Amebiasis and Infection with Free-Living Amebae
Amebiasis is an infection caused by Entamoeba histolytica, an intestinal protozoan. Its spectrum of clinical syndromes ranges from asymptomatic colonization (90% of cases) to invasive amebiasis, which accounts for 10% of affected individuals. Invasive amebiasis frequently presents as intestinal colitis (dysentery or diarrhea) or as extraintestinal amebiasis, in which abscesses of the liver are more commonly found than involvement of the lungs or brain.
LIFE CYCLE AND TRANSMISSION
E. histolytica is acquired by ingestion of viable cysts from fecally contaminated water, food, or hands (Fig. 218-1). Food-borne exposure is most prevalent and is particularly likely when food handlers are shedding cysts or food is being grown with feces-contaminated soil, fertilizer, or water. Besides the drinking of contaminated water, less common means of transmission include oral and anal sexual practices and—in rare instances—direct rectal inoculation through colonic irrigation devices. Motile trophozoites are released from cysts in the small intestine and, in most patients, remain as harmless commensals in the large bowel. After encystation, infectious cysts are shed in the stool and can survive for several weeks in a moist environment. In some patients, the trophozoites invade either the bowel mucosa, causing symptomatic colitis, or the bloodstream, causing distant abscesses of the liver, lungs, or brain. The trophozoites may not encyst in patients with active dysentery, and motile hematophagous trophozoites are frequently present in fresh stools. Trophozoites are rapidly killed by exposure to air or stomach acid, however, and therefore cannot transmit infection.
Life cycle of Entamoeba histolytica. GI, gastrointestinal; RBCs, red blood cells.
E. histolytica infection typically affects tropical underdeveloped regions with poor sanitation systems and hygiene, occurring particularly often in children <5 years of age. This infection is widespread on the Indian subcontinent and in Africa, parts of East Asia (Thailand), and Central and South America (Mexico and Colombia). According to the Global Burden of Disease 2015 study, amebiasis accounts for 67,900 all-age deaths, including 15,500 children <5 years old.
In contrast, the main groups at risk for amebiasis in developed countries are returned travelers, recent immigrants, men who have sex with men (MSM), military personnel, and inmates of institutions. Data for 1997–2011 from the GeoSentinel Surveillance Network, which encompasses information from tropical medicine clinics on six continents, showed that, among long-term travelers (trip duration, >6 months), diarrhea due to E. histolytica was among the most common diagnoses. In fact, amebiasis may be considered an emerging infectious disease in developed countries such as Japan, where the number of reported cases among HIV-positive patients, and particularly among MSM, has increased.
Worldwide, E. histolytica is the second most common cause of death related to parasitic infection (after malaria). ...