Why is Blastocystis hominis so controversial regarding pathogenicity?

Why is Blastocystis hominis so controversial regarding pathogenicity?

What we currently call Blastocystis hominis (morphologically) appears to be approximately 10 different strains or species, some of which are pathogenic and some are nonpathogenic. Based on continuing molecular studies, changes in the classification of this group are probable. If all strains or species currently make up what is called “Blastocystis hominis” – this could explain the controversy regarding pathogenicity and the fact that some patients are symptomatic, while some are asymptomatic. Unfortunately, all of the strains or species are morphologically identical; therefore, the correct identification reported should remain Blastocystis hominis, and the number should be quantitated from the permanent stained smear (rare, few, moderate, many, packed). Some laboratories have begun reporting Blastocystis spp., with a report comment explaining the pathogenic vs nonpathogenic controversy.

This is from http://www.med-chem.com
in the Parasitology section.

From 1912, when Blastocyctis was described up to the 1980's Blastocystis was thought to be a commensal yeast that occured in about half of the world's peoples. DNA analysis proved that it is a protozoa, one of a large and diverse group of mostly free living protozoa. In about the 1990's medical scientists began to consider that B. Hominis might be an opportunistic pathogen. The identification of the strains of B. hominis has led to the current understanding stated above. People who have been trained in parasite identification in the past 10 years would be able to note the presence on their reports but if their training has been before that they probably would not recognize it as a possible pathogen unless they happened to have taken continuing ed. courses in parasitology. At present there are no readily available advanced tests to guide a Dr. in diagnosis and treatment. A Lab report that states B. hominis is present (few to packed field)does not tell the Dr. whether this is the cause of the patient's problems. Tweet