involving staining and microscopic examination of fresh stool smears for trophozoites that contain ingested red blood cells (see the image below).[67] The presence of intracytoplasmic RBCs in trophozoites is diagnostic of E histolytica infection, although some studies have demonstrated the same phenomenon with E dispar.[18, 44]

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.

Examination of a single stool sample has a sensitivity of only 33-50%; however, examination of 3 stool samples over no more than 10 days can improve the detection rate to 85-95%. It should be kept in mind that routine microscopy cannot be relied on to distinguish the pathogenic E histolytica from the nonpathogenic E dispar, E bangladeshi, and E moshkovskii.

Stool leukocytes may be found, but in fewer numbers than in shigellosis.

Stool examination findings in patients with amebic liver abscess are usually negative. Repeated stool sampling in patients with proven amebic liver abscess is positive in 8-40% of cases. Identification of the parasite in a liver abscess aspirate is only 20% sensitive.

The World Health Organization (WHO) recommends that intestinal amebiasis be diagnosed with an E histolytica–specific test, thus rendering the classic ova-and-parasite stool examination obsolete.

Culture

Cultures can be performed either with fecal or rectal biopsy specimens or with liver abscess aspirates.[18] Culture has a success rate of 50-70%, but it is technically difficult. Overall, culture is less sensitive than microscopy.

Xenic cultivation, first introduced in 1925, is defined as the growth of the parasite in the presence of an undefined flora. This technique still is in use today, using modified Locke-egg media. Axenic cultivation, first achieved in 1961, involves growing the parasite in the absence of any other metabolizing cells. Only a few strains of E dispar have been reported to be viable in axenic cultures.

Antigen detection

Enzyme-linked immunosorbent assay (ELISA) is used to detect antigens from E histolytica in stool samples. Several kits are commercially available.[18]

Antigen-based ELISA kits using monoclonal antibodies against the galactose/N -acetylgalactosamine (GAL/GalNAc)–specific lectin of E histolytica (E histolytica II, TechLab, Blacksburg, VA) yield an overall sensitivity of 71-100% and a specificity of 93-100%. One study showed a much lower sensitivity (14.2%). In patients with amebic liver abscess, serum and liver aspirate antigen detection using the same kit was shown to yield a sensitivity of 96% in serum and 100% in liver aspirate.

Other stool detection kits use monoclonal antibodies against the serine-rich antigen of E histolytica (Optimum S; Merlin Diagnostika, Bornheim-Hersel, Germany) or against other specific antigens (Entamoeba CELISA-PATH, Cellabs, Brookvale, Australia; ProSpecT EIA, Remle Inc, Lenexa, KY).

No specific antigen tests are available for the detection of E dispar and E moshkovskii from clinical samples.

Polymerase Chain Reaction

Real-time PCR is considerably more sensitive than stool antigen detection.[68]  Gastrointestinal panels such as the xTAG Gastrointestinal Pathogen Panel (Luminex, Austin, TX) and the BioFire FilmArrray GI Panel (BioFire Diagnostics, Salt Lake City, UT) are currently approved by FDA for the detection of E. histolytica in stool samples.[69, 70]   

Antibody detection

Serum antibodies against amebae are present in 70-90% of individuals with symptomatic intestinal E histolytica infection.[18] Antiamebic antibodies are present in as many as 99% of individuals with liver abscess who have been symptomatic for longer than 1 week. Serologic examination should be repeated 1 week later in those with negative test on presentation. However, serologic tests do not distinguish new from past infection, because the seropositivity persists for years after an acute infection.

ELISA, the assay most commonly used worldwide, measures the presence of serum antilectin antibodies (immunoglobulin G [IgG]). The galactose lectin antigen is present in the serum of 75% of subjects with amebic liver abscess and may be particularly useful in patients presenting acutely, before an IgG antiamebic antibody response occurs. The sensitivity of ELISA for detection of antibodies to E histolytica in patients with amebic liver abscess is 97.9%, and its specificity is 94.8%. False-negative results can occur within the first 7-10 days after infection.

Immunofluorescent assay (IFA) is also rapid, reliable, and reproducible. In the setting of amebic liver abscess, the sensitivity and specificity of IFA were shown to be 93.6% and 96.7%, respectively.

Indirect hemagglutination assay (IHA) detects antibody specific for E histolytica. The antigen used in IHA consists of a crude extract of axenically cultured organisms. Antibody titers of more than 1:256 to the 170-kd subunit of the galactose-inhibitable adherence lectin are noted in approximately 95% of patients with extraintestinal amebiasis, 70% of patients with active intestinal infection, and 10% of asymptomatic individuals.

IHA is very specific (99.1%), but it is less sensitive than ELISA. It is not useful in differentiating acute infection from previous infection, because high titers may persist for years after successful treatment. False-positive reactions at titers higher than 1:256 are rare. ELISA has replaced IHA in most laboratories.[71]

Immunoelectrophoresis, counterimmunoelectrophoresis (CIE), and immunodiffusion (ID) use the precipitation property of antigen-antibody complexes in agar. CIE is time-consuming but has a sensitivity of 100% in invasive amebiasis. ID is simple to perform and thus ideal for laboratories that only rarely perform amebic serology; however, it requires a minimum of 24 hours, compared with 2 hours for IHA or ELISA. ID is slightly less sensitive than IHA and ELISA but is equally specific. Complement fixation (CF) is less sensitive than other techniques.

Although detection of immunoglobulin M (IgM) antibodies specific for E histolytica has been reported, sensitivity in patients with current invasive disease is only about 64%.

Polymerase chain reaction assay

A wide variety of PCR-based methods targeting different genes, including a small-subunit rRNA gene (18S rDNA), a 30-kd antigen gene, a serine-rich protein gene, a chitinase gene, a hemolysin gene, and extrachromosomal circular DNA, have been described for the detection and differentiation of E histolytica, E dispar, and E moshkovskii.[18]

Sensitivities can vary according to sampling and the specific target gene used. Field studies that directly compared PCR with stool culture or antigen-detection tests for the diagnosis of E histolytica infection found these methods to be comparably effective. PCR assay also can be used for detection of E histolytica in liver aspirates for the diagnosis of amoebic liver abscess.[72]

PCR-based tests have been strongly endorsed by the WHO. However, application of PCR-based methods in routine diagnosis still is very limited[73, 74, 75] ; the generation of nonspecific DNA fragments from environmental and clinical samples often leads to false-positive results.

Loop-mediated isothermal amplification assay

The loop-mediated isothermal amplification (LAMP) assay has been applied to the detection of E histolytica in cases of hepatic amebiasis. A study that compared this test with PCR testing in 50 patients with clinical suspicion of amebic liver abscess found that LAMP assay detected five additional abscesses that were missed by PCR assay.[76] The rapidity, operational simplicity, high specificity and sensitivity, and high yield of LAMP assay suggest that it may prove to be a better diagnostic tool than PCR assay for diagnosis of hepatic amebiasis.

Chest radiography may reveal an elevated right hemidiaphragm and a right-side pleural effusion in patients with amebic liver abscess.

On ultrasonograms, amebic liver abscesses usually appear as a solitary homogenous hypoechoic round lesion in the posterosuperior aspect of the right lobe of the liver (70-80% of cases), though multiple abscesses may occur in some patients. In an ultrasonographic evaluation of 212 patients, 34 (16%) had multiple abscesses, 75 (35%) had an abscess in the left lobe, and the remaining 103 (49%) had a solitary abscess in the right lobe.

CT plays an important role in the diagnosis of amebic colitis by demonstrating the presence of colitis with the typical involvement of the cecum and rectum.[77]  Both ultrasonography and CT scanning are sensitive but nonspecific for amebic liver abscess. Ultrasonography is preferred for the evaluation of amebic liver abscess because of its low cost, rapidity, and lack of adverse effects. CT may be slightly more sensitive than ultrasonography. In cerebral amebiasis, CT shows irregular lesions without a surrounding capsule or enhancement.

On CT scans with intravenous (IV) contrast, amebic liver abscess can appear as a rounded, low-attenuation lesion with an enhancing rim. Furthermore, the abscess may be homogenous or septated, with or without observable fluid levels.

Magnetic resonance imaging (MRI) reveals high signal intensity on T2-weighted images. Perilesional edema and enhancement of rim are noted after injection of gadolinium (86% of cases).

Complete resolution of liver abscess may take as long as 2 years. Repeat imaging is not indicated if the patient is otherwise doing well.

Ultrasound- or CT-guided needle aspiration of the liver should be performed when a diagnosis must be established very rapidly; pyogenic liver abscess can present and appear similarly to amebic liver abscess.

Liver abscess aspirate is usually an odorless thick yellow-brown liquid classically referred to as “anchovy paste.” This liquid lacks white blood cells (WBCs) as a result of lysis by the parasite. Amebae are visualized in the abscess fluid in a minority of patients with amebic liver abscess (see the image below). Liver aspiration is indicated only if abscesses are large (> 12 cm), abscess rupture is imminent, medical therapy has failed, or abscesses are present in the left lobe.

Entamoeba histolytica in liver aspirate, trichrome stain. Image courtesy of Centers for Disease Control and Prevention.

The aspirate can be sent for microscopy, culture, antigen detection, and PCR, where available. A Gram stain should also be performed if a pyogenic etiology is suspected clinically.

Rectosigmoidoscopy and colonoscopy with biopsy or scraping at the margin of a colonic mucosal ulcer provide valuable materials for diagnostic information in intestinal amebiasis. Tissue can be sent for microscopic evaluation, culture, and PCR assay, where available.

Indications for endoscopy in suspected intestinal amebiasis include the following:

• Stool examination findings are negative, but serum antibody test findings are positive

• Stool examination findings are negative, but immediate diagnosis is required

• Stool examination and antibody test results are negative, but amebiasis is strongly suspected

• Evaluation of chronic intestinal syndromes or mass lesions is desired

Fulminant colitis is a relative contraindication to colonoscopy, because it increases the risk for intestinal perforation.

On endoscopic examination, small mucosal ulcers covered with yellowish exudates are observed. The mucosal lining between the ulcers appears normal. The mucosa resembles that seen in inflammatory bowel disease (IBD). Biopsy results and a scraping of ulcer edge may reveal trophozoites. Ameboma (a carcinomalike annular lesion) can also be seen, usually in the cecum and ascending colon.[4, 78]

Rectosigmoidoscopy and colonoscopy should be considered before steroids are used in patients with suspected IBD. In a multivariate analysis, the best combination of findings for predicting amebic colitis was the combination of cecal lesions, multiple lesions, and exudates.[79]

The intestinal biopsy specimen should be taken from the edge of ulcers and evaluated for motile trophozoites.

Histopathologic findings may include nonspecific mucosal thickening, multiple discrete ulcers separated by regions of normal-appearing colonic mucosa, diffusely inflamed and edematous mucosa, necrosis, or wall perforation.

Gross pathology of intestinal ulcers due to amebiasis. Image courtesy of Centers for Disease Control and Prevention.

Amebic invasion through the mucosa and into submucosal tissues is the hallmark of amebic colitis; lateral extension through the submucosal tissues gives rise to the classic flask-shaped ulcer of amebic colitis.

Histopathology of amebiasis. Image courtesy of Centers for Disease Control and Prevention.

Histopathology of typical flask-shaped ulcer of intestinal amebiasis. Image courtesy of Centers for Disease Control and Prevention.

Different chemical stains can be used, including periodic acid−Schiff stain, which makes E histolytica appear magenta in color.

Trichrome stain of Entamoeba histolytica cyst in amebiasis. Each cyst has 4 nuclei with characteristically centrally located karyosomes. Cysts measure 12-15 mm.

Treatment of amebiasis includes pharmacologic therapy, surgical intervention, and preventive measures, as appropriate. Most individuals with amebiasis may be treated on an outpatient basis. Several clinical scenarios may favor inpatient care, as follows:

• Severe colitis and hypovolemia requiring intravenous (IV) volume replacement

• Liver abscess that is of uncertain etiology or is not responding to empiric therapy

• Fulminant colitis requiring surgical evaluation

• Peritonitis and suspected amebic liver abscess rupture

Intestinal amebiasis may be mistakenly treated as if it were inflammatory bowel disease (IBD). Accordingly, in all patients with suspected IBD, lower gastrointestinal (GI) endoscopy should be performed before treatment with steroids is initiated.

The following consultations may be helpful:

• Infectious disease specialist

• General surgeon

• GI specialist

Follow-up stool examination after therapy completion is recommended to ensure intestinal eradication. No special diet is recommended.

In endemic areas, asymptomatic infections are not treated. In nonendemic areas, however, asymptomatic infection should be treated[80] ; luminal agents that are minimally absorbed by the GI tract (eg, paromomycin, iodoquinol, and diloxanide furoate) are best suited for such therapy.[81, 82] This recommendation is based on two arguments: first, that invasive disease may develop, and second, that shedding of E histolytica cysts in the environment is a public health concern.[4]

Asymptomatic E dispar infections should not be treated, but because this organism is a marker of fecal-oral contamination, educational efforts should be initiated.[4]

Metronidazole is the mainstay of therapy for invasive amebiasis.[80, 83, 84] Tinidazole has been approved by the US Food and Drug Administration (FDA) for intestinal or extraintestinal amebiasis. Other nitroimidazoles with longer half-lives (ie, secnidazole and ornidazole) are currently unavailable in the United States.

Nitroimidazole therapy leads to clinical response in approximately 90% of patients with mild-to-moderate amebic colitis. Because intraluminal parasites are not affected by nitroimidazoles, nitroimidazole therapy for amebic colitis should be followed by treatment with a luminal agent (eg, paromomycin or diloxanide furoate) to prevent a relapse.[4] A 2019 Cochrane Database Review reported that tinidazole may be more effective than metronidazole and is associated with fewer adverse events.[85] Combination drug therapy may be more effective for reducing parasitological failure compared with metronidazole alone.[85]

Amebic liver abscess of up to 10 cm can be cured with metronidazole without drainage.[86] Drainage is reserved for larger abscesses, for patients who do not exhibit a clinical response to medical therapy by about 72 hours, or for left-lobe abscesses, which could rupture into the pericardium. Resolution of the abscess cavity on serial imaging takes months.[87] Treatment with a luminal agent should also follow.[4]

Chloroquine has also been used for patients with hepatic amebiasis. Dehydroemetine (available from the Centers for Disease Control and Prevention [CDC] Drug Service [404-639-3670]) has been successfully used but, because of its potential myocardial toxicity, is not preferred.

Broad-spectrum antibiotics may be added to treat bacterial superinfection in cases of fulminant amebic colitis and suspected perforation. Bacterial coinfection of amebic liver abscess has occasionally been observed (both before and as a complication of drainage), and adding antibiotics to the treatment regimen is reasonable in the absence of a prompt response to nitroimidazole therapy.

A better understanding of the parasite's metabolic and cellular pathways has been useful in identifying promising drug targets and drug molecules for the treatment of amebiasis.[88]

Loperamide should be avoided in the treatment of amebic colitis.[89]

Surgical intervention is required for acute abdomen that is due to any of the following[90] :

• Perforated amebic colitis

• Massive GI bleeding

• Toxic megacolon (rare and typically associated with the use of corticosteroids)

Surgical intervention is usually indicated in the following clinical scenarios[4] :

• Uncertain diagnosis (possibility of pyogenic liver abscess)

• Concern about bacterial suprainfection in amebic liver abscess

• Failure to respond to metronidazole after 4 days of treatment

• Empyema after amebic liver abscess rupture

• Large left-side amebic liver abscess representing risk of rupture in the pericardium

• Severely ill patient with imminent amebic liver abscess rupture

Unlike pyogenic liver abscess, uncomplicated amebic liver abscess generally responds to medical therapy alone; drainage is seldom necessary and is usually best avoided. When drainage is necessary, image-guided percutaneous intervention (ie, needle aspiration or catheter drainage[91] ) has replaced surgical intervention as the procedure of choice. The indications for drainage of amebic liver abscess include the following:

• Presence of a left-lobe abscess more than 10 cm in diameter

• Impending rupture and abscess that does not respond to medical therapy within 3-5 days

Percutaneous catheter drainage improves treatment outcomes in amebic empyema and is life-saving in amebic pericarditis. It should be used judiciously in the setting of localized intra-abdominal fluid collections.

Amebiasis is prevented by eradicating fecal contamination of food and water through improved sanitation, hygiene, and water treatment. In nonendemic areas, disease transmission can be reduced by early treatment of carriers.

Amebic cysts are not killed by soap or low concentrations of chlorine or iodine; therefore, water in endemic areas should be boiled for more than 1 minute and vegetables should be washed with a detergent soap and soaked in acetic acid or vinegar for 10-15 minutes before consumption.

Avoiding sexual practices that involve fecal-oral contact may reduce the risk of sexual transmission of infective cysts. Because reinfection is possible, family members or close contacts of an index case should be screened.

Household contacts of patients with amebiasis should be screened to prevent spread.[52] Patients with a new diagnosis of inflammatory bowel disease and a history of travel to an endemic area should be screened for amebiasis prior to corticosteroid or other immunosuppressive therapy, as fulminant amebic colitis may result from misdiagnosis.[52]

In humans, natural E histolytica infection does not seem to result in long-term immunity: individuals with a previous amebic liver abscess are as susceptible to a new infection as other members of the population are.[4]

Development of a vaccine for invasive amebiasis is still in its infancy.[92, 93, 94] Many components of the ameba are immunogenic and may serve as targets for a future vaccine, including the galactose/N -acetylgalactosamine lectin, the serine-rich E histolytica protein, cysteine proteinases, lipophosphoglycans, amebapores, and the 29-kd protein. Quach et al reviewed current strategies involved in the development of a vaccine against E histolytica.[95]

Asymptomatic amebiasis in nonendemic areas should be treated with a luminal agent (iodoquinol, paromomycin, or diloxanide furoate) to eradicate infection. Asymptomatic Entamoeba dispar infections should not be treated, but education should be pursued.

Amebic colitis is treated first with a nitroimidazole derivative and then with a luminal agent to eradicate colonization. Paromomycin is safe, well tolerated, and effective in the treatment of intestinal amebiasis, including in patients with HIV infection.[96] Diloxanide is a dichloroacetamide derivative that is amebicidal against trophozoite and cyst forms of E histolytica. It is not available in the United States.

Amebic liver abscess can be cured without drainage by using metronidazole. Treatment with a luminal agent should also follow. Disseminated amebiasis should be treated with metronidazole, which can cross the brain-blood barrier. Empirical antibacterial therapy should be used concomitantly if perforated bowel is a concern.

Class Summary

Several agents are active against anaerobic bacteria and protozoa. Metronidazole is the drug of choice for symptomatic, invasive disease; paromomycin is the drug of choice for noninvasive disease. Because parasites persist in the intestines of 40-60% of patients treated with metronidazole, this drug should be followed with paromomycin to cure luminal infection. Do not give the 2 medications at the same time; the diarrhea that often results from paromomycin might be confused with continuing active intestinal disease from the parasite.

Metronidazole (Flagyl)

Metronidazole kills trophozoites of Entamoeba histolytica in intestines and tissue but does not eradicate cysts from intestines. It appears to be absorbed into cells. Intermediate metabolized compounds are formed and bind DNA and inhibit protein synthesis, causing cell death. Antimicrobial effect may be due to production of free radicals. Metronidazole is indicated for invasive amebiasis.

Tinidazole (Tinidazole)

Tinidazole is a 5-nitroimidazole derivative used to treat susceptible protozoal infections. The mechanism by which it acts against Giardia and Entamoeba species is not known. Tinidazole is indicated for treatment of intestinal amebiasis and amebic liver abscess caused by E histolytica in adults and children aged 3 years and older.

Paromomycin

Paromomycin is an amebicidal and antibacterial aminoglycoside obtained from a strain of Streptomyces rimosus; it is poorly absorbed and is active only against the intraluminal form of amebiasis. Used to eradicate cysts of E histolytica after treatment with metronidazole or tinidazole for invasive disease.

Parasite biochemical pathways are sufficiently different from those of the human host to allow selective interference by chemotherapeutic agents in relatively small doses.

Iodoquinol (Yodoxin)

Iodoquinol is a halogenated hydroxyquinoline that is a luminal amebicide; it acts primarily in the bowel lumen because it is poorly absorbed. It is best tolerated when given with meals. Because iodoquinol is active only against intraluminal form of amebiasis, it is used to eradicate cysts of E histolytica after treatment of invasive disease.

Chloroquine phosphate (Aralen)

Chloroquine phosphate inhibits growth by concentrating within acid vesicles of the parasite, thereby increasing the organism's internal pH. It also inhibits the organism's hemoglobin utilization and metabolism.

In vitro studies involving trophozoites of E histolytica demonstrate that chloroquine possesses amebicidal activity comparable to that of emetine. It is highly effective in treating amebic liver abscess when administered with emetine or dehydroemetine; however, like emetine and dehydroemetine, it is not effective against luminal forms. Irreversible retinal damage does not occur with the dosage and treatment duration used for treatment of hepatic amebiasis.

Dehydroemetine (Mebadin)

Dehydroemetine is preferred to emetine because it is less toxic. It eradicates amebic tissue infections, including liver abscess, but does not act on luminal forms. To eradicate a bowel luminal infection, a luminal amebicide must be used as well. Dehydroemetine is effective only against trophozoites, not against cysts. In the United States, it is available only from the Parasitic Disease Drug Service of the Centers for Disease Control and Prevention (CDC).