Amebiasis

Glenn G. Fort, M.D., M.P.H.

Basic Information

Definition

Amebiasis is an infection caused by the protozoal parasite Entamoeba histolytica. Although primarily an infection of the colon, amebiasis may cause extraintestinal disease, particularly liver abscess.

Synonyms

Amebic dysentery (when severe intestinal infection)

ICD-10CM CODES
A06.9	Amebiasis, unspecified
A06.1	Chronic intestinal amebiasis
A06.7	Cutaneous amebiasis

Epidemiology & Demographics

Incidence (In U.S.)

1.2 cases per 100,000 U.S. population. Highest in institutionalized patients, and travelers to/immigrants from developing nations.

Prevalence (In U.S.)

4% (80% of infections asymptomatic)

Predominant Sex

•

Equal sex distribution in general
•

Striking male predominance of liver abscess

Predominant Age

2nd through 6th decades

Peak Incidence

Peaks at age 2 to 3 yr and >40 yr

Physical Findings & Clinical Presentation

•

Often nonspecific
•
Approximately 20% of cases symptomatic
1. 1.
  
  Diarrhea, which may be bloody
2. 2.
  
  Abdominal and back pain
•

Abdominal tenderness in 83% of severe cases
•

Fever in 38% of severe cases
•

Hepatomegaly, right upper quadrant tenderness, and fever in almost all patients with liver abscess (may be absent in fulminant cases)

Etiology

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Caused by the protozoal parasite E. histolytica, E. dispar, and E. moshkovskii. The latter two are 10 times more common but nonpathogenic and difficult to distinguish from E. histolytica morphologically.
•

Transmission by the fecal-oral route
•

Infection usually localized to the large bowel, particularly the cecum where a localized mass lesion (ameboma) may form
•

Extraintestinal infection in which the organism invades the bowel mucosa and gains access to the portal circulation

Diagnosis

Differential Diagnosis

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Severe intestinal infection possibly confused with ulcerative colitis or other infectious enterocolitis syndromes, such as those caused by Shigella, Salmonella, Campylobacter, or invasive Escherichia coli
•

In elderly patients: ischemic bowel possibly producing a similar picture

Workup

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Stool antigen testing is more sensitive than ova and parasite examination for the diagnosis of amebiasis
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Three stool specimens over a period of 7 to 10 days to search for cysts or trophozoites has a sensitivity of 85% to 95%, but microscopy cannot differentiate between the species
•

Concentration and staining the specimen with Lugol’s iodine or methylene blue to increase the diagnostic yield
•

Fecal leukocytes not always present

Laboratory Tests

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Fecal ELISA antigen detection is specific for E. histolytica (87% percent sensitivity and >90% specificity as compared with culture) and also useful for diagnosis of liver abscess.
•

PCR-based assays on stool: 90%-95% sensitive; 95%-100% specific.
•

Mucosal biopsy is occasionally necessary to look for cysts or trophozoites.
•

Serum antibody assays specific for E. histolytica are available and are particularly sensitive and specific for extraintestinal infection or severe intestinal disease but may not distinguish recent from remote infection.
•

Aspiration of abscess fluid is used to distinguish amebic from bacterial abscesses.

Imaging Studies

Abdominal imaging studies (sonography or CT scan) to diagnose liver abscess

Treatment

Acute General Rx

•

Entamoeba histolytica causes amoebic dysentery and requires treatment. Other species of Entamoeba may colonize the gastrointestinal tract but are not pathogenic and do not mandate treatment.
•

Table E1 summarizes drug treatment options for amebiasis in adults and children.
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Liver abscess is generally responsive to medical management but surgical intervention indicated for extension of liver abscess into pericardium or for toxic megacolon.

TABLEE1 Drug Treatment for AmebiasisFrom Kliegman RM et al.: Nelson textbook of pediatrics, ed 19, Philadelphia, 2011, Saunders.

Medication	Adult Dosage (Oral)	Pediatric Dosage (Oral)^∗
Invasive Disease
Metronidazole	Colitis or liver abscess: 750 mg tid for 7-10 days	Colitis or liver abscess: 35-50 mg/kg/day in 3 divided doses for 7-10 days
or
Tinidazole	Colitis: 2 g once daily for 3 days Liver abscess: 2 g once daily for 3-5 days	Colitis: 50 mg/kg/day once daily for 3 days Liver abscess: 50 mg/kg/day once daily for 3-5 days
Followed by:
Paromomycin (preferred)	500 mg tid for 7 days	25-35 mg/kg/day in 3 divided doses for 7 days
or
Diloxanide furoate^† or	500 mg tid for 10 days	20 mg/kg/day in 3 divided doses for 7 days
Iodoquinol	650 mg tid for 20 days	30-40 mg/kg/day in 3 divided doses for 20 days
Asymptomatic Intestinal Colonization
Paromomycin (preferred) or Diloxanide furoate^† or Iodoquinol	As for invasive disease	As for invasive disease

Disposition

Host immunity incomplete and reinfection rate high for patients remaining at risk

Referral

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For consultation with infectious diseases specialist for extraintestinal infection or persistent or relapsing intestinal infection
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For surgical consultation:
1. 1.
  
  For toxic megacolon
2. 2.
  
  For impending rupture of or extension of liver abscess into adjacent structures

Pearls & Considerations

Comments

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Infection with other intestinal parasites, particularly Giardia lamblia, may coexist with amebiasis.
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There is a high prevalence of E. dispar in homosexual males, which is nonpathogenic but may be difficult to distinguish from the pathogen E. histolytica.

Ferri – Amebiasis