Etiology and pathogenesis
The disease is prevalent in tropical countries, Central Asia, and the South Caucasus. The source of infection is an individual shedding parasite cysts in feces. Transmission occurs via the alimentary route through the consumption of contaminated vegetables, greens, fruits, and water.
Swallowed cysts of Entamoeba histolytica reach the large intestine, where amebas invade the submucosal layer. The local inflammatory process progresses rapidly, leading to tissue necrosis and the formation of characteristic ulcers. Thinning of the intestinal wall creates a risk of perforation resulting in purulent peritonitis. Furthermore, penetration of the parasite into intestinal blood vessels leads to hematogenous dissemination of the infection to internal organs.
Intestinal amebiasis and its complications
The classic presentation of intestinal amebiasis resembles bacterial dysentery. Patients present with fever, general weakness, and malaise. Stool becomes frequent, containing mucus and blood, acquiring a characteristic «raspberry jelly» appearance. The diagnosis is confirmed by detecting amebas in the stool and specific serological tests.
Surgical complications of the intestinal form include:
- Intestinal perforation and gangrene: lead to purulent peritonitis and bacterial-toxic shock. Signs of peritoneal irritation may be masked. Peritonitis is an indication for emergency surgery, whereas gangrene or pronounced necrosis around the ulcer requires intestinal resection with colostomy formation.
- Amebic granuloma (ameboma): a rare complication of a prolonged, untreated course. It is an inflammatory infiltrate (often in the cecum or ascending colon) palpable as a painful, firm-elastic mass. It consists of zones of necrosis and eosinophilic infiltration. It may suppurate (with a risk of fecal peritonitis) or cause obturation obstruction.
- Intestinal hemorrhage: a consequence of extensive ulcers. Managed conservatively (hemostatics, volume resuscitation, antiamebic drugs). Surgery (resection) is a last resort after colonoscopic verification of the source.
- Appendicitis: specific ulcers in the vermiform appendix combined with secondary flora. It mimics typical acute appendicitis.
Amebic liver abscess
This is the most frequent surgical complication of amebiasis (occurring in 2–10% of cases). The right lobe of the liver is affected in 80–90% of cases. A key morphological feature is the absence of a pyogenic membrane. The contents of the abscess consist of liquefied tissue in the form of odorless coffee-colored liquid pus. When bacterial flora is superadded, the color changes to greenish/whitish, and a foul odor appears.
The acute phase is characterized by an extremely severe condition, liver failure, and exacerbation of intestinal symptoms. There is severe pain in the right upper quadrant radiating to the scapula and shoulder. Objective findings reveal skin hyperesthesia, intercostal tenderness, and hepatomegaly.
Chronic abscess presents with signs of cachexia: patients lose weight, the skin becomes dry and grayish-earthy, sclerae are icteric, and pedal edema develops. The liver is markedly enlarged, firm, but less tender than in the acute phase.
Treatment is predominantly puncture-based (under US/CT guidance) with pus aspiration and instillation of medications. Open surgical drainage is used if punctures are ineffective or if there is a risk of rupture (into the abdominal or pleural cavities, or biliary tract).
Pulmonary and cerebral involvement
Extraintestinal dissemination may also affect other organs:
- Amebic lung abscess: metastasis of the pathogen leads to specific pneumonia or abscess formation. The clinical and radiological presentation is indistinguishable from non-specific suppurations. Treatment includes bronchological lavage and therapeutic punctures with the administration of antiamebic agents.
- Brain abscess: a rare but critical complication. Multiple foci typically form. Diagnosed based on clinical presentation and CT/US findings. Treated with a combination of specific medications and surgical removal of the abscesses along with the capsule.
In any complications, surgical intervention must be supplemented with pharmacotherapy (emetine, metronidazole, chloroquine).
Briefly: answers to topical questions
What stages of the life cycle does Entamoeba histolytica go through in the human body?
The life cycle of Entamoeba histolytica in the human body includes vegetative and cyst stages.
- Cyst — the resting stage, swallowed upon infection, which overcomes the acidic barrier of the stomach.
- Precystic form — a transitional stage formed in the small intestine.
- Small vegetative (luminal) form (forma minuta) — a non-pathogenic stage residing in the intestinal lumen.
- Large vegetative (tissue) form (forma magna) — a pathogenic stage that invades the intestinal wall and transforms into the tissue form.
Which specific serological reactions and methods are used for laboratory confirmation of amebiasis?
Laboratory confirmation of amebiasis relies on methods aimed at detecting specific antibodies and amebic antigens. The following serological and immunological reactions are used in clinical practice:
- IHA — indirect hemagglutination assay for antibody detection.
- ELISA — enzyme-linked immunosorbent assay for antibody detection.
- IFA — indirect immunofluorescence assay (immunofluorescence method).
- CFT — complement fixation test.
Detection of amebic antigens using monoclonal antibodies is also employed.
With which conditions must differential diagnosis of colonic ameboma be performed?
Differential diagnosis of colonic ameboma (amebic granuloma) must be performed against oncological neoplasms. Clinically and radiologically, this specific lesion—representing a proliferation of annular granulations in the intestinal wall tissues—manifests as a true intestinal tumor. For this reason, the pathology is primarily differentiated from colorectal cancer. When large, an ameboma can cause intestinal stricture and bowel obstruction, which also requires ruling out malignant growth.
For quick memorization of key amebiasis markers, use a «color» association: stool in the intestinal form resembles «raspberry jelly», and pus in a liver abscess resembles «coffee».
Class and exam questions
What is an ameboma?
It is an inflammatory perifocal infiltrate of the large intestine wall (usually the cecum or ascending colon) developing around a chronic amebic ulcer.
What is the management strategy for intestinal hemorrhage in the setting of amebiasis?
The mainstay is conservative therapy (hemostatics, infusions, antiamebic agents). Surgery (intestinal resection) is a last resort.
Which drugs are included in specific therapy?
Emetine, metronidazole, and chloroquine are used for all forms of complications.
What else to study on this topic
- Pathophysiology of masked symptoms of peritonitis during the perforation of amebic ulcers.
- Differential diagnosis of amoeboma and malignant neoplasms of the large intestine.
- Mechanisms of Entamoeba histolytica metastasis to the brain and lungs.
- Features of preoperative preparation in patients with bacterial-toxic shock due to intestinal gangrene.
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