Common enteric parasites

Clinical Parasitology

This list captures a typical top ten common enteric parasites of medical importance, though the order of importance will vary around Australia. Information on Entamoeba histolytica is given, as this species is a common reason for a test request even if a positive result is relatively uncommon. The pathogenic role of several of the other parasite species is debatable, and detection may be more useful as an indication of poor hygiene, recent international travel or immune compromise. Further details at the CDC.

Species Diagnostic stage   Size Key features
Blastocystis hominis round-oval form  6-40um Large central vacuole surrounded by small nuclei
Cryptosporidium parvum  oocyst  4-5um Round, pink on modified acid-fast stain
Endolimax nana  cyst & trophozoite  Cyst, 5-10m; troph, 6-12um Oval cyst – 4 nuclei; troph variable chromatin
Entamoeba coli  cyst & trophozoite  Cyst, 10-35um; troph, 15-  50um Cyst – 8-16 nuclei; troph – uneven periph chromatin, large karyosome
Entamoeba histolytica  cyst & trophozoite  Cyst, 10-20um; troph, 16-  20um Cyst – 4 nuclei; troph – even periph chromatin, central karyosome,  may have ingested erythrocytes
Enterobius vermicularis egg  50-60um long, 20-40um  wide Flattened on one side, colourless
Giardia lamblia Cyst & trophozoite  Cyst, 8-19um long; troph,  10-20um Oval or round cyst; troph pear-shaped, 2 nuclei, flagellate
Hymenolepis nana egg  30-45um 6-hook oncosphere at centre of thin walled egg
Iodamoeba butschlii cyst & trophozoite  Cyst,  5-20um; troph, 8-  20um cyst – large glycogen vacuole; troph – large karyosome
Schistosoma spp. egg  S.mansoni egg = 115-  175um  long; S.haematobium egg =  112-170um long S.mansoni egg, prominent lateral spine; S.haematobium egg , terminal spine
Strongyloides stercoralis first stage larva  180-380um long Short buccal cavity, mid-body genital primordium


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