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Amoebiasis - Overview
Last reviewed: 05.07.2025
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Amebiasis is an anthropozoonotic protozoan disease with a fecal-oral transmission mechanism. Amebiasis is characterized by ulcerative lesions of the colon, a tendency to chronic recurrent course, extraintestinal complications in the form of abscesses of the liver and other organs.
[ Epidemiology of amoebiasis
The source is a person (mainly a carrier of lumen forms), excreting mature cysts of amoebas with feces. The transmission mechanism is fecal-oral. The routes of transmission are water, alimentary, contact-household. The factors of transmission are water, food products (mainly vegetables and fruits that have not been heat-treated), household items. Cysts can be spread by mechanical carriers: flies and cockroaches, in whose digestive system amoebas remain viable for several days.
Susceptibility is relative. No seasonality is observed; some increase in morbidity in the warm season is associated with exacerbations of intestinal amoebiasis caused by various reasons, primarily the layering of acute intestinal infections. In countries with a temperate climate, infection with E. dispar is 10 times higher than with E. histolytica; the latter predominates in tropical countries. Specific antibodies do not play a significant protective role in E. histolytica invasion. Immunity in amoebiasis does not protect against relapses and reinfection, since it is unstable and non-sterile.
High prevalence of amoebiasis is observed in Southeast Asia, South and Central America, South and West Africa. The disease amoebiasis is common in the CIS countries, Transcaucasia and Central Asia. Approximately 480 million people are carriers of E. histolytica, 48 million of them develop colitis and extraintestinal abscesses, more than 50 thousand patients die. In Russia, sporadic cases, mainly imported, are detected in all regions; the risk of amoebiasis is higher in the southern regions of the country.
What causes amebiasis?
Amebiasis is caused by Entamoeba histolytica, which belongs to the kingdom Protozoa, subphylum Sarcodina, class Rhizopoda, order Amoebina, family Entamoebidae.
The life cycle of E. histolytica includes two stages - vegetative (trophozoite) and resting stage (cyst). The small vegetative form (luminal form, or forma minuta) has dimensions from 7 to 25 μm. The division of the cytoplasm into ecto- and endoplasm is poorly expressed. This non-pathogenic, commensal form lives in the lumen of the human colon, feeds on bacteria by endocytosis, is mobile, and reproduces vegetatively. The tissue form (20-25 μm) is found in the affected tissues and organs of the host. It has an oval nucleus, well-defined glassy ectoplasm and granular endoplasm, is very mobile, and forms wide blunt pseudopodia. The large vegetative form (forma magna) is formed from the tissue form.
Pathogenesis of amebiasis
The reason why E. histolytica passes from the luminal state to tissue parasitism is not fully understood. It is believed that the main virulence factor in E. histolytica is cysteine proteinases, which are absent in E. dispar. In the development of invasive forms of amoebiasis, such factors as the intensity of invasion, changes in the physicochemical environment of the intestinal contents, immunodeficiency, starvation, stress, etc. are important. Relatively frequent development of invasive forms in women during pregnancy and lactation, in persons infected with HIV is noted. Probably, amoebas pass to tissue parasitism with the acquisition of properties characteristic of other pathogenic microorganisms, such as adhesiveness, invasiveness, the ability to affect the host's defense mechanisms, etc. It has been established that trophozoites attach to epithelial cells due to a specific lectin - galactose-N-acetylgalactosamine. E. histolytica has been found to contain hemolysins, proteases, and in some strains, hyaluronidase, which may play a significant role in the destruction of the epithelial barrier by amoebae.
What are the symptoms of amebiasis?
In countries where E. histolytica is widespread, 90% of infected individuals have noninvasive amoebiasis and are thus asymptomatic carriers of luminal forms of amoebas, and only 10% of infected individuals develop invasive amoebiasis.
Invasive amoebiasis has two main forms - intestinal and extraintestinal.
When lesions are localized in the rectosigmoid region of the colon, the symptoms may correspond to a dysentery-like syndrome with tenesmus and occasionally with an admixture of mucus, blood and pus in the stool. When lesions are localized in the cecum, constipation with pain in the right iliac region and symptoms characteristic of the clinical picture of chronic appendicitis are noted (in some cases, appendicitis actually develops). In the ileum, amoebic lesions are relatively rare.
How is amoebiasis diagnosed?
The most reliable diagnostic test for intestinal amoebiasis is a microscopic examination of feces to detect vegetative forms (trophozoites) and cysts. Trophozoites are best detected in patients with diarrhea, and cysts in formed stool. Primary microscopy involves examining native preparations from fresh fecal samples with physiological saline. To identify amoeba trophozoites, native preparations are stained with Lugol's solution or buffered methylene blue. To identify cysts, native preparations prepared from fresh or preservative-treated fecal samples are stained with iodine. Detection of amoebas is more effective with immediate examination of feces after administration of a laxative.
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How is amoebiasis treated?
Amebiasis is treated with drugs that can be divided into two groups - contact (luminal) drugs that affect intestinal luminal forms, and systemic tissue amebicides.
Non-invasive amoebiasis (asymptomatic carriers) is treated with luminal amebicides. They are also recommended to be prescribed after completion of treatment with tissue amebicides to eliminate any amoebae that may remain in the intestine. If reinfection cannot be prevented, the use of luminal amebicides is inappropriate. In these situations, luminal amebicides should be prescribed according to epidemiological indications, for example, to persons whose professional activities may contribute to the infection of others, in particular, employees of food establishments.
Drugs
Prevention of amebiasis
Amebiasis can be prevented by protecting water bodies from faecal contamination and ensuring a high-quality water supply; preventing contamination of food with amoebic cysts; early detection and treatment of amoebiasis and asymptomatic carriers; and systematic health education. Boiling water is a more effective method of killing amoebic cysts than using chemicals.
What is the prognosis for amebiasis?
Currently, amoebiasis is considered to be a virtually completely curable disease, provided that it is diagnosed early and treated adequately. However, developing complications of intestinal amoebiasis and liver abscesses remain the main cause of death.