Amoebiasis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Yazan Daaboul, M.D.; Serge Korjian M.D.

Overview

E. histolytica cyst is usually transmitted by the fecal-oral route (through contaminated drinking water or food) or by direct contact with infected individuals. Following transmission, E. histolytica trophozoites undergoes excystation in the small intestine, after which it migrates to the large intestine using pseudopods. The trophozoites invades the intestinal mucosa and migrates into the bloodstream. Simultaneously, they form resistant cysts in the large intestines that are then excreted in human stools. E. histolytica trophozoites secrete proteases and glycosidases to degrade the intestinal layers. Once cellular degradation occurs, upregulation of IL-8 and TNF-alpha results in the recruitment of neutrophils, macrophages, and eosinophils. The host immune system does not usually result in adequate destruction of the E. histolytica trophozoite, and the parasite continues to invade until it reaches the bloodstream, whereby it can then migrate to other organs and cause multisystem disease. On gross pathology, amebiasis may be characterized by wavy surface epithelium and formation of nodular and irregular ulcerations. On microscopic pathology, flask-shaped ulcers are characteristic, but interglandular ulcerations, hyperemia, mucosal thickening, reactive glandular hyperplasia, and neutrophilic infiltration are common findings. In late stages, amebiasis often results in tissue fibrinoid necrosis and formation of granulation tissue.

Transmission

E. histolytica cyst is usually transmitted by the fecal-oral route through contaminated drinking water or food
E. histolytica cyst may also be transmitted through direct contact with infected individuals.

Pathogenesis

Following transmission, E. histolytica trophozoites undergoes excystation in the small intestine, after which it migrates to the large intestine using pseudopods.
In the large intestine, the trophozoites invades the intestinal mucosa into the bloodstream. Simultaneously, they form resistant cysts in the large intestines that are then excreted in human stools.
Once in the bloodstream, the trophozoite migrates into the portal circulation and develops amebic liver abscess.^[1]

Invasion of Intestinal Mucosa

E. histolytica trophozoites secrete proteases, which induce the release of mucin from goblet cells, resulting in glandular hyperplasia.^[1]
E. histolytica is also thought to contain glycosidases that cleave glycsolyated mucin molecules, resulting in mucin degradation.^[2]^[3]
Once the mucin layer is degraded, E. histolytica then adheres to the enterocyte plasma membrane and uses lectins, amebapores, and proteases to cause damage by a characteristic "hit and run" phenomenon.^[1]

Lectin: responsible for adhesion of the parasite on Gal-GalNAc residues of the enterocyte
Amebapore: Protein that forms channels that induce cytolysis in a process similar to perforin-mediated cytolysis of cytotoxic T-cells
Protrease: Enzymes that metabolize cellular proteins

As the trophozites creates interglandular lesions and degrades the extracellular matrix, it is propelled forward by locomotion.^[1]^[4]^[5]

Activation of Host Immune System

As the trophozoites invade, IL-8 and TNF-alpha secretion is upregulated, and the host immune cells are activated.^[6]^[7]
Neutrophils migrate to the site of invasion and contribute to the inflammatory damage induced by E. histolytica, but are generally incapable of destroying the organism. The mechanism may which E. histolytica evades neutrophils is unknown.^[6]^[7]^[1]
Once neutrophils are recruited, macrophages and eosinophils are also activated.^[1]
The tropohozites can then invade into the bloodsteam, whereby they can ingest red blood cells (erythrophagocytosis) and migrate into distant organs (e.g. liver, brain, lungs).

Pathology

Gross Pathology

On gross pathology, the following findings may be present in patients with amebiasis:^[1]

Wavy surface epithelium (results from focal release of mucin and spasm of the muscular layer)
Nodular and/or irregular ulcers in the cecum (most common), sigmoid colon, and rectum. Early ulcers are usually in the interglandular epithelium.

Nodular: small (sub-centrimetric), rounded, elevated lesions with necrotic center and edematous rim
Irregular: large (1-5 cm), shallow with broad elevated margins

Note: the mucosal folds may occasionally hide small colonic ulcers (false-negative results)

Microscopic Pathology

On microscopic pathology, amebiasis is characterized by formation of multiple flask-shaped ulcers (deep, microhemorrhagic ulceration involving the submucosa), which are findings associated with advanced disease.^[1]
Additional findings may be present in patients with amebiasis:^[1]