The infectious agent
- The causative agent of amoebiasis is a pathogenic protozoa (amoeba) called Entamoeba histolytica. E. histolytica is now recognized as a separate species from the closely related non-pathogenic Entamoeba dispar.
- Distribution is worldwide with a higher prevalence in tropical and subtropical regions. The annual number of patients is approximately 40–50 million, of which about 10% are symtomatic. The overall mortality is around 0.2%. Some of the deaths result from the development of an amoebic abscess 1, but amoebic colitis that causes ulcerations may also lead to bloody diarrhoea (dysentery) and death (usually in less than 2% of patients with amoebic colitis).
Transmission and clinical pictures
- E. histolytica lives in the large intestine and produces cysts into the faeces. Contamination occurs when cysts are ingested.
- E. histolytica may penetrate the intestinal mucous layer, destroy epithelial cells and cause crater-shaped ulcers.
- Sometimes the organisms invade the peritoneum by penetrating the intestinal wall and cause peritonitis.
- As the amoebae disseminate through the venous circulation e.g. into the liver an amoebic abscess may develop 2.
- Other possible, though rare, intestinal disease forms include toxic megacolon and amoeboma, i.e. a tumour-like lesion in the colonic wall consisting of granulomatous tissue and causing local obstruction.
- The incubation period from contracting the infection until symptom onset is from 1 week to 4 months.
- It was previously thought that almost all of carriers of E. histolytica were asymptomatic, but according to current knowledge, most of those being asymptomatic are carriers of the nonpathogenic E. dispar. Nevertheless, about 90% of the patients infected with E. histolytica still appear to remain asymptomatic or only have mild symptoms.
- The clinical picture of intestinal amoebiasis varies from very slight diarrhoea to bloody diarrhoea, i.e. dysentery, which may be life threatening.
- In addition to diarrhoea, the symptoms include abdominal pain, cramps, fatigue, low grade fever, loss of appetite, headache and low back pain.
- On examination the patient usually has a tender abdomen and about 10–30% of the patients have fever. Visible blood in stools is often present.
- Diagnosis is based on detecting E. histolytica from a stool sample or a colonic biopsy.
- In severe diarrhoea, no cysts are formed but the majority of the amoebae will be shed as trophozoites. The primary investigation when amoebiasis is suspected is either nucleic acid detection test or antigen detection test on an untreated stool sample. Both tests identify specifically the pathogenic E. histolytica only and not the non-pathogenic E. dispar.
- If amoebiasis is suspected in association with chronic gastrointestinal complaints, formalin-fixed samples collected on three different days may be used as a screening test, as an alternative to the nucleic acid detection test.
- Samples (curettage or biopsy) taken during colonoscopy can also be examined for amoebae by antigen or nucleic acid detection or staining.
- In differential diagnostics, other intestinal infections, particularly those causing bloody diarrhoea, as well as ulcerative colitis, Crohn’s disease 3 and irritable bowel syndrome should be considered.
- Sometimes a stool or intestinal sample for E. histolytica may reveal the presence of other amoebae. Entamoeba coli, E. hartmanni, E. dispar, Endolimax nana and Iodamoeba bütschlii are all non-pathogenic and do not therefore require treatment with antimicrobials.
Treatment and prognosis
- The aim of the treatment is to eliminate symptoms and to prevent further transmission of the infection.
- The most effective treatment of amoebic colitis and extraintestinal amoebiasis is metronidazole 400–800 mg × 3 for 7–10 days (children: 12–16 mg/kg × 3) B. To remove intestinal cysts, additional treatment with either diloxanide furoate 500 mg × 3 for 10 days (children: 7 mg/kg × 3), iodoquinol 650 mg × 3 for 20 days (children: 10–13 mg/kg × 3, up to 2 g/day), or with paromomycin 8–11 mg/kg × 3 for 7 days is required.
- The treatment of asymptomatic carriers of gastrointestinal E. histolytica is aimed at eliminating the source of infection and thus preventing later development of a symptomatic disease. The first-line drug for the treatment of asymptomatic patients is diloxanide furoate, iodoquinol or paromomycin, which all are more effective against cysts than metronidazole. The treatment of carriers of E. dispar is not indicated. It is usually warranted to investigate with nucleic acid or antigen detection test stool samples from persons living in the same household with patients with a severe clinical picture.
- Relapses do sometimes occur and a repeat stool sample should therefore be submitted 4 weeks after the treatment.
- Adding chlorine to drinking water does not destroy the cysts of E. histolytica, but good filtering systems, treating water with iodine tablets, freezing it to –20°C or heating it (5 min. +50°C) will eliminate the cysts.
- Cochrane reviews 1
1. Pritt BS, Clark CG. Amebiasis. Mayo Clin Proc 2008;83(10):1154-9; quiz 1159-60. [PMID:18828976]
2. Stanley SL Jr. Amoebiasis. Lancet 2003;361(9362):1025-34. [PMID:12660071]
3. Tanyuksel M, Petri WA Jr. Laboratory diagnosis of amebiasis. Clin Microbiol Rev 2003 Oct;16(4):713-29. [PMID:14557296]
4. Haque R, Huston CD, Hughes M, Houpt E, Petri WA Jr. Amebiasis. N Engl J Med 2003 Apr 17;348(16):1565-73. [PMID:12700377]
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