Amebiasis
2008; Elsevier BV; Volume: 83; Issue: 10 Linguagem: Inglês
10.4065/83.10.1154
ISSN1942-5546
AutoresBobbi S. Pritt, C Graham Clark,
Tópico(s)Gallbladder and Bile Duct Disorders
ResumoAmebiasis is defined as infection with Entamoeba histolytica, regardless of associated symptomatology. In resource-rich nations, this parasitic protozoan is seen primarily in travelers to and emigrants from endemic areas. Infections range from asymptomatic colonization to amebic colitis and life-threatening abscesses. Importantly, disease may occur months to years after exposure. Although E histolytica was previously thought to infect 10% of the world's population, 2 morphologically identical but genetically distinct and apparently nonpathogenic Entamoeba species are now recognized as causing most asymptomatic cases. To avoid unnecessary and possibly harmful therapies, clinicians should follow the diagnostic and treatment guidelines of the World Health Organization. Amebiasis is defined as infection with Entamoeba histolytica, regardless of associated symptomatology. In resource-rich nations, this parasitic protozoan is seen primarily in travelers to and emigrants from endemic areas. Infections range from asymptomatic colonization to amebic colitis and life-threatening abscesses. Importantly, disease may occur months to years after exposure. Although E histolytica was previously thought to infect 10% of the world's population, 2 morphologically identical but genetically distinct and apparently nonpathogenic Entamoeba species are now recognized as causing most asymptomatic cases. To avoid unnecessary and possibly harmful therapies, clinicians should follow the diagnostic and treatment guidelines of the World Health Organization. Amebiasis is defined by the World Health Organization (WHO) and Pan American Health Organization (PAHO) as infection with Entamoeba histolytica, regardless of symptomatology.1WHO/PAHO/UNESCO report: a consultation with experts on amoebiasis: Mexico City, Mexico 28-29 January, 1997.Epidemiol Bull. 1997; 18: 13-14Google Scholar This protozoan parasite has a global distribution and an especially high prevalence in countries where poor socioeconomic and sanitary conditions predominate.2Stanley Jr, SL Amoebiasis.Lancet. 2003; 361: 1025-1034Abstract Full Text Full Text PDF PubMed Scopus (1027) Google Scholar In resource-rich nations, infections may be seen in travelers to and emigrants from endemic areas.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar Most infections are asymptomatic, but tissue invasion may result in amebic colitis, life-threatening hepatic abscesses, and even hematogenous spread to distant organs.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar, 4Salles JM Moraes LA Salles MC Hepatic amebiasis.Braz J Infect Dis. 2003; 7 (Epub 2003 Nov 19.): 96-110Crossref PubMed Scopus (84) Google Scholar Importantly, disease can occur months to years after exposure5Blessmann J Ali IK Nu PA et al.Longitudinal study of intestinal Entamoeba histolytica infections in asymptomatic adult carriers.J Clin Microbiol. 2003; 41: 4745-4750Crossref PubMed Scopus (114) Google Scholar and must remain in the differential diagnosis in at-risk populations. Advances in molecular technologies have revolutionized our understanding of this organism.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar Most notably, 2 additional Entamoeba species that are morphologically indistinguishable from E histolytica have been recognized in humans. As our knowledge of the global epidemiology and pathogenicity of Entamoeba spp increases, new clinical algorithms are developed.1WHO/PAHO/UNESCO report: a consultation with experts on amoebiasis: Mexico City, Mexico 28-29 January, 1997.Epidemiol Bull. 1997; 18: 13-14Google Scholar The latest nomenclature and recommendations, although unfamiliar and confusing to many, are important for appropriate patient care. Our review discusses what is known about these 3 Entamoeba spp and clarifies the currently accepted recommendations for diagnosis and treatment. It is a long-held misconception that 10% of the world's population is infected with E histolytica. In fact, most of these infections should be attributed to the morphologically identical but nonpathogenic E dispar. Emile Brumpt6Brumpt E Étude sommaire de l'Entamoeba dispar n. sp. Amibe à kystes quadrinucléés, parasite de l'homme.Bull Acad Med (Paris). 1925; 94: 943-952Google Scholar first proposed the existence of 2 indistinguishable Entamoeba spp, one pathogenic and one nonpathogenic, in 1925. However, not until 1978 was evidence for the existence of 2 separate entities provided by new technology (isoenzyme analysis).7Sargeaunt PG Williams JE Grene JD The differentiation of invasive and non-invasive Entamoeba histolytica by isoenzyme electrophoresis.Trans R Soc Trop Med Hyg. 1978; 72: 519-521Abstract Full Text PDF PubMed Scopus (254) Google Scholar More recent studies using methodologies capable of distinguishing the 2 species suggest that E dispar is up to 10 times more prevalent in asymptomatic patients than E histolytica in endemic regions.8Gathiram V Jackson TF Frequency distribution of Entamoeba histolytica zymodemes in a rural South African population.Lancet. 1985; 1: 719-721Abstract PubMed Scopus (87) Google Scholar, 9Haque R Ali IM Petri Jr, WA Prevalence and immune response to Entamoeba histolytica infection in preschool children in Bangladesh.Am J Trop Med Hyg. 1999; 60: 1031-1034PubMed Google Scholar, 10Solaymani-Mohammadi S Rezaian M Babaei Z et al.Comparison of a stool antigen detection kit and PCR for diagnosis of Entamoeba histolytica and Entamoeba dispar infections in asymptomatic cyst passers in Iran.J Clin Microbiol. 2006; 44: 2258-2261Crossref PubMed Scopus (62) Google Scholar, 11Huston CD Petri WA Amebiasis: clinical implications of the recognition of Entamoeba dispar.Curr Infect Dis Rep. 1999; 1: 441-447Crossref PubMed Scopus (23) Google Scholar Little is currently known about their epidemiology in resource-rich nations, where the incidence of both is rare, but previous reports of infection with E histolytica based only on morphology likely represent E dispar. The discovery of a third morphologically identical Entamoeba spp further complicated our understanding of the epidemiology of E histolytica. The new species, named E moshkovskii, was first recognized as a ubiquitous free-living organism in 194112Tshalaia LE On a species of Entamoeba detected in sewage effluents.Med Parazit (Moscow). 1941; 10: 244-252Google Scholar; it has been reported in humans from both resource-rich and resource-poor nations.13Clark CG Diamond LS The Laredo strain and other ‘Entamoeba histolytica-like' amoebae are Entamoeba moshkovskii.Mol Biochem Parasitol. 1991; 46: 11-18Crossref PubMed Scopus (141) Google Scholar, 14Fotedar R Stark D Marriott D Ellis J Harkness J Entamoeba moshkovskii infections in Sydney, Australia.Eur J Clin Microbiol Infect Dis. 2008 Feb; 27 (Epub 2007 Oct 24.): 133-137Crossref PubMed Scopus (57) Google Scholar Although largely nonpathogenic, some recent evidence suggests that it may have a role in human intestinal disease. Much remains unknown regarding the pathogenicity and epidemiology of E moshkovskii. E histolytica is the pathogenic species responsible for amebic colitis throughout the world. It infects people of both sexes and all ages; however, populations at risk may vary with geographic location, host susceptibility, and differences in organism virulence. People in highly endemic areas probably have recurrent asymptomatic infections, thus accounting for the high reported prevalence.5Blessmann J Ali IK Nu PA et al.Longitudinal study of intestinal Entamoeba histolytica infections in asymptomatic adult carriers.J Clin Microbiol. 2003; 41: 4745-4750Crossref PubMed Scopus (114) Google Scholar, 15Gathiram V Jackson TF A longitudinal study of asymptomatic carriers of pathogenic zymodemes of Entamoeba histolytica.S Afr Med J. 1987; 72: 669-672PubMed Google Scholar In developed countries, amebic colitis is most commonly found in travelers to or emigrants from endemic countries, institutionalized persons, and patients infected with human immunodeficiency virus.16Stauffer W Ravdin JI Entamoeba histolytica: an update.Curr Opin Infect Dis. 2003; 16: 479-485Crossref PubMed Scopus (114) Google Scholar, 17Ali IK Clark CG Petri Jr, WA Molecular epidemiology of amebiasis.Infect Genet Evol. 2008 May 14; ([Epub ahead of print])PubMed Google Scholar, 18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Men who have sex with men were previously thought to have an increased incidence of infection, but this supposition was based on morphologic studies. New evidence suggests that these men were colonized primarily with E dispar rather than E histolytica.19Weinke T Friedrich-Jänicke B Hopp P Janitschke K Prevalence and clinical importance of Entamoeba histolytica in two high-risk groups: travelers returning from the tropics and male homosexuals.J Infect Dis. 1990; 161: 1029-1031Crossref PubMed Scopus (71) Google Scholar, 20Allason-Jones E Mindel A Sargeaunt P Williams P Entamoeba histolytica as a commensal intestinal parasite in homosexual men.N Engl J Med. 1986; 315: 353-356Crossref PubMed Scopus (125) Google Scholar The simple life cycle of E histolytica begins when infectious cysts are ingested in fecally contaminated food or water.21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar This association with poor sanitation explains why resource-poor nations carry the bulk of the world's disease. After ingestion and passage through the stomach, the organism excysts and emerges in the large intestine as an active trophozoite. Trophozoites multiply by simple division and encyst as they move further down the large bowel. Cysts are then expelled with the feces and may remain viable in a moist environment for weeks to months.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar, 21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Amebae typically subsist on a diet of intestinal bacteria and partially digested host food but are capable of tissue invasion and dissemination. Most infections (≥90%) remain asymptomatic,5Blessmann J Ali IK Nu PA et al.Longitudinal study of intestinal Entamoeba histolytica infections in asymptomatic adult carriers.J Clin Microbiol. 2003; 41: 4745-4750Crossref PubMed Scopus (114) Google Scholar, 15Gathiram V Jackson TF A longitudinal study of asymptomatic carriers of pathogenic zymodemes of Entamoeba histolytica.S Afr Med J. 1987; 72: 669-672PubMed Google Scholar suggesting that tissue invasion is an aberration rather than a typical behavior. Invasive intestinal disease may occur days to years after initial infection and is characterized classically by abdominal pain and bloody diarrhea.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Watery or mucus-containing diarrhea, constipation, and tenesmus may also occur.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar This clinical picture corresponds histologically with trophozoites invading and laterally undermining the intestinal surface to form the so-called flask-shaped ulcers (Figure 1). The right side of the colon is commonly involved.4Salles JM Moraes LA Salles MC Hepatic amebiasis.Braz J Infect Dis. 2003; 7 (Epub 2003 Nov 19.): 96-110Crossref PubMed Scopus (84) Google Scholar Severe cases of amebic colitis are characterized by copious bloody diarrhea, diffuse abdominal pain, and (rarely) fever. Extensive fulminant necrotizing colitis, the most severe form of intestinal disease, is often fatal.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Patients at increased risk of severe disease include those who are very young, very old, malnourished, or pregnant and those who are receiving corticosteroids.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar Some evidence suggests that patients infected with human immunodeficiency virus are at increased risk of severe disease,22Hung CC Deng HY Hsiao WH et al.Invasive amebiasis as an emerging parasitic disease in patients with human immunodeficiency virus type 1 infection in Taiwan.Arch Intern Med. 2005; 165: 409-415Crossref PubMed Scopus (78) Google Scholar but this is not universally accepted.23Morán P Ramos F Ramiro M et al.Infection by human immunodeficiency virus-1 is not a risk factor for amebiasis.Am J Trop Med Hyg. 2005; 73: 296-300PubMed Google Scholar Complications of intestinal disease include stricture, rectovaginal fistulas, formation of an annularintraluminal mass (ameboma), bowel obstruction, perianal skin ulceration, toxic megacolon, perforation, peritonitis, shock, and death.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar, 18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Chronic intestinal amebiasis is also well described; patients with this condition can have years of intermittent abdominal pain, diarrhea, and weight loss.21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar On rare occasions, E histolytica trophozoites enter the bloodstream and disseminate to other body sites, most commonly the liver via spread from the intestine through the portal vein. The right lobe is 4 times more likely to be involved than the left because it receives the bulk of the venous drainage from the right colon.4Salles JM Moraes LA Salles MC Hepatic amebiasis.Braz J Infect Dis. 2003; 7 (Epub 2003 Nov 19.): 96-110Crossref PubMed Scopus (84) Google Scholar Adult men aged 20 to 40 years are most frequently affected, although people of both sexes and all ages may develop an amebic liver abscess (ALA).17Ali IK Clark CG Petri Jr, WA Molecular epidemiology of amebiasis.Infect Genet Evol. 2008 May 14; ([Epub ahead of print])PubMed Google Scholar, 18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Disease can occur years after exposure and may follow the onset of immunosuppression.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Hepatic invasion by amebic trophozoites results in marked tissue destruction with neutrophil recruitment, cellular necrosis, and formation of microabscesses that gradually coalesce.4Salles JM Moraes LA Salles MC Hepatic amebiasis.Braz J Infect Dis. 2003; 7 (Epub 2003 Nov 19.): 96-110Crossref PubMed Scopus (84) Google Scholar Most patients (65%-75%) present with a single abscess; however, multiple abscesses may also be formed.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Abscesses consist of soft, necrotic, acellular yellow-brown debris, described as “anchovy paste.”21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Amebae are seldom identified in aspirates because they are located at the periphery of the lesion.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar White blood cells are also not usually seen, presumably because they have been destroyed by the amebic trophozoites. Clinical presentation of ALA is highly variable and commonly includes tender hepatomegaly and pain in theright upper quadrant.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Unlike amebic colitis, ALA is commonly accompanied by fever,3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar as well as by rigors, chills, and profuse sweating.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Most patients do not have concurrent colitis and cysts, and trophozoites are not always seen on fecal smears,3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar posing an important diagnostic challenge. Jaundice is not typically present; elevated bilirubin levels are seen in less than 50% of patients, but elevated alkaline phosphatase levels are common.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Complications include secondary bacterial infection; perforation into peritoneal, pleural, and pericardial cavities; septic shock; and death.4Salles JM Moraes LA Salles MC Hepatic amebiasis.Braz J Infect Dis. 2003; 7 (Epub 2003 Nov 19.): 96-110Crossref PubMed Scopus (84) Google Scholar, 18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Perhaps the most serious complication is amebic metastasis from the liver. Rarely, trophozoites end up in other regions of the body, such as the brain, spleen, lungs, and genitourinary tract, through hematogenous or direct spread.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar Brain abscesses are extremely rare and are associated with high mortality rates.2Stanley Jr, SL Amoebiasis.Lancet. 2003; 361: 1025-1034Abstract Full Text Full Text PDF PubMed Scopus (1027) Google Scholar Like patients with ALA, those with disseminated disease do not usually have concomitant amebic colitis.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar Disseminated disease is not an adaptive mechanism for the parasite because its life cycle cannot be completed outside the intestine. When amebiasis is suspected, radiologic and endoscopic examination may lend further support for a diagnosis. Colonoscopy can provide a wide spectrum of findings, from rare large-bowel ulcers in mild disease to diffuse mucosal ulceration, hemorrhage, colonic stricture, and presence of an ameboma.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Grossly, these findings may resemble those seen with inflammatory bowel disease; therefore, correlation with histopathology and laboratory results is essential.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar, 21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Endoscopy is contraindicated in patients with evidence of peritonitis, severe dehydration, or shock.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Radiologic studies may also be helpful in evaluating a patient with possible ALA. Chest and abdominal radiography often reveal a pleural effusion and raised hemi-diaphragm overlying the involved liver lobe.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Ultrasonography reveals lesions that are typically hypoechoic and well defined with rounded edges.4Salles JM Moraes LA Salles MC Hepatic amebiasis.Braz J Infect Dis. 2003; 7 (Epub 2003 Nov 19.): 96-110Crossref PubMed Scopus (84) Google Scholar Computed tomography and magnetic resonance imaging can further characterize an abscess and allow for better detection of smaller lesions. All 3 techniques may facilitate guided needle biopsy and drainage if indicated.4Salles JM Moraes LA Salles MC Hepatic amebiasis.Braz J Infect Dis. 2003; 7 (Epub 2003 Nov 19.): 96-110Crossref PubMed Scopus (84) Google Scholar An abscess can usually be distinguished from solid lesions and biliary tract disease, but the differentiation between bacterial and amebic abscesses is less clear. Gallium scans may have a role in this differential diagnosis because amebic abscesses are usually “cold” on scan because of the lack of white blood cells in the abscess, whereas bacterial abscesses are typically “hot.”21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Clinically, it is desirable to definitively distinguish E histolytica from E dispar and E moshkovskii because, of the 3, it is the only proven human pathogen.17Ali IK Clark CG Petri Jr, WA Molecular epidemiology of amebiasis.Infect Genet Evol. 2008 May 14; ([Epub ahead of print])PubMed Google Scholar The diagnosis of invasive amebiasis is usually suggested by the patient's presenting symptoms, exposure history, and radiologic findings but should be confirmed with microbiological laboratory results. Many laboratory methods exist for identification of E histolytica, E dispar, and/or E moshkovskii, and the clinician should be aware that tests vary considerably in price, sensitivity, specificity, and the ability to definitively differentiate among the 3 species. Light microscopic examination of fecal specimens (ie, “ova and parasite” examination) is often the first step in diagnosis3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar; the characteristic trophozoites and cysts can often be identified through direct, concentrated, and/or permanently stained smears (Figure 2). Because organisms may appear intermittently, current recommendations call for submission of 3 stool specimens on different days during a period of 10 days.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar As mentioned previously, stool specimens from patients with disseminated disease may not contain cysts and trophozoites, despite repeated examinations.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar If stool cannot be examined in the fresh state (within 15 minutes) for motile trophozoites, then it should be placed immediately in an appropriate fixative to prevent deterioration of organisms.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar Unfortunately, microscopy alone cannot differentiate E histolytica from E dispar and E moshkovskii; additional tests are required for definitive speciation. The rare exception is when trophozoites containing ingested red blood cells are identified; they are strongly (but not definitively) indicative of invasive amebiasis.1WHO/PAHO/UNESCO report: a consultation with experts on amoebiasis: Mexico City, Mexico 28-29 January, 1997.Epidemiol Bull. 1997; 18: 13-14Google Scholar Trophozoites may also be identified in intestinal biopsy specimens, scrapings, or aspirates,21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar allowing a diagnosis of amebiasis to be made if mucosal invasion and ulceration are also observed. When only examination of stool specimens is available, the WHO/PAHO recommends that morphologically consistent cysts and trophozoites receive the nonspecific diagnosis E histolytica/E dispar,1WHO/PAHO/UNESCO report: a consultation with experts on amoebiasis: Mexico City, Mexico 28-29 January, 1997.Epidemiol Bull. 1997; 18: 13-14Google Scholar which could now be augmented to include E moshkovskii. The clinician must then interpret this laboratory result in the context of the individual patient and determine whether treatment is warranted. When possible, E histolytica should be definitively identified.1WHO/PAHO/UNESCO report: a consultation with experts on amoebiasis: Mexico City, Mexico 28-29 January, 1997.Epidemiol Bull. 1997; 18: 13-14Google Scholar Identification methods include biopsy, serology, antigen detection, and molecular assays. Culture may be performed by some large specialty laboratories but is technically challenging and time-consuming. Furthermore, a negative culture result from intestinal samples does not exclude E histolytica1WHO/PAHO/UNESCO report: a consultation with experts on amoebiasis: Mexico City, Mexico 28-29 January, 1997.Epidemiol Bull. 1997; 18: 13-14Google Scholar because sensitivity is less than 100%. Culture followed by isoenzyme analysis is the criterion standard in diagnosis; however, it will likely be replaced by molecular assays in the near future.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar, 11Huston CD Petri WA Amebiasis: clinical implications of the recognition of Entamoeba dispar.Curr Infect Dis Rep. 1999; 1: 441-447Crossref PubMed Scopus (23) Google Scholar Serologic tests detect the presence of species-specific antibodies in the patient's serum. They are particularly useful in nonendemic countries where prevalence is low and have a good sensitivity and specificity for detecting invasive intestinal disease. They are also the test of choice for ALA because titers are typically high and test sensitivities and specificities exceed 95% with most assays.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar, 21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar The primary disadvantage of serologic tests is that they cannot distinguish between past and current infection unless IgM is detected; IgM antibodies to E histolytica are short-lived and rarely detected. In contrast, IgG antibodies are long-lived but highly prevalent in endemic settings because of past exposure.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar Serologic assays, which are also less sensitive in asymptomatic infection, take 7 to 10 days to appear in the bloodstream, resulting in possible false-negative results.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar Enzyme-linked immunosorbent assay is the most popular test in the diagnostic setting because of its speed and ease of use.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar Fecal antigen detection tests use specific monoclonal or polyclonal antibodies to detect E histolytica antigens. They are rapid, highly sensitive, and widely used in the diagnostic laboratory.11Huston CD Petri WA Amebiasis: clinical implications of the recognition of Entamoeba dispar.Curr Infect Dis Rep. 1999; 1: 441-447Crossref PubMed Scopus (23) Google Scholar Antigen tests are useful for confirming microscopic findings and providing a diagnosis in patients with negative fecal smear results. They are also helpful for interpreting positive results on amebic serology in patients from endemic countries because positive results on an antigen test indicate current rather than past infection.11Huston CD Petri WA Amebiasis: clinical implications of the recognition of Entamoeba dispar.Curr Infect Dis Rep. 1999; 1: 441-447Crossref PubMed Scopus (23) Google Scholar, 21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Some antigen detection kits can also be used on serum and material obtained from aspirated abscesses, offering greater sensitivity than microscopy for extraintestinal disease.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Not all commercial kits are capable of speciation; some demonstrate cross-reactivity between E histolytica and E dispar. Antigen detection methods are also not as sensitive as polymerase chain reaction assays3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar, 11Huston CD Petri WA Amebiasis: clinical implications of the recognition of Entamoeba dispar.Curr Infect Dis Rep. 1999; 1: 441-447Crossref PubMed Scopus (23) Google Scholar and may have low specificity in nonendemic regions.24Paul J Srivastava S Bhattacharya S Molecular methods for diagnosis of Entamoeba histolytica in a clinical setting: an overview.Exp Parasitol. 2007 May; 116 (Epub 2006 Dec 26.): 35-43Crossref PubMed Scopus (19) Google Scholar Clinicians should be familiar with the specifications of the kits used in their laboratory and confirm a suspected diagnosis if indicated. The highest sensitivity and specificity for the diagnosis of E histolytica are offered by DNA-based tests. Many assays are available, including conventional and real-time polymerase chain reaction formats3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar; however, they are currently used primarily by research and reference laboratories. Like most molecular amplification assays, they remain impractical for resource-limited settings because of their equipment, personnel, and facility requirements. The differential diagnosis of amebic colitis must include bacterial (eg, Salmonella and Shigella spp, Mycobacterium tuberculosis), parasitic (eg, Schistosoma mansoni, Balantidium coli), and noninfectious (eg, inflammatory bowel disease, carcinoma, ischemic colitis, diverticulitis) causes of dysentery.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar, 18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar When present, amebomas may mimic carcinoma, tuberculosis, or an appendiceal mass.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Diagnostic tests in the work-up of patients with dysentery might include stool cultures for bacteria, ova, and parasites (other than E histolytica) and assays for bacterial toxins.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar Biopsy specimens of intestinal ulcers are useful for confirming the presence of trophozoites and for excluding other etiologies. Given its varied clinical presentation and possible delay of onset, the diagnosis of ALA may not be straightforward. The differential diagnosis includes bacterial abscess, echinococcal cyst, tuberculosis, and primary or metastatic tumor,18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar all which would have vastly different treatments. Radiology can differentiate between many noninfectious and infectious etiologies; however, bacterial and amebic abscesses may appear remarkably similar. In comparison with bacterial abscesses, ALAs are more likely to be solitary, subcapsular, and located in the right lobe of the liver, but these findings are not always reliable.4Salles JM Moraes LA Salles MC Hepatic amebiasis.Braz J Infect Dis. 2003; 7 (Epub 2003 Nov 19.): 96-110Crossref PubMed Scopus (84) Google Scholar, 18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Occasionally, ALA may cause a pneumonia-like presentation with pleuritic pain, cough, and dyspnea.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Radiologic imaging, clinical history, findings on physical examination, and serologic results are essential for including or excluding the diagnosis of ALA. The WHO/PAHO recommendations state that, when possible, E histolytica should be differentiated from morphologically similar species and treated appropriately. Given the small but substantial risk of invasive disease and the potential to transmit the infection to others, WHO/PAHO recommends treating all cases of proven E histolytica, regardless of symptoms.1WHO/PAHO/UNESCO report: a consultation with experts on amoebiasis: Mexico City, Mexico 28-29 January, 1997.Epidemiol Bull. 1997; 18: 13-14Google Scholar If E dispar is the only species identified, then no treatment should be given and other causes should be sought as appropriate.1WHO/PAHO/UNESCO report: a consultation with experts on amoebiasis: Mexico City, Mexico 28-29 January, 1997.Epidemiol Bull. 1997; 18: 13-14Google Scholar, 11Huston CD Petri WA Amebiasis: clinical implications of the recognition of Entamoeba dispar.Curr Infect Dis Rep. 1999; 1: 441-447Crossref PubMed Scopus (23) Google Scholar In resource-poor countries, the standard but less optimal approach is to treat all patients with cysts and trophozoites identified on stool examination without additional testing for speciation.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar This method results in vast overtreatment and may hasten the development of drug resistance in E histolytica.3Fotedar R Stark D Beebe N Marriott D Ellis J Harkness J Laboratory diagnostic techniques for Entamoeba species.Clin Microbiol Rev. 2007; 20: 511-532Crossref PubMed Scopus (338) Google Scholar Thus, WHO/PAHO recommends withholding treatment from asymptomatic patients when only a morphologic diagnosis by stool examination is available (ie, E histolytica/E dispar/E moshkovskii), unless another reason to suspect E histolytica infection exists.1WHO/PAHO/UNESCO report: a consultation with experts on amoebiasis: Mexico City, Mexico 28-29 January, 1997.Epidemiol Bull. 1997; 18: 13-14Google Scholar Even if patients diagnosed as being infected with E histolytica/E dispar/E moshkovskii have symptoms, other causes of disease, such as bacterial colitis, should not be excluded until further testing is done.1WHO/PAHO/UNESCO report: a consultation with experts on amoebiasis: Mexico City, Mexico 28-29 January, 1997.Epidemiol Bull. 1997; 18: 13-14Google Scholar Prophylaxis for E histolytica infection with amebicides is not recommended under any circumstances.1WHO/PAHO/UNESCO report: a consultation with experts on amoebiasis: Mexico City, Mexico 28-29 January, 1997.Epidemiol Bull. 1997; 18: 13-14Google Scholar The medications recommended to treat confirmed amebiasis vary with clinical manifestation. Asymptomatic intestinal infection with E histolytica should be treated with luminal amebicides, such as paromomycin and diloxanide furoate.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar These medications will eradicate the luminal amebae and prevent subsequent tissue invasion and spread of the infection through cysts.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar, 21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Paromomycin, more widely available in the United States, has the advantage of not being absorbed in the bowel.21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Abdominal cramps and nausea are the most commonly reported adverse effects. A 10-day course at 30 mg/kg per day (divided into 3 daily doses) is typical. Some recommend follow-up stool examination to confirm eradication of cysts.21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Compared with asymptomatic infection, intestinal and extraintestinal invasive disease are aerobic processes and should be treated with tissue amebicides, such as 5-nitroimidazoles (eg, metronidazole), which are readily absorbed into the bloodstream.1WHO/PAHO/UNESCO report: a consultation with experts on amoebiasis: Mexico City, Mexico 28-29 January, 1997.Epidemiol Bull. 1997; 18: 13-14Google Scholar Metronidazole (750 mg, 3 times a day, for 5-10 days) is the most commonly used drug in the United States for invasive amebiasis.21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Because little metronidazole reaches the lumen of the colon, treatment should be followed by administration of a luminal agent to eradicate any potential intestinal reservoirs.21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Most uncomplicated cases respond to a 5-day course of metronidazole; however, a 10-day course is useful in severe cases.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Metronidazole may also be given parenterally to critically ill patients and can be supplemented with an antibiotic to cover secondary sepsis with bowel flora. The most common adverse effects of metronidazole are abdominal discomfort and nausea; most patients, however, are able to complete a full 5- to 10-day course. Serious adverse drug reactions include confusion, ataxia, and seizures.4Salles JM Moraes LA Salles MC Hepatic amebiasis.Braz J Infect Dis. 2003; 7 (Epub 2003 Nov 19.): 96-110Crossref PubMed Scopus (84) Google Scholar A promising new regimen for invasive amebiasis is a 3-day course of nitazoxanide. This drug is effective against both luminal and invasive forms and has the added benefit of eliminating other intestinal parasites, including helminths.21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Surgery may be necessary in cases of perforation, abscess, obstruction, stricture, or toxic megacolon. However, given the friable nature of the inflamed mucosa, bowel repair is risky and should be avoided when possible.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar, 21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Like amebic colitis, ALA typically responds well to a 5- to 10-day course of metronidazole, which should also be followed with a luminal amebicide.21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar Metronidazole is the drug of choice in this setting, given its fast intestinal absorption, excellent bioavailability in tissue, and good abscess penetration.4Salles JM Moraes LA Salles MC Hepatic amebiasis.Braz J Infect Dis. 2003; 7 (Epub 2003 Nov 19.): 96-110Crossref PubMed Scopus (84) Google Scholar Surgical or percutaneous drainage of ALAs is generally not recommended because of the risk of content spillage and/or bacterial superinfection; exceptions are cases of imminent rupture, failure to respond to treatment after 4 to 5 days, and secondary bacterial infection.4Salles JM Moraes LA Salles MC Hepatic amebiasis.Braz J Infect Dis. 2003; 7 (Epub 2003 Nov 19.): 96-110Crossref PubMed Scopus (84) Google Scholar, 21Ravdin JI Stauffer WM Entamoeba histolytica (amebiasis).in: Mandell GL Bennett JE Dolin R 6th ed. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Vol 2. Churchill Livingstone, Philadelphia, PA2005: 3097-3111Google Scholar After treatment, ultrasonography may be used to monitor abscess regression, which occurs slowly during a period of 3 to 12 months.4Salles JM Moraes LA Salles MC Hepatic amebiasis.Braz J Infect Dis. 2003; 7 (Epub 2003 Nov 19.): 96-110Crossref PubMed Scopus (84) Google Scholar Small cystic defects may remain indefinitely.18Gill GV Beeching NJ Tropical Medicine. 5th ed. Blackwell Publishing, Oxford, UK2004: 153-159Google Scholar Amebae rarely disseminate beyond the portal circulation. Given the small number of cases, no definitive treatment guidelines are available for management of extraintestinal, extrahepatic disease. As mentioned previously, infections with E dispar do not require treatment. Less is known about E moshkovskii, but it is likely that this infection also would not require treatment in most cases. Recent discoveries have revolutionized our understanding of the epidemiology of Entamoeba spp infections and have led to important treatment and diagnostic recommendations. To avoid unnecessary and possibly harmful therapies, clinicians should follow the precise guidelines promulgated by the WHO/PAHO in 1997, including definitive differentiation of E histolytica from morphologically identical nonpathogenic species. Such definite differentiation is especially important in countries with adequate sanitation measures, where the predominant organism identified from morphologic stool examination will be E dispar. Because they have the highest sensitivity and specificity, molecular technologies offer the greatest diagnostic potential for laboratories in resource-rich countries at this juncture; however, some antigen detection tests can also provide reliable speciation. When speciation is impossible, we recommend using the phrase “E histolytica/E dispar/E moshkovskii” to describe the morphologically identical species seen on stool examination. Continued use of new technologies will be crucial in elucidating the true epidemiology and pathogenesis of Entamoeba spp, including the less well-studied E moshkovskii. Continued development of affordable, sensitive, and specific diagnostic tools will be required for use in resource-poor settings, where the incidence of disease is highest.
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