Sharon Roy, Barbara L. Herwaldt, Stephanie P. Johnston

Infectious Agent

Amebiasis is caused by the protozoan parasite Entamoeba histolytica.

Mode of Transmission

Transmission occurs via the fecal–oral route, either directly by person-to-person contact (e.g., diaper changing, sexual practices) or indirectly by eating or drinking fecally contaminated food or water.

Occurrence

• Amebiasis occurs worldwide, but it is more common in areas of poor sanitation, particularly in the tropics. Most infections, morbidity, and mortality occur in Africa, Asia, and Central and South America.
• Only an estimated 10%–20% of individuals infected with E. histolytica become symptomatic. Among these, approximately 50 million cases of invasive E. histolytica disease occur each year, with up to 100,000 deaths. Prevalence and presentation of symptomatic amebiasis vary geographically (e.g., amebic colitis may be the predominant presentation in one country, whereas amebic liver abscesses may predominate in another country).
• The prevalence of asymptomatic infection also varies geographically, ranging from 1% to 21% in persons in developing countries based on stool tests.

Risk for Travelers

• E. histolytica can infect persons of all ages.
• Persons at high risk for severe disease include pregnant women, immunocompromised individuals, and patients receiving corticosteroids. Associations with diabetes and alcohol use have also been reported.
• The rate of amebic diarrhea in returning travelers varies by travel destination. One study found rates of 1.5% in travelers returning from Southeast Asia and 3.6% in those returning from Central America. The overall rate in travelers returning from all regions was 2.7%. Other studies among travelers to the tropics provided similar estimates.
• Risk of infection for both travelers and residents is highest in settings with poor sanitation where barriers between human feces and food and water (including ice) are inadequate.

Clinical Presentation

• The clinical spectrum of E. histolytica ranges from asymptomatic infection to amebic diarrhea and dysentery to fulminant colitis and peritonitis to extraintestinal amebiasis.
• Acute amebiasis can present as amebic dysentery, with frequent, urgent, small bloody stools.
• Chronic amebiasis can present with alternating diarrhea and constipation every few days, combined with fatigue and weight loss.
• The incubation period is commonly 2–4 weeks but ranges from a few days to years.
• Occasionally, the parasite may spread to other organs (extraintestinal amebiasis), most commonly the liver (amebic liver abscess). Amebic liver abscess presents with fever and right upper quadrant abdominal pain, usually in the absence of diarrhea.

Diagnosis

• Microscopy does not distinguish between the amebas E. histolytica (pathogenic) and E. dispar (nonpathogenic). Enzyme immunoassay (EIA) or polymerase chain reaction (PCR) is needed to confirm the diagnosis of E. histolytica. Contact your state health department reference laboratory for recommendations on E. histolytica-specific testing.
• The recognition of two identical-appearing species, one pathogenic and one not, may explain the observation that some people with apparent E. histolytica infection were “asymptomatic cyst passers.” Based on this new knowledge, some people passing apparent E. histolytica cysts, but having no symptoms, may be infected with E. dispar and not require treatment.
• The sensitivity of serologic tests varies depending on clinical presentation (approximately 90% extraintestinal and 70% intestinal) and cannot distinguish between current and past infection.

Treatment

• Travelers with either asymptomatic E. histolytica infection or symptomatic E. histolytica disease should be treated if the organism can be proven to be E. histolytica. Otherwise, asymptomatic travelers do not need to be treated.
• For asymptomatic infection, iodoquinol or paromomycin are the drugs of choice.
• For symptomatic intestinal infection and extraintestinal disease, treatment with metronidazole or tinidazole should be followed by treatment with iodoquinol or paromomycin.

Preventive Measures for Travelers

No vaccine is available. Travelers to developing countries should be advised to follow food and water precautions.

References

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