Amebiasis

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-9CM CODES

006.9 Amebiasis

EPIDEMIOLOGY & DEMOGRAPHICS

INCIDENCE (IN U.S.):

Highest in institutionalized patients, sexually active homosexual menPREVALENCE (IN U.S.):

4% (80% of infections asymptomatic)

PREDOMINANT SEX:

• Equal sex distribution in general
• Striking male predominance of liver abscess

PREDOMINANT AGE:

Second through sixth decades

PEAK INCIDENCE:

Peaks at age 2 to 3 yr and >40 yr

GENETICS:

Infection more likely to be fulminant in young infants

PHYSICAL FINDINGS & CLINICAL PRESENTATION

• Often nonspecific
• Approximately 20% of cases symptomatic
1. Diarrhea, which may be bloody
2. Abdominal and back pain
• Abdominal tenderness in 83% of severe cases
• Fever in 38% of severe cases
• Hepatomegaly, RUQ tenderness, and fever in almost all patients with liver abscess (may be absent in fulminant cases)

Mature cyst of Entamoeba histolytica. Three of the four nuclei are seen in the plane of focus of this photomicrograph

ETIOLOGY
• Caused by the protozoal parasite E. histolytica
• 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

	•	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

	•	Three stool specimens over a period of 7 to 10 days to exclude the diagnosis (sensitivity 50% to 80%)
	•	Concentration and staining the specimen with Lugol’s iodine or methylene blue to increase the diagnostic yield
	•	Available culture (rarely necessary in routine cases)

LABORATORY TESTS

	•	Stool examination is generally reliable.
	•	Mucosal biopsy is occasionally necessary.
	•	Serum antibody may be detected and is particularly sensitive and specific for extraintestinal infection or severe intestinal disease.
	•	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

	•	Metronidazole (750 mg PO tid for 10 days) is used in the treatment of mild to severe intestinal infection and amebic liver abscess; it may be administered intravenously when necessary.
	•	Follow with iodoquinol (650 mg PO tid for 20 days) to eradicate persistent cysts.
	•	For asymptomatic patients with amebic cysts on stool examination, use iodoquinol or paromomycin (500 mg PO tid for 7 days).
	•	Avoid antiperistaltic agents in severe intestinal infections to avoid risk of toxic megacolon.
	•	Liver abscess is generally responsive to medical management but surgical intervention indicated for extension of liver abscess into pericardium or for toxic megacolon.

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

REFERRAL

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