December 15 - "Blastocystis hominis"

Single-celled organisms are difficult to classify. They don't have very much when it comes to morphology and so for a long time were just put into the large, amorphous group called "Protozoa" and treated as descending from a common ancestor. We now know that they are very divergent groups and today's parasite is a perfect example of the challenges of taxonomy. Blastocystis was originally thought to be closely related to yeasts, but then was moved to a large group called Sporozoa, which includes many well known parasites. DNA sequence data have shown, though that these parasites are part of another group known as the stramenopiles, which includes the diatoms, brown algae and Phytophthora infestans, the cause of Irish potato blight. Species of Blastocystis were classified based on the host that they had been found in, hence Blastocystis hominis. However, genetic studies showed that there is not a single species that infects humans, but rather nine or ten different subtypes, which have not yet been formally described (thus the quotation marks on the name.) Even more confusing than the taxonomy is the pathology. The protists live in the GI tract and are thought to produce typical types of GI-tract symptoms (do I have to list them?), but the symptoms reported are extremely varied and not everyone that tests positive for it shows symptoms.