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.
There are also some other pathogenic amoeba species, which do not cause intestinal infections and the infections caused by them are rare. Acanthamoeba species, which are also found in Northern Europe, can cause keratitis, primarily, however, in individuals using contact lenses. Acanthamoebas and amoeba-like flagellate species in the genus Naegleria can also cause severe infections of the central nervous system, but these are very rare. These other amoebas are not discussed in more detail in this article.
Occurrence
Distribution is worldwide with a higher prevalence in tropical and subtropical regions. The annual number of cases 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 Amoebic Abscess, 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 Amoebic Abscess.
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.
Symptoms
The incubation period from contracting the infection until symptom onset is from 1 week to 4 months.
It was previously thought that almost all 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 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
Diagnosis is based on detecting E. histolytica from a stool sample or a colonic biopsy.
The primary investigation when amoebiasis is suspected is a nucleic acid detection test on a stool sample. The test identifies specifically the pathogenic E. histolytica only and not the non-pathogenic E. dispar. Tests that are based on microscopy or antigen detection are less sensitive and hence clearly in a secondary role.
Samples (curettage or biopsy) taken during colonoscopy can also be examined for amoebae by antigen or nucleic acid detection or staining.
Sometimes, when looking for other amoebae in a stool or intestinal sample, E. histolytica may be found in microscopic examination. Entamoeba coli, E. hartmanni, E. dispar, Endolimax nana and Iodamoeba bütschlii are all non-pathogenic and do not therefore require treatment with antimicrobials.
To remove intestinal cysts, additional treatment with paromomycin 8-11 mg/kg 3 times daily 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 paromomycin, which is 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 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.
Prevention
Adding chlorine to drinking water does not destroy the cysts of E. histolytica, but good filtering systems, treating water with iodine, freezing it to -20°C or heating it (5 min. +50°C) will eliminate the cysts.
References
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Stanley SL Jr. Amoebiasis. Lancet 2003;361(9362):1025-34. [PubMed]
Tanyuksel M, Petri WA Jr. Laboratory diagnosis of amebiasis. Clin Microbiol Rev 2003 Oct;16(4):713-29. [PubMed]
Haque R, Huston CD, Hughes M, Houpt E, Petri WA Jr. Amebiasis. N Engl J Med 2003 Apr 17;348(16):1565-73. [PubMed]