ENTAMOEBA HISTOLYTICA: MORPHOLOGY, LIFE CYCLE, SYMPTOMS - BIOLOGY - 2025

Entamoeba histolytica is a parasitic microorganism of the human intestine. It can parasitize canids and other vertebrates. It is the causative agent of amoebic dysentery or amoebiasis.

It is an anaerobic organism that can live as a commensal in the large intestine or invade the mucosa causing significant lesions. From the intestine it can infect extrainstestinal liver, lung and even brain tissues. Pathogenic and non-pathogenic strains can exist.

Amoebic dysentery is one of the parasitic diseases with the highest morbidity and mortality in humans in tropical countries. It is considered the third leading cause of death after malaria and schistosomiasis.

Factors such as inadequate faecal waste management systems, potable water supply and inadequate food handling contribute to the existence of endemic areas in the world.

Biological characteristics

E. histolytica presents two parasitic forms: the cyst and the trophozoites. The cyst is the infectious form, it does not have locomotion and is resistant in the external environment; trophozoites represent the vegetative form, being mobile and active.

E. histolytica feeds by phagocytosis, that is, it emits pseudopods with which it introduces the small particles that make up its food into its cellular content where it is digested.

In its development the trophozoite and cyst phases are present. Trophozoites are the mobile, amoeboid form. The cyst is the non-active form, resistant to adverse conditions.

Morphology

E. histolytica is morphologically indistinguishable from commensal amoebae E. dispar and E. moshkovskii. It can be distinguished from E. coli, another species present in humans, because the latter does not emit pseudopods.

The trophozoite has a central mass called the endoplasm and an outer layer known as the ectoplasm. They have a nucleus with a central karyosome and regularly distributed peripheral chromatin.

It has an anterior end that can form pseudopods and a posterior end that presents a bulb or uroid with a tuft of filopodia for the accumulation of waste. It presents a system that consists of a network of digestive vacuoles and ribosomes.

The trophozoites can be in two forms: magna and minuta. The magna form measures 20-30 microns and can emit thick pseudopodia; the minute form measures 19-20 microns and can emit shorter pseudopods.

Cysts are round or spherical in shape. Under the microscope they show refractive, it can be seen that the membrane contains one to four nuclei depending on the maturity.

Metacysts have a thinner membrane. The nuclei are rod-shaped with rounded ends and glycogen vacuoles. In the cytoplasm, chromatid bodies can be seen, which are glycogen inclusions in the cytoplasm.

Lifecycle

E. histolitica

The parasitized person may remain asymptomatic, or present mild or severe symptoms. Mild cases are the most common, representing 90% of them.

Mild symptomatic cases show nausea, diarrhea, weight loss, fever, and abdominal pain. In chronic cases, colic can occur, including ulcers and the presence of blood in the stool.

When extra-intestinal invasion occurs, the most common condition is liver abscess, which causes fever and pain in the upper abdomen.

Diagnosis

The diagnosis is made by examining feces under a light microscope. In the samples, forms of the parasite are identified, in cases positive to amoebiasis. Serial examinations with a minimum of three samples analyzed on successive days are recommended.

The use of PCR or serology with specific antibodies are also useful techniques in diagnosis.

In extraintestinal cases the diagnosis can be made by CT images.

Mucus and blood may occur in the stool depending on the severity of the infection.

Treatment

Metronidazole, paromomycin, and tinidazole delivery have been used. In cases of extraintestinal invasion, such as liver abscesses, surgery has been a used technique.

It is recommended to verify the diagnosis well to avoid false identifications due to the presence of species such as E. dispar and E. moshkovskii. The misapplication of commonly used drugs leads to the formation of resistant strains.

Control and prevention

In the world, health strategies focus on the application of measures that seek to interrupt the biological cycle of the parasite, through the participation of the different social actors involved.

In this, the conscious participation of the communities is very important, mainly in areas of epidemiological risk. Among others we can mention:

• Education of the population about amebiasis, its life cycle and the risks of contagion
• Maintenance of adequate sanitary systems for the deposition and treatment of feces.
• Maintenance of adequate supply systems and access to drinking water.
• Availability of infrastructure and accessibility for the population to diagnostic services and care for affected people.

References

1. Chacín-Bonilla, L. (2013). Amebiasis: clinical, therapeutic and diagnostic aspects of the infection. Medical Journal of Chile, 141 (5): 609-615.
2. Diamond, LS & Clark, CG (1993). A redescription of Entamoeba histolytica Schaudinn, 1903 (emended Walker, 1911) separating it from Entamoeba dispar Brumpt, 1925. Journal of Eukaryotic Microbiology, 40: 340-344.
3. Elsheikha, HM, Regan, CS & Clark, CG (2018). Novel Entamoeba Findings in Nonhuman Primates. Trends in Parasitology, 34 (4): 283-294.
4. Gómez, JC, Cortés JA, Cuervo, SI &, López, MC (2007). Intestinal amebiasis. Infectio, 11 (1): 36-45.
5. Showler, A. & Boggild, A. (2013). Entamoeba histolytica. Canadian Medical Association Journal, 185 (12): 1064.