Amoebic abscess

Causative agent

  • Amoebic abscess and intestinal amoebiasis are caused by the amoeba Entamoeba histolytica, which is a pathogenic protozoan. The closely related E. dispar, which was previously mistaken for E. histolytica, is not pathogenic.

Distribution

  • Worldwide, but principally in the tropical and subtropical regions. The worldwide incidence of serious amoebiasis is estimated at 40–50 million cases annually. About 4% of persons with amoebic colitis will develop an amoebic abscess.

Transmission

  • E. histolytica lives in the human colon where it forms cysts that are excreted in the faeces. The infection is acquired through ingestion of the cystic form of the protozoan. The cysts remain viable for a considerable length of time, and the transmission usually occurs via faecally contaminated food or drink.
  • Upon colonising the intestines, the E. histolytica amoebas invade the intestinal wall where they may cause crater-like ulcers through which the amoebas gain access to the venous blood stream. They may subsequently spread to the liver, and other sites, and form an amoebic abscess.

Significance worldwide

  • The mortality rate in amoebiasis is about 0.1–0.2%. The great majority of deaths are due to the development of an amoebic abscess.
  • The mortality rate in amoebic abscess is 2–5%.
  • An amoebic abscess should be considered in the differential diagnosis if a febrile patient returning from a tropical region presents with upper abdominal pain or if the imaging studies are suggestive of an abscess in the liver.

Signs and symptoms

  • The incubation period from infection to symptom emergence varies from 2 weeks to 5 months. In travellers, symptoms usually start within 2–3 months after return.
  • The symptoms of a liver abscess are fever and upper abdominal or flank pain. In a slowly progressing, subacute type of the disease the symptoms may also consist of nausea and weight loss.
  • In addition to fever, a physical examination usually reveals tenderness of the upper abdomen and hepatomegaly.
  • Leucocytosis as well as increased concentrations of alkaline phosphatase, alanine aminotransferase and CRP are common.

Diagnosis

  • The differential diagnosis of an amoebic abscess includes, among others, a bacterial abscess, cholecystitis, a tumour, echinococcosis and a cyst.
  • The diagnosis is based on imaging studies of the liver and the detection of antibodies against E. histolytica in the serum.
  • If a bacterial abscess cannot be excluded or if the anti-amoebic drugs have no effect, a diagnostic percutaneous aspiration is indicated. The aspirate should be cultured for bacteria, examined for amoeba either under the microscope or by testing to detect nucleic acids or antigens, and if a tumour is suspected, with cytology.
  • Stool samples must always be sent for analysis from patients with an amoebic abscess since the patient may also harbour intestinal amoebiasis, or be an asymptomatic carrier, and thus require medication that is effective against the cystic form of the protozoan. Amoeba is found in the intestinal tract of about one third of all patients with an amoebic abscess.

Treatment, follow-up and prognosis

  • An amoebic abscess can usually be cured with drug treatment, and surgical intervention has even been considered to be contraindicated due to the risk of spread into the abdominal cavity.
  • The most effective treatment is metronidazole (in severe disease intravenously 500 mg 3 times daily; for children 35–50 mg/kg divided into 3 doses), after which the medication is continued with oral metronidazole 400–800 mg 3 times daily. The total duration of treatment is 10 days and the maximum daily dose of metronidazole is 2,400 mg. In addition to medication, percutaneous drainage of the abscess may be indicated. For the eradication of possible intestinal cysts, additional medication is required; see (Amoebiasis).
  • The aim of treatment in asymptomatic carriers of E. histolytica is to prevent both the dissemination of the protozoans to extra-intestinal sites and the subsequent development of a symptomatic disease. At the same time, the treatment prevents the spread of the disease into the environment.
  • Healing of an amoebic abscess is followed up clinically and by imaging studies. Normalization of the imaging findings may take 3–12 months.
  • If E. histolytica was detected in a faeces sample, the effect of the treatment on the amoeba cysts should be controlled with faecal samples about 2 weeks after the treatment.

Prevention

  • E. histolytica cysts are resistant to the standard chlorination of water, but the cysts can be eradicated from the water supply by adequate filtration, treatment with iodine, freezing to –20°C or heating (5 min at +50°C).

References

1. Pritt BS, Clark CG. Amebiasis. Mayo Clin Proc 2008;83(10):1154-9; quiz 1159-60.  [PMID:18828976]


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