Monday, March 6, 2017

Answer to Case 437

Answer: Blastocystis species

I had posed the question about nomenclature to see how labs are currently reporting this parasite when it is identified in stool specimens by microscopic examination.

Originally, only B. hominis was thought to infect humans, while other species infected other animals. However, we now know that humans can be infected with the same organisms as many other hosts, and differentiation between these species is not possible by morphologic examination alone. Therefore, the most accurate way to report this organism when observed in human stool specimens is "Blastocystis sp." rather than "Blastocystis hominis".

This may cause some confusion among clinicians and therefore, providing some education about the change is recommended.

Some other fun taxonomic facts about Blastocystis spp.:
  • It was first described by Alexeev in 1911
  • It was thought to be a yeast due to its morphologic appearance
  • Later, Blastocystis was re-classified as a sporozoan protozoal parasite 
  • Blastocystis spp. is now believed to be a stramenopile - a group that includes diatoms, brown algae and mildew. The only other stramenopile to infect humans is Pythium. (HERE is a nice article in the Journal of Clinical Microbiology about human infection with P. insidiosum)
For more information about Blastocystis spp., check out the excellent BLOG by Dr. Christen Rune Stenvold. Many thanks to Dr. Graham Clark for his assistance with describing Blastocystis taxonomy is a comprehensible fashion.

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