Amebiasis Workup

  • Author: Alexandre Lacasse, MD, MSc; Chief Editor: Burke A Cunha, MD   more...
 
Updated: Jan 10, 2012
 

Laboratory Studies

  • Microscopy[12]
    • Microscopic stool examination for trophozoites from a single stool sample in amebic colitis is only 33-50% sensitive. Examination of 3 stool samples over no more than 10 days can improve the detection rate to 85-95%. See the image below. Trichrome stain of Entamoeba histolytica trophozoiTrichrome 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.
    • 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 presence of intracytoplasmic red blood cells in trophozoites is diagnostic of E histolytica infection, although recent studies demonstrated the same phenomenon with E dispar.
    • The World Health Organization (WHO) recommends that intestinal amebiasis be diagnosed with an E histolytica -specific test, thus rendering the classic stool ova and parasite examination obsolete.
  • Culture[12]
    • Xenic cultivation, first introduced in 1925, is defined as the growth of the parasite in the presence of an undefined flora. This technique is still in use today using modified Locke-egg media.
    • Axenic cultivation, first achieved in 1961, involves growth of 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.
    • Cultures can be performed using fecal or rectal biopsy specimens and liver abscess aspirates. The success rate is between 50% and 70%, but the technique is technically difficult. Overall, it is less sensitive than microscopy.
  • Antigen detection[12]
    • Enzyme-linked immunosorbent assay (ELISA) is used to detect antigens from E histolytica in stool samples. Several kits are commercially available.
    • Antigen-based ELISA kits using monoclonal antibodies against the GAL/GalNAc–specific lectin of E histolytica (E histolytica II, TechLab, Blacksburg, VA) yield an overall sensitivity of 71%-100% and 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% and 100%, respectively.
    • Other stool detection kits use monoclonal antibodies against the serine-rich antigen of E histolytica (Optimum S kit, Merlin Diagniostika, 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.
  • Serology[12]
    • Multiple serologic assays are available for the diagnosis of amebiasis.
    • ELISA is the most used assay throughout the world and is used to measure the presence of serum antilectin antibodies (IgG). The sensitivity for detection of antibodies to E histolytica in patients with amebic liver abscess is 97.9%, whereas the specificity is 94.8%. False-negative results can occur within the first 7-10 days following infection.
    • Immunofluorescent assay (IFA) is also rapid, reliable, and reproducible. In the setting of amebic liver abscess, the sensitivity and specificity of IFA was shown to be 93.6% and 96.7%, respectively.
    • Indirect hemagglutination (IHA) is very specific (99.1%) but is less sensitive than ELISA.
    • Immunoelectrophoresis, counter-immunoelectrophoresis (CIE), and immunodiffusion tests use the precipitation property of antigen-antibody complexes in agar. CIE is time-consuming but has shown a sensitivity of 100% in invasive amebiasis.
    • Complement fixation (CF) is less sensitive than other techniques.
    • The seropositivity prevalence is very high in endemic areas, limiting antibody-based testing for diagnosing currently active disease, since antibodies can persist for years after infection.
  • Polymerase chain reaction[12]
    • E histolytica can be identified in various types of clinical specimens, including feces, tissues, and liver abscess aspirates.
    • A wide variety of polymerase chain reaction (PCR) methods targeting different genes, including a small-subunit rRNA gene (18S rDNA), 30-kDa antigen gene, serine-rich protein gene, chitinase gene, hemolysin gene, and extrachromosomal circular DNA, have been described for the detection and differentiation of E histolytica, E dispar, and E moshkovskii.
    • Sensitivities can vary according to sampling and the specific target gene used. Performed on feces, PCR yields a sensitivity that is similar to that of stool antigen-based assay.
    • PCR-based tests have been strongly endorsed by the WHO.
    • Application of PCR-based methods in routine diagnosis is still very limited, as the generation of nonspecific DNA fragments from environmental and clinical samples often leads to false-positive results
  • Nonspecific laboratory tests
    • Amebic liver abscess
      • Leukocytosis without eosinophilia (80%)
      • Elevated alkaline phosphatase (80%)
      • Mildly elevated transaminases
      • Mild anemia
      • Elevated erythrocyte sedimentation rate
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Imaging Studies

  • Both ultrasonography and CT scanning are sensitive but nonspecific for amebic liver abscess. Lesions are usually solitary and located in the right hepatic lobe (70-80%), although multiple abscesses are possible.
  • On sonograms, amebic liver abscesses usually appear as a homogenous hypoechoic round lesion.
  • On CT scans with intravenous 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.
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Procedures

  • Liver aspiration
    • Ultrasound- or CT-guided needle aspiration should be performed when a diagnosis must be established very rapidly, since pyogenic liver abscess can present and appear in a similar fashion.
    • Liver abscess aspirate is usually an odorless thick yellow-brown liquid classically referred to as "anchovy paste."
    • 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.
  • Colonoscopy
    • Colonoscopy can be performed for biopsy in the setting of negative findings on stools studies, including antigen testing. Tissue can be sent for microscopic evaluation, culture, and PCR, where available.
    • Fulminant colitis is a relative contraindication to colonoscopy since the risk of intestinal perforation is increased.
    • A friable and diffusely ulcerated mucosa resembling inflammatory bowel disease can be observed.
    • A carcinomalike annular lesion called ameboma can also be seen, usually in the cecum and ascending colon.[1, 20]
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Histological Findings

  • The intestinal biopsy specimen should be taken from the edge of ulcers and evaluated for motile trophozoites.
  • Histopathological findings can comprise of mucosal thickening, multiple discrete ulcers separated by regions of normal-appearing colonic mucosa, diffusely inflamed and edematous mucosa, necrosis, or wall perforation.
  • 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.
  • Different chemical stains can be used, including periodic acid-Schiff stain, which makes E histolytica appear magenta in color. See the images below.Trichrome stain of Entamoeba histolytica trophozoiTrichrome 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. Trichrome stain of an Entamoeba histolytica cyst iTrichrome stain of an Entamoeba histolytica cyst in amebiasis. Each cyst has 4 nuclei with characteristically centrally located karyosomes. Cysts measure 12-15 mm.
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Contributor Information and Disclosures
Author

Alexandre Lacasse, MD, MSc  Internal Medicine Faculty, Assistant Director, Medicine Clinic, Infectious Disease Consultant, St Mary's Health Center

Alexandre Lacasse, MD, MSc is a member of the following medical societies: American College of Physicians, American Medical Association, Association of Program Directors in Internal Medicine, Infectious Diseases Society of America, and Society for Healthcare Epidemiology of America

Disclosure: Nothing to disclose.

Coauthor(s)

Kerry O Cleveland, MD  Associate Professor of Medicine, University of Tennessee College of Medicine; Consulting Staff, Department of Internal Medicine, Division of Infectious Diseases, Methodist Healthcare of Memphis

Kerry O Cleveland, MD is a member of the following medical societies: American College of Physicians, Infectious Diseases Society of America, and Society for Healthcare Epidemiology of America

Disclosure: Nothing to disclose.

J Robert Cantey, MD  Professor, Department of Medicine, Division of Infectious Diseases, Medical University of South Carolina

J Robert Cantey, MD is a member of the following medical societies: Alpha Omega Alpha, American Society for Clinical Investigation, American Society for Microbiology, Infectious Diseases Society of America, International Society of Travel Medicine, Musculoskeletal Infection Society, Phi Beta Kappa, and Southern Society for Clinical Investigation

Disclosure: Nothing to disclose.

Specialty Editor Board

Klaus-Dieter Lessnau, MD, FCCP  Clinical Associate Professor of Medicine, New York University School of Medicine; Medical Director, Pulmonary Physiology Laboratory; Director of Research in Pulmonary Medicine, Department of Medicine, Section of Pulmonary Medicine, Lenox Hill Hospital

Klaus-Dieter Lessnau, MD, FCCP is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, American Medical Association, American Thoracic Society, and Society of Critical Care Medicine

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD  Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

Michael Stuart Bronze, MD  Professor, Stewart G Wolf Chair in Internal Medicine, Department of Medicine, University of Oklahoma Health Science Center

Michael Stuart Bronze, MD is a member of the following medical societies: Alpha Omega Alpha, American College of Physicians, American Medical Association, Association of Professors of Medicine, Infectious Diseases Society of America, Oklahoma State Medical Association, and Southern Society for Clinical Investigation

Disclosure: Nothing to disclose.

Eleftherios Mylonakis, MD  Clinical and Research Fellow, Department of Internal Medicine, Division of Infectious Diseases, Massachusetts General Hospital

Eleftherios Mylonakis, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians, American Society for Microbiology, and Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Chief Editor

Burke A Cunha, MD  Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital

Burke A Cunha, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, and Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Additional Contributors

The authors and editors of eMedicine gratefully acknowledge the contributions of previous author Robert Swords, MD, to the development and writing of this article.

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