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Amoebiasis (Entamoeba histolytica)


Amoebiasis is caused by infection with Entamoeba histolytica which is transmitted primarily through the faecal-oral route. Symtoms may include: abdominal cramps, bloody diarrhoea or diarrhoea with mucus, nausea and vomiting, loss of weight and intermittent fever.

Extra-intestinal amoebiasis can occur if the amoebae spreads to other organs, most commonly the liver where it causes amoebic liver abscess. Amoebic liver abscess often present with fever and right upper quadrant abdominal pain.

In patients with a travel or exposure history compatible with intestinal amoebiasis, it is advised that the following investigations are performed prior to starting immunosuppression or surgery for suspected inflammatory bowel disease: hot stool examination for trophozoites, PCR of stool for E. histolytica, amoebic serology and rectal scrapings/biopsies.

Positive results should be discussed with a member of the Parasitology clinical team.


Detection of Entamoeba histolytica/Entamoeba dispar cysts and trophozoites by microscopy

  • Sample type: Standard stool sample – cysts may be identified in stool samples, but Entamoeba histolytica and Entamoeba dispar cysts are indistinguishable by microscopy.
  • Sample type: Hot stool sample – Examination for trophozoites requires that the stool is examined within 15 to 20 minutes of voiding. Stool samples for examination can be sent by conventional means so long as they arrive within this time frame. Please phone the laboratory to inform them of expected sample arrival in advance of submission.
  • Sample type: Rectal scrapings – Microscopy for these samples must be arranged with the laboratory in advance.

 

Detection of Entamoeba histolytica, Cryptosporidium species and Giardia intestinalis by PCR

  • Sample type: Standard stool sample must NOT be in any fixative as this may cause false negatives
  • Sample type: Liver aspirates for the molecular test must NOT be in any fixative as this may cause false negatives (please note this test has not been validated for this sample type but will still be performed upon request).
  • Sample type: Duodenal biopsy or fluid for the molecular test must NOT be in any fixative as this may cause false negatives (please note this test has not been validated for this sample type but will still be performed upon request).

This test offers several advantages over standard microscopy based diagnostics. The assay is significantly more sensitive (greater than ten fold improvement in the limit of detection for some species) than light microscopy. In addition, the assay is semi-quantitative and can therefore reveal detailed information on the response of a patient’s parasite load to subsequent drug therapy.

For Entamoeba histolytica, the assay also has the advantage of being specific for this pathogen, and does not pick up morphologically related but non pathogenic cysts such as those of Entamoeba dispar.

Finally, the assay can be run on a much wider range of samples, such as biopsies and liver aspirates, as it does not rely on the presence of morphologically intact parasites, although the assay is not currently validated for anything other than stool samples.


Detection of antibodies to E. histolytica by serology

  • Sample type: A minimum of 0.5ml of serum is required (can be transported at room temperature).

The Entamoeba histolytica ELISA is an essential test in cases of suspected amoebic liver abscess (ALA). The reported sensitivity of this ELISA is 100% in patients suffering from an amoebic abscess. The specificity of this test is reported as: 96% in uninfected Swiss blood donors; 89% in patients suspected to have amoebiasis where the disease has been ruled out; and 80% in patients with other parasitic infections. Cross-reactivity mainly occurs in patients with leishmaniasis, malaria, filariasis and strongyloidiasis.

Amoebic serology may be negative in amoebic dysentery, especially in the early stage. If clinical suspicion remains, please send a further serum sample 2 weeks after the first sample. In addition, for all suspected cases of intestinal amoebiasis, faecal microscopy and PCR for Entamoeba histolytica should be performed.

Diagnosis of amoebic liver abscess is based on imaging plus serology. Amoebic serology can take up to 2 weeks to become positive in amoebic liver abscess. Therefore, a further serum sample should be sent if the first one is negative, if clinical suspicion remains. In the interim, empirical treatment with metronidazole or tinidazole should be considered.

In cases of diagnostic uncertainty where liver abscess aspiration has been performed, please send an aliquot of the aspirate for Entamoeba histolytica PCR. Light microscopy alone is not adequate to confirm or exclude the presence of amoebae in liver abscess pus.