Diagnosis and Management of Amebiasis
- 1Departments of Medicine, Microbiology, and Pathology, University of Virginia, Charlottesville, Virginia
- 2Department of Microbiology, Stanford University, Stanford, California
- Reprints or correspondence: Dr. William A. Petri, Jr., MR-4, Room 2115, Department of Medicine, University of Virginia School of Medicine, Charlottesville, Virginia 22908 (wap3g{at}virginia.edu).
Amebiasis in developed countries is most common in immigrants and travelers returning from the third world, and less common in men who have sex with men and immunosuppressed individuals. Infection is acquired by ingestion of food or water containing the cyst form of the parasite Entamoeba histolytica, which is the cause of amebic colitis and liver abscess. The trophozite invades the intestinal epithelium and causes disease by destroying host tissues. Amebic colitis commonly has a subacute onset with weight loss, and is characterized by diarrhea that usually contains occult or gross blood. Because E. histolytica is identical in appearance to the nonpathogenic parasite Entamoeba dispar, amebic colitis is best diagnosed by detection of E. histolytica in stool. Amebic liver abscess is 10 times more common in men than women. A history of alcohol abuse is common, and patients usually present with several weeks of fever and right upper quadrant pain. Most patients with liver abscess do not have concurrent dystentery. Standard treatment with metronidazole plus a luminal agent cures most patients with invasive amebiasis, and drug resistance has yet to be encountered.
E. dispar is the new species name for what had been called “noninvasive” or “nonpathogenic” E. histolytica [1–4]. (Earlier reports that E. histolytica and E. dispar could “convert” in culture [5] were an artifact of culture contamination [6].) E. dispar, Entamoeba moshkovskii, Entamoeba coli, Entamoeba hartmanni, and Endolimax nana are nonpathogenic parasites of the human intestine. There has been no correlation established between infection with these organisms and gastrointestinal symptoms, and antiamebic treatment is not warranted. Dientamoeba fragilis and Entamoeba polecki have been occasionally implicated as a cause of diarrhea, and Entamoeba gingivalis is associated with periodontal disease [7].
E. histolytica is a pseudopod-forming nonflagellated protozoan parasite. The E. histolytica life cycle consists of an infective cyst and an invasive trophozoite form. The quadrinucleate cyst is resistant to gastric acidity and desiccation and can survive in a moist environment for several weeks. Only the trophozoite form invades human tissue. Some interesting differences between the biochemical pathways of E. histolytica and those of higher eukaryotes include its lack of glutathione, its use of pyrophosphate instead of ATP at several steps in glycolysis [8], a unique alcohol-aldehyde reductase [9], and its inability to synthesize purine nucleotides de novo. Molecular phylogenetic analysis of eukaryotic organisms based on sequence comparisons of small subunit rRNA has placed Entamoeba on the lowermost branches of the eukaryotic tree, closest to Dictyostelium. However, E. histolytica shares with higher branching eukaryotes mitochondrial genes that are contained within an apparently biochemically inert remnant organelle [10, 11]. Control of gene expression is fundamentally different from that of other eukaryotes, with a novel conserved sequence (GAAC element) in the RNA polymerase II promoter that specifies the rate and site of mRNA transcription [12]. The genome size of the parasite has recently been estimated to be slightly <20 megabases [13], and the sequencing of the entire genome is under way.
Pathogenesis
Infection occurs on ingestion of the cyst form of the parasite. Understanding of the mechanism of formation of the cyst has come from studies of the distantly related reptilian parasite Entamoeba invadens. Novel proteins and glycoconjugates are produced by E. invadens during encystation, and parasitegalactose-containing molecules and receptors appear to be involved in regulating encystation [14]. The quadrinucleate cyst releases 8 trophozoites in the intestine, which colonize the intestine by adhering to colonic mucin glycoproteins via a galactose and N-acetyl-D-galactosamine (Gal/GalNAc)–specific lectin [15].
Killing of host cells by the extracellular amebic trophozoites requires contact via the parasite Gal/GalNAc lectin. Inhibition of the lectin with Gal/GalNAc prevents host-cell destruction, and cells that lack surface Gal/GalNAc are resistant to adherence and cytolysis [16, 17]. The Gal/GalNAc lectin is a multigene family of 260-kDa heterodimers consisting of heavy (170-kDa) and light (35/31-kDa) subunits linked by disulfide bonds [18]. By expression cloning, the carbohydrate recognition domain was identified within the lectin heavy subunit cysteine-rich region. Interestingly for a hepatic parasite, the carbohydrate recognition domain sequence was identitical to the receptor-binding domain of hepatocyte growth factor, and it competed with hepatic growth factor for binding to the c-Met hepatic growth factor receptor [19]. Genetic proof that the lectin is required for virulence has come from the use of dominant negative mutants: inducible expression of the lectin cytoplasmic tail interfered with “inside-out” regulation of lectin activity and decreased liver abscess size by 90% [20].
Host cells are killed via the induction of an apoptotic cascade (figure 1). The parasite therefore does not so much kill the host as induce it to commit suicide [21]. Apoptotic killing occurs by a novel pathway that is not blocked by bcl-2 and does not require fas or the TNF-α receptor [21, 22]. Recently the amebae have been demonstrated to activate “effector” caspases immediately before destruction of the host cell. Inhibition of these human caspases blocks killing by the amebae [23]. Trophozoites also contain a pore-forming protein that is probably involved in the destruction of endocytosed bacteria. The purified protein is also capable of inducing necrotic death of eukaryotic cells [24]. Parasites that have invaded humans resist destruction by the complement arm of the innate immune system via lectin-mediated inhibition of assembly of the membrane attack complex [25]. Invasion is probably also promoted by the cytoskeleton-induced motility and by the secretion of proteases that degrade the extracellular matrix and antibody and that may also be involved in endocytosis [26–29].
Apoptotic killing of host by Entamoeba histolytica: electron microscopic appearance of murine myeloid cells incubated in vitro with E. histolytica for 30 min at 37°C. Compacted chromatin, cytoplasmic condensation and convolution, membrane blebbing, and nuclear envelope swelling are illustrated. Magnification, ×9425. From [21] (used with permission).
Epidemiology
Amebic colitis and liver abscess are much more common in developing nations than in industrialized countries such as the United States. E. histolytica infection is probably second only to malaria as a protozoan cause of death. The best estimate is that 40–50 million cases of amebic colitis and liver abscess occur annually in the world, resulting in 40,000–110,000 deaths [1]. Most amebic infections occur in Central and South America, Africa, and Asia. The 1987–1988 Mexican national serosurvey, for example, demonstrated an 8.4% seropositivity for E. histolytica. In the year of the serosurvey there were an estimated 1 million cases of amebiasis and 1216 deaths due to E. histolytica infection in Mexico [30]. The prevalence of disease in the developing world is due to fecal-oral spread of infection via contaminated food and water. The vulnerability of the developed world to epidemics has recently been demonstrated by a large and apparently waterborne outbreak in Tbilisi in the former Soviet Republic of Georgia [31].
In the United States, immigrants from and travelers to developing countries are those most likely to develop amebiasis (table 1). A total of 2970 cases of amebiasis in the United States were reported to the Centers for Disease Control and Prevention in 1993; 33% of the patients were Hispanic immigrants and 17% immigrants from Asia or the Pacific Islands [32]. Travelers to the tropics are at a low but definite risk for acquiring amebic infection [33, 34]. One study of 2700 German citizens returning from tropical areas demonstrated a 0.3% incidence of E. histolytica infection [33]. Residents of institutions for the mentally retarded are also at increased risk for amebic colitis and liver abscess [35]. Men who have sex with men were in the past predominantly infected with the nonpathogenic ameba E. dispar [36], but recently invasive amebiasis has been seen in this group (with and without HIV infection) [37].
Gross and microscopic pathology of amebic colitis. A, Colonic ulcers of ∼1 mm diameter. B, Cross-section of one of the ulcers, demonstrating characteristic flask shape. C, Entamoeba histolytica trophozoites in ulcer containing ingested red blood cells and surrounded by eosinophilic debris due to amebic destruction of submucosa. (From collection of the late Dr. Harrison Juniper.)
Antigen detection test for Entamoeba histolytica. Antigen test uses monoclonal antibodies specific for Gal/GalNAc lectin of E. histolytica, allowing rapid identification of E. histolytica (○) in stool. Closely related but nonpathogenic parasite Entamoeba dispar (◊) is not recognized by monoclonal antibodies.
Imaging of hepatic amebic abscess by CT. Patient was young Hispanic man from Charlottesville, Virginia, who had last lived in Mexico a year before.
Table 1
Persons at increased risk for amebiasis.
Amebic liver abscess is 7–12× more common in men than in women. In children, it is an unusual manifestation of amebiasis, and its distribution between the sexes is equal. The typical patient in the United States with amebic liver abscess is a Hispanic immigrant man aged 20–40 years. Infection with E. histolytica presents clinically within a year of immigration to the United States, although in unusual cases, it presents ⩽12 years after immigration [37–42].
Clinical Findings
Asymptomatic colonization. Asymptomatic infection with E. histolytica is defined as the presence of E. histolytica in stool in the absence of colitis or extraintestinal infection (table 2). Colonization with the morphologically identical parasite E. dispar is 3× more common in developing countries and at least 10× more common in developed nations [43–45]. E. histolytica colonization is frequently observed in high-risk settings. For example, in an urban refugee camp in Dhaka, Bangladesh, asymptomatic infection with E. histolytica was present in 5% and E. dispar in 13% of children aged 2–5 years. Colonization with E. histolytica carried a low but definite risk of development of invasive amebiasis: 2 of 17 children developed dysentery during a 1-year follow-up; the other children cleared the infection without specific treatment [46]. Since the stool ova and parasite microscopic examination cannot differentiate E. histolytica from E. dispar or E. moshkovskii [47], a diagnostic test specific for E. histolytica should be used (see Diagnosis, below). Only E. histolytica infection requires treatment [46, 48].
Table 2
Clinical manifestations of Entamoeba histolytica and Entamoeba dispar infection.
Dysentery or colitis. Patients with amebic colitis typically present with a several-week history of gradual onset of abdominal pain and tenderness, diarrhea, and bloody stools [49–52] (table 3). In one series, patients with amebic colitis had an average duration of pre-hospital illness of 21 days, compared with 4 days for patients with shigellosis [50]. Because of the gradual onset, weight loss is a common finding. Surprisingly, fever is present in only the minority (8%–38%) of patients with amebic colitis. Colonic lesions can vary from mucosal thickening only to flask-shaped ulcerations to necrosis of the intestinal wall (figure 2). In at least 70% of cases, the stool is positive for blood (gross or microscopic). Infection with Shigella dysenteriae and Shigella flexneri was more common in children in Dhaka with E. histolytica or E. dispar infection, potentially complicating the management of amebiasis [43].
Table 3
Clinical findings for patients with amebic colitis.
The differential diagnosis of an illness with diarrhea containing gross or occult blood should include infectious etiologies (including amebiasis and infections with Shigella, Salmonella, Campylobacter, and enteroinvasive and enterohemorrhagic Escherichia coli) and noninfectious causes (including inflammatory bowel disease, ischemic colitis, arteriovenous malformation, and diverticulitis). It can be difficult to make the diagnosis of amebic colitis, since the presentation of the illness may be insidious or chronic, bleeding may occur without diarrhea, and fever is an unusual finding. Once the diagnosis has been entertained, conclusive diagnosis can also be problematic, because a single stool examination for parasites is insensitive, histopathologic confirmation of infection on biopsy specimens may be difficult, and serological tests for antibodies to amebae are not always positive. The best initial diagnostic approach at this time is detection of E. histolytica antigen in stool (figure 3).
Unusual manifestations of amebic colitis include acute necrotizing colitis, ameboma (granulation tissue in colonic lumen mimicking colonic cancer in appearance), cutaneous amebiasis, and rectovaginal fistulas. Acute fulminant or necrotizing colitis occurs in ∼0.5% of cases, usually requires surgical intervention, and has a mortality of >40%. Abdominal pain and distension and rebound tenderness are present in most patients with fulminant colitis, and indications for surgery include perforation and persistence of abdominal distension and tenderness while undergoing antiamebic therapy. Partial or total colectomy with exteriorization of the ends is recommended over primary anastomosis, since anastomoses may fail because of the friable condition of the bowel wall [51, 52].
Liver abscess. Roughly 90% of patients with amebic liver abscess are young adult males, although among children and infants the male/female ratio is equal. Liver abscess may present acutely, with fever and right upper abdominal tenderness and pain, or subacutely, with prominent weight loss and less frequent fever and abdominal pain [37–42] (table 4). Most often, patients with liver abscess will present without concurrent colitis, although they sometimes have a history of dysentery within the last year. A history of alcohol abuse is common. A more chronic presentation of 2–12 weeks of weight loss, fever, and abdominal pain has been reported in a subset of patients with single abscesses. In a recent series of patients from San Francisco, one-third were HIV-infected and did not have a history of residence or travel to an area of endemicity [37].
Table 4
Clinical findings for patients with amebic liver abscess.
Findings from physical and laboratory examination include right upper quadrant pain, fever of 38.5°C–39.5°C, leukocytosis, and elevated levels of serum transaminases and alkaline phosphatase. An elevated right hemidiaphragm is a common finding on chest radiograph. Early evaluation of the hepatobiliary system with ultrasound, CT, or MRI is essential to demonstrate the abscess in the liver.
The differential diagnosis of the lesion in the liver should include pyogenic abscess (less probable if the gallbladder and ducts appear normal), hepatoma, and echinococcal cyst (usually an incidental finding unrelated to the acute fever and abdominal pain) (table 5). Eighty percent of the time, amebic abscess is single and is in the right lobe of the liver (figure 4). However, the most common location for a pyogenic abscess is also in the right lobe, so location is not helpful in distinguishing the etiology of an abscess. Patients with pyogenic abscesses are more likely to be aged >50 years, to have preexisting illness, to present with jaundice, pruritus, sepsis, or shock, and to have a palpable mass.
Table 5
Comparison of findings for patients with pyogenic vs. amebic liver abscess.
Aspiration of the abscess is occasionally required to diagnose amebiasis. Amebae are visualized in the pus in only the minority of cases. If the abscess is pyogenic, however, the responsible bacteria should be identified by gram staining and/or culture. Antibodies to E. histolytica are present in the serum of 86%–97% of patients on acute presentation with amebic liver abscess and therefore are very useful diagnostically. Because a significant proportion of the population in developing countries is seropositive, antibody tests are less specific in residents of or immigrants from the developing world (see Diagnosis, below).
Complications of amebic liver abscess include abscess rupture and secondary bacterial infection of the abscess. Impending abscess rupture was associated in 1 study with dyspnea, elevated right hemidiaphragm and pleural effusion, jaundice, anemia, and diabetes mellitus. Patients with these associated risk factors and who do not respond within 72 hours to antiamebic therapy may benefit from percutaneous drainage of the abscess. Aspiration of the abscess is otherwise not required as part of the treatment and can be complicated by secondary bacterial infection. Intrathoracic and intraperitoneal rupture of an amebic liver abscesses can be adequately treated with antiamebic therapy without surgery if secondary bacterial infection is absent.
Unusual extraintestinal manifestations of amebiasis include direct extension of the liver abscess to pleura or pericardium, cutaneous amebiasis, and brain abscess [53].
Laboratory Diagnosis of Amebiasis
The World Health Organization has recommended that intestinal infection be diagnosed with an E. histolytica–specific test [1]. The classic stool ova and parasite examination is therefore obsolete. The TechLab (Blacksburg, VA) E. histolytica stool antigen detection test is the only one available that is specific for the pathogenic ameba E. histolytica [4, 43, 54] (figure 3). All other antigen detection tests currently available cross-react with E. dispar [54], which is a problem because E. dispar infection is 3–10× more common than E. histolytica infection. Published experience with the TechLab E. histolytica stool antigen detection test has shown a sensitivity of 87% and a specificity of >90% compared with culture. Stool ova and parasite examination, in addition to being nonspecific, misses one-half to two-thirds of all E. histolytica colonic infections detected by culture [4, 43, 46]. A second-generation E. histolytica stool antigen test with improved sensitivity will be available shortly [55]. At this time culture and PCR detection of the parasite are research tools and not practical or approved for clinical diagnostic use [4, 54].
An important adjunct to antigen detection is the detection of serum antibodies to amebae. Especially in the case of amebic liver abscess, in which most patients do not have detectable parasites in stool, the presence of antibodies to amebae can be very useful in diagnosis. Tests for antibodies to amebae are ∼90% sensitive for amebic liver abscess and 70% sensitive for amebic colitis. A major problem with current serological tests is that the patient continues to test positive for years after an episode of amebiasis. As a result, currently available serological tests show that a substantial number (10%–35%) of residents of developing countries have antibodies to amebae. Since the vast majority of patients with invasive amebiasis in developed countries are immigrants from developing nations, serological tests may not be as specific as one would hope.
Colonoscopy may be helpful in the diagnosis of amebic colitis if antigen detection tests are negative. Colonoscopy is preferable to sigmoidoscopy for the diagnosis of amebic colitis because disease may be localized to the cecum or ascending colon. Cathartics or enemas should not be used to prepare the patient because they will interfere with the identification of the parasite. Wet preparations of material aspirated or scraped from the base of ulcers should be examined for motile trophozoites. The appearance of amebic colitis may resemble that of inflammatory bowel disease, with granular, friable, and diffusely ulcerated mucosa. Large ulcers with sharply defined borders and pseudomembranes may also be present. The detection rate of trophozoites on histopathologic examination of colonic biopsy specimens from patients with amebic colitis varies in different reports from all to only some of the patients. Biopsy specimens should be taken from the edge of the ulcers. Periodic acid–Schiff stains the parasites a magenta color, increasing the ease of detection in biopsies [56]. E. histolytica has been shown to invade carcinomas, which causes diagnostic confusion.
Ultrasound, CT, and MRI studies of the liver are equally sensitive at detecting amebic abscesses and equally incapable of specifically differentiating an amebic from a pyogenic abscess. Characteristically, an amebic liver abscess will appear on ultrasound as a homogeneous hypoechoic round or oval lesion. On contrast CT scan, amebic abscesses usually appear as rounded, well-defined, low-attenuation lesions, the wall commonly enhancing with contrast. The appearance of the abscess cavity on CT is variable: some appear homogeneous, and others show septations or observable levels of fluid or debris. Follow-up imaging studies are not indicated: 6 months after successful treatment, ultrasonography showed that only one-third to two-thirds of amebic liver abscesses had disappeared [57, 58].
Treatment
Colonization with E. histolytica should be treated with a luminal agent alone; E. dispar infection does not require treatment (table 6) [59]. Oral drugs that are effective against luminal infection include diloxanide furoate (available only through the Centers for Disease Control and Prevention; causes frequent gastrointestinal disturbances and rare diplopia), paromomycin (rarely causes ototoxicity and nephrotoxicity but frequently causes gastrointestinal disturbances), and iodoquinol (rarely causes optic neuritis and atrophy with prolonged use). The recommended duration of treatment with paromomycin is 7 days, with diloxanide furoate is 10 days, and with iodoquinol is 20 days [60]. In a case in which luminal agents cannot be used, it seems a reasonable (if unproven) approach to treat luminal infection with metronidazole and test for cure with the stool antigen detection test.
Table 6
Treatment of amebiasis.
Invasive amebiasis (e.g., colitis, liver abscess) should be treated with metronidazole for 10 days. Although metronidazole has some unpleasant side effects, such as headache, nausea, metallic taste, and a disulfiram-like reaction to alcohol, reaction is rarely severe. Uncommon neurological side effects, such as vertigo or encephalitis, or neutropenia may necessitate discontinuation of treatment. Therapy with metronidazole should be followed with a luminal agent, since patients are otherwise at risk of relapsing from residual infection in the intestine [60]. The majority of patients with amebic liver abscess defervesce after 3–4 days of treatment with metronidazole. Chloroquine and/or percutaneous drainage of the liver abscess are options in addition to metronidazole treatment for the rare patient who does not respond to metronidazole alone [61, 62].
Prevention
Prevention of amebiasis at present requires interruption of the fecal-oral spread of the infectious cyst stage of the parasite. Because cysts are resistant to low doses of chlorine or iodine, in developing countries water must be boiled before it is safe to drink, and raw vegetables must be washed with soap and then soaked in vinegar for 15 min before they can be eaten. Since amebiasis often spreads through a household, it is prudent to screen family members of an index case for intestinal E. histolytica infection.
On the horizon is the development of a vaccine to prevent disease in residents of and travelers to the developing world. Both the amebic adherence lectin and serine-rich antigen have proven effective in the prevention of liver abscess in animal models of the disease [62–67]. The lectin is a particularly attractive candidate antigen because it is required to initiate contact-dependent cytolysis, mediates evasion of the complement membrane attack complex, and is antigenically conserved among geographically distinct isolates of E. histolytica.
Current obstacles to vaccine development include an incomplete understanding of mechanisms of immunity in humans and in animal models of the disease. Protection against liver abscess can be partially transferred with antibodies in rodent models [19, 64]; however, it is likely that protection will also require cellular immune responses against the ameba. For example, macrophages and neutrophils that were activated with IFN-γ and TNF-α could kill E. histolytica trophozoites in vitro, whereas in the absence of the cytokines these cells were themselves killed by the amebae [68, 69]. Another obstacle to be overcome is the lack of well-studied intestinal models of infection, which has blocked testing of any of the current prototype vaccines for protection from intestinal infection. Progress in this field continues, however, to move at a rapid pace. It is exciting to consider that since humans are the only significant reservoir of infection, a vaccine that blocked colonization could lead to elimination of amebiasis.
Footnotes
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Publication of this State-of-the-Art Clinical Article has been made possible by an educational grant from Roche Laboratories.
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Financial support: Work in the authors' laboratories was supported by the National Institutes of Health (grant AI-26649), by a Burroughs Wellcome Fund Career Award in the Biomedical Sciences (to U.S.), and by a Burroughs Wellcome Fund Scholar Award in Molecular Parasitology (to W.P.). The University of Virginia has licensed technology for amebiasis diagnosis to TechLab, Inc.
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The “Conflict-of-Interest Policy” of the Office of Continuing Medical Education, University of California–Los Angeles School of Medicine, requires that faculty participating in a CME activity disclose to the audience any relationship with a pharmaceutical or equipment company which might pose a potential, apparent, or real conflict of interest with regard to their contribution to the program. Neither of the the authors has a conflict of interest.
- © 1999 by the Infectious Diseases Society of America
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