Entamoeba histolytica

The gastrointestinal pathogen Entamoeba histolytica causes amoebiasis

Clinical severity ranges from the asymptomatic passage of cysts in the stool to fulminant dysentery. Potentially fatal extraintestinal amoebiasis, including amoebic liver abscess, complicates 1%–3% of infections.¹ Pregnancy, immunocompromise, corticosteroid use, alcohol abuse and diabetes are risk factors for severe disease.² Transmission can be sexual or occur via fecal contamination of food and water.³ Most infections are acquired abroad in tropical and subtropical areas,⁴ although household contacts of a patient with this infection, men who have sex with men and residents of institutions are also at risk.³

Standard stool examinations for ova and parasites cannot reliably distinguish pathogenic E. histolytica from nonpathogenic Entamoeba dispar

Stool microscopy reports typically state “Entamoeba histolytica/dispar present.” Although morphologically identical to E. histolytica, E. dispar is a nonpathogenic intestinal amoeba. False-negative result rates for stool microscopy approach 40%, but decrease to 5%–15% with 3 or more specimens examined.¹ Testing to confirm E. histolytica by polymerase chain reaction or enzyme immunoassay should be done on a separate, fresh and unpreserved specimen.⁵ In developed countries, the prevalence of E. histolytica/E. dispar is 2.4% among immigrants, 4% among travellers and 27% among men who have sex with men.⁴ Most cases represent E. dispar colonization; in 1 Canadian study, only 4.6% of cases involved E. histolytica.⁴

Infection with E. histolytica requires treatment; colonization with E. dispar does not

Entamoeba histolytica is a nationally and provincially notifiable disease; E. dispar is a harmless commensal with no public health implications. Colonization of the stool with E. dispar suggests potential exposure to other pathogens with fecal–oral transmission but does not cause diarrhea.

Empiric treatment is rarely warranted

Current guidelines recommend treatment only when E. histolytica is identified, or if dysentery of unknown cause persists despite empiric treatment of Shigella with consecutive 2-day courses of different antibiotic agents.⁶ Suspicion of amoebic liver abscess based on epidemiology and characteristic imaging should prompt empiric treatment.²

Invasive disease requires dual therapy

Symptomatic amoebiasis requires a 2-drug regimen: an amoebicidal agent, such as metronidazole, and a luminal-acting cysticidal agent, such as iodoquinol.⁷ A recent meta-analysis concluded that monotherapy is inadequate for most cases of amoebiasis.⁷ Asymptomatic cyst passage is treated with a luminal cysticide to prevent transmission and progression to invasive disease. A summary of therapeutic options is shown in Appendix 1 (available at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.121576/-/DC1).