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Amebiasis

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Reporting Obligations

Confirmed and suspected cases shall be reported to the local Health Unit.

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Epidemiology

Aetiologic Agent

Amebiasis is an enteric infection caused by Entamoeba histolytica, a microscopic intestinal parasite excreted as cysts or trophozoites in stools of infected people.

Clinical Presentation

Clinical syndromes associated with E. histolytica infection include non-invasive intestinal infection, intestinal amebiasis, ameboma (amebic granulomata), and liver abscess. Most infections are asymptomatic.

Persons with non-invasive intestinal infection may be asymptomatic or may have non-specific intestinal tract complaints. Persons with intestinal amebiasis (amebic colitis) generally have 1 to 3 weeks of increasingly severe diarrhea progressing to grossly bloody dysenteric stools with lower abdominal pain and tenesmus. Weight loss and fever may be present.

An ameboma may occur as an annular lesion of the cecum or ascending colon that may be mistaken for colonic carcinoma or as a tender extra-hepatic mass, mimicking a pyogenic abscess. Amebomas usually resolve with anti-amebic therapy and do not require surgery.

In a small proportion of people, extraintestinal disease may occur usually in the liver but can occur in the lungs, pleural space, pericardium, brain skin and genitourinary tract. Liver abscess may be acute with fever, abdominal pain, tachycardia, liver tenderness and hepatomegaly or chronic with weight loss, vague abdominal symptoms and irritability.

Modes of Transmission

Mainly through ingestion of fecally contaminated food or water containing amoebic cysts, which are relatively chlorine-resistant. Cysts can survive in moist environmental conditions for weeks to months. Transmission may occur sexually by fecal-oral contact with a chronically ill or asymptomatic cyst passer, or direct rectal inoculation through colonic irrigation devices. During the acute phase of the illness, those infected tend to shed more trophozoites than cysts and pose only limited danger to others because of the absence of cysts in dysenteric stools and the fragility of trophozoites.

The infective dose in humans is reported to be fewer than 10 cysts.

Incubation Period

From a few days to several months or years; commonly 2–4 weeks.

Period of Communicability

During the period that E. hystolytica cysts are passed, which may continue for years.

Risk Factors/Susceptibility

Sexual transmission via anal-oral contact.

Close contact with case.

Consumption of raw unwashed produce.

Consumption of potentially contaminated water.

Travel outside province/country.

Poor hand hygiene.

Diagnosis and Laboratory Testing

E. hystolytica is morphologically identical to non-pathogenic E. dispar.

Ova and parasite (O&P) screening on stool samples preserved in Sodium acetate-acetic acid-formalin (SAF) fixative. If positive for E.histolytia/dispar by screen, then stool antigen detection using ELISA on unpreserved stool sample to distinguish between E. histolytica from E. Dispar.

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Treatment and Case Management

Treatment is under the direction of the attending health care provider.

Provide information to patients on personal prevention measures (careful hand hygiene after defecation, sexual contact and before preparing or eating food) including advice to avoid public swimming pools when symptomatic. Household members should be assessed for symptoms.

Inform patients that symptomatic cases will be excluded from conducting activities in high-risk settings such as the food industry, healthcare, or daycare, for 24 hours after diarrhea resolves or for 48 hours after completion of antibiotic treatment.

Patient Information

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Additional Resources

Heymann, D.L. Control of Communicable Disease Manual (19th Ed.). Washington, American Public Health Association, 2008.

Public Health Ontario. “Amebiasis”.

References

Ministry of Health and Long-Term Care, Infectious Diseases Protocol, 2019.