Intestinal parasites of humans can be divided into mainly helminths ('worms' including cestodes, nematodes and trematodes), and single-celled eukaryotic organisms. Most single-celled intestinal parasites belong to one of four main groups:
- Archamoebae or Amoeboids (e.g. Entamoba, Iodamoeba, Endolimax)
- Ciliates (e.g. Balantidium)
- Sporozoa (e.g. Cryptosporidium, Cyclospora, Cystoisospora; even microsporidia)
- Flagellates (e.g. Giardia, Chilomastix, Enteromonas, Pentatrichomonas, Retortamonas, Dientamoeba (unflagellated flagellate!))
Traditionally, these four groups have been referred to as
protozoa.
However, the most common, single-celled intestinal parasitic eukaryote,
Blastocystis, does not belong in any of these four categories. Taxonomically,
Blastocystis belongs to the heterogeneous group of
Stramenopiles, which includes slime nets, diatoms, water moulds and brown algae. Most stramenopiles are free-living organisms.
Blastocystis is an atypical stramenopile not only as this group is named for the straw-like tubular hairs on the flagella and sometimes the cell body -
Blastocystis has no flagella and lacks any tubular hairs - but also due to its parasitic nature.
Often,
Blastocystis is referred to as a 'protozoon', although '
protist' is more appropriate. Protists can be defined basically as any eukaryote that is not a plant, an animal or a fungus.
One of the closest relatives of
Blastocystis identified to date is
Proteromonas lacertae, a parasite of reptiles.
Interestingly,
Proteromonas does have flagella and hairs on the cell body. For comparison, the image below shows
Blastocystis (culture)
- appearing almost amoeboid, only with very limited morphological
hallmarks (note examples of binary fission and the eccentrically located
nuclei and mitochondrion-like organelles).
Blastocystis is one of two Stramenopiles known to infect humans, the other being
Pythium insidiosum, which has been associated with keratitis and dermatological lesions mainly in SE Asia.
Other organisms with close relation to
Blastocystis include
Karotomorpha,
Cepedea,
Protoopalina and
Opalina.
For further information, please visit
