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Common Enteric Protozoal Infections

1. Non-pathogenic protozoa

Amoeba: Entamoeba hartmanii, Entamoeba coli, Entamoeba polecki, Entamoeba dispar (see below), Endolimax nana, Iodamoeba butschlii

Flagellates: Trichomnas hominis, Chilomastix mesnili

If non-pathogenic protozoa are reported exclusively, a thorough search for pathogenic organisms is important as the former implies a breakdown of faecal-oral hygiene. Consider:

Are more stool specimens needed? At least 3 collections on separate days, preferably more, should be taken as the excretion of parasites can be intermittent

Has your patient collected them properly? Specimens for bacterial culture must be placed into the fridge (not freezer) soon after collection to prevent the proliferation of commensal organisms at room temperature.

The recovery of protozoa is enhanced by collection of stool into fixative by the patient (see Dientamoeba fragilis below) and permanent staining of the slide by the Laboratory.

Does the laboratory need specific requests? Cryptosporidia requires an acid-fast stain. Ask for Clostridium difficile toxin and/or culture if antibiotics have been prescribed in the last 6 weeks.

2. Entamoeba histolytica/dispar complex

Pathogenic and non-pathogenic strains of Entamoeba histolytica can now be differentiated based on specific parasite iso-enzymes. The non-pathogenic strain of E. histolytica has been renamed E. dispar.

In stool, light microscopy can not differentiate the 2 species, hence the term E. histolytica/dispar complex. If this is reported a request should be made for EIA for Entamoeba histolytica (some labs need a new specimen).

E. histolytica serology is positive in invasive disease such as liver abscess and colitis.

No treatment is needed for E. dispar.

Treatment Regimes:

Flagyl 800 mg tds 7 days, for liver abscess give 10 days therapy

Flagyl doesn’t eliminate cysts from the colon, cyst eradication should be considered in bad cases of colitis and liver abscess where relapse is best avoided. For cyst eradication use:

Diloxanide furoate 500 mg tds 10 days. Special Access Scheme approval needed; Fax 02 62328112

These latter 2 medications are available from West Lindfield Pharmacy Ph 02 9416 2642

4 weeks after finishing therapy repeat the stool parasite analysis.

3. Dientamoeba fragilis

This is now a recognised pathogen, which can cause acute and chronic gastrointestinal symptoms. A few case reports also suggest a causative role in colitis.

The recovery of these protozoa from stool is sub-optimal unless specific collection and processing guidelines are followed. This is because D. fragilis does not have a cyst stage and the trophozoites die easily, hence its name. Stool must be collected into fixative (sodium acetate-acetic acid formalin fixative) by the patient followed by the laboratory permanently staining the slides for analysis.

Treatment Regimes:

Paromomycin 500mg tds for 7 days. Special Access Scheme approval needed; Fax 02 62328112

Avaliable from West Lindfield Pharmacy Ph 94162642

Flagyl is sub-optimal, sometimes Doxycycline 100 mg bd works.

4 weeks after finishing therapy repeat the stool parasite analysis.

4. Blastocystis hominis

Debate remains whether this is a commensal or true pathogen. There is considerable genetic diversity so perhaps there is more than 1 strain with differing levels of pathogenicity. The concentration of organisms in stool is not correlated with disease.

Before symptoms are ascribed to B. hominis ensure other enteric pathogens and non-infectious causes of diarrhoea have been thoroughly investigated.

Treatment Regimes:

Nitroimidazoles; Flagyl 800 mg tds 10 days or Fasigyn 500 mg tds 10 days


Bactrim DS 1 bd 10 days


A combination of Nitroimidazole and Bactrim