Blastocystis spp., also known as Blastocystis hominis, is an enigma with regard to classification, pathology, and forms. It has been classified with several groups, including yeast, sporozoa, and amoeba. Although it possibly has an amoeboid form, it is generally classified with the non-motile organisms. A number of recent sources indicate that it belongs to a group of organisms known as Stramenpiles or Heterokonta. This group encompasses organisms such as certain algae, slime molds, and diatoms. Though often known as Blastocystis hominis, it includes genetically diverse, but morphologically identical species, some of which may be pathogenic.
It is found worldwide in both humans and animals and is one of the most common parasites seen in human feces. In certain areas, up to 40% of fecal specimens may contain Blastocystis. It is thought to infect humans via the fecal-oral route, although human-to-human, animal-to-human, and insect-to-human transmissions are possible.
It may cause diarrhea, cramps, fever, nausea, abdominal pain and urticaria. It has also been suggested that it can cause intestinal obstruction in some patients.
Blastocystis spp. can have four or five forms or stages (not fully decided yet) and the infective stage has not been definitively described.
