Non-E. histolytica intestinal amoebae
At least 10 different amoeba species are found in the intestinal lumen or mouth. Some consider all amoebae apart from E. histolytica as non-pathogenic commensals, but more investigation is needed to clarify some issues especially regarding Blastocystis hominis and Dientamoeba fragilis. Pathogenicity is probably due to strain differences that are increasingly investigated. Genetic analysis indicates that D. fragilis is actually more closely related to Trichomonas than to amoebae.
- Entamoeba histolytica
- Entamoeba dispar
- Entamoeba moshkovskii
- Entamoeba hartmanni
- Entamoeba coli
- Entamoeba polecki
- Entaoeba chattoni
- Entamoeba gingivalis
- Endolimax nana
- Iodamoeba butschlii
- Blastocystis hominis
- Dientamoeba fragilis
E. dispar and E. moshkovskii are morphological identical with E. histolytica. In order to distinguish between E. histolytica and E. dispar molecular tools such as PCR technology are used. Most antigen-detection tests cannot distinguish the two organisms, although one test (Wampole E. histolytica test) uses reagents that differentiate between E. histolytica and E. dispar. If trophozoites in stool contain RBCs, they are pathogenic E. histolytica, but if the trophozoites do not contain RBCs no species identification can be reached. Limited research has been carried out on E. moshkovskii. At present there are no good practical tests to distinguish this organisms from the two other look-alikes. Its presence is suspected especially in people who have E. histolytica / E. dispar-like cysts in the stools, but who test negative for E. histolytica/E. dispar antigen. E. moshkovskii is highly resistant to the current amoebicidal drugs. The existence of these non-pathogenic look-alikes often results in clinical doubt and leads to overtreatment. Infections with non-pathogenic amoebae are much more frequent than infections with pathogenic E. histolytica.
E. hartmanni is a non-pathogenic intraluminal parasite which can only be distinguished from E. histolytica forms by its smaller dimensions.
Entamoeba coli is a non-pathogenic organism that is commonly mistaken for a pathogenic E. histolytica. Trophozoites move slowly and never contain red blood cells. E. coli cysts are larger (10-30 µm) and may contain up to eight nuclei.
Endolimax nana is non-pathogenic. The trophozoites are small (up to 10 µm), move slowly with blunt hyaline pseudopods.
Iodamoeba butschlii also has small cysts, about 9 µm. These have only one nucleus and a glycogen mass which stains with iodine (Lugol), from which it gets its name.
Dientamoeba fragilis is an amoeboflagellate. The fact that it can develop flagella puts it in a different taxonomic group from the above-mentioned amoebae. It is more closely related to Trichomonas sp than to Entamoeba histolytica. It is a non-invasive intestinal parasite. Many infections are asymptomatic, but it has been associated with non-specific diarrhoea. It is very difficult to demonstrate with the microscope because the vegetative form is easily damaged (fragilis = breakable). No cyst stage is known and it is unclear if transmission via trophozoites can take place. One hypothesis as to how transmission of such a fragile microorganism is possible is that Enterobius vermicularis (pinworms) could function as vectors but solid evidence is lacking. If the faeces cannot be brought quickly to the laboratory (ideally < 10 min), they should be fixed in PVA (polyvinyl alcohol) or SAF (sodium acetate formalin), otherwise the parasite will most likely not be detected. There seems to be wide genetic variability between isolates, e.g. as demonstrated by differences in DNA melting temperature or variability of certain DNA markers. As more information will become available in the future; it is possible we will encounter a scenario like the one with Entamoeba histolytica (being pathogenic) and Entamoeba dispar (non-pathogenic): i.e. a heterogeneous species with genetic variants that have similar morphologies but different pathogenicities. It is clear that more study is needed. Dientamoeba fragilis infections can be treated with a 5-day course of metronidazole, but a single 2-gram dose (adult patient) of ornidazole it is easier and gives less side-effects. Paromomycin and iodoquinol can also be used and actually give higher cure rates.
For Blastocystis hominis, see below (separate chapter).
