In a laboratory specimen, Blastocyctis is typically a microscopic cell with a ring of cytoplasm and a handful of nuclei wrapped around a central body that resembles a vacuole. It is commonly seen in stool specimens submitted for ova and parasite examination. When found in humans, it is traditionally called Blastocystis hominis, and for years, no one has been sure what significance it has, if any.
Taxonomy of Blastocystis
The scientific classification of Blastocystis remains uncertain. Textbooks usually lump Blastocystis with the protozoa. In appearance, it is similar to the amoebae, but as early as 1993, research indicated that it might belong with the Apicomplexa, a taxonomic phylum that also includes Cryptosporidium, Toxoplasma gondii, and the parasites that cause malaria.
With the recent widespread use of DNA and RNA investigation methods, scientists have shown that Blastocystis may actually be more closely related to algae and diatoms than protozoa. RNA analysis also reveals that the same species of Blastocystis that infects humans also infects many species of animals; thus, the species name hominis – referring to humans – is probably inappropriate.
Does Blastocystis Cause Disease?
The medical significance of Blastocystis has been hotly debated, both in research and medicine. Though commonly seen in stool specimens for ova and parasites, many laboratories have only considered it a significant finding if it is present in large numbers.
Reasons for doubt that Blastocystis causes disease include:
- Many people have Blastocystis in stool specimens but have no symptoms.
- Treatment with antiparasitic drugs sometimes alleviates symptoms, but Blastocystis is not eliminated from the intestine.
- Mixed protozoan infections are common, but protozoa can be difficult to detect with routine laboratory methods. Symptoms are often attributed to a second undetected parasite.
- Some studies have found that infection with Blastocystis is associated with symptoms, while other studies have found the opposite.
- The long-standing controversy about Blastocystis has made it an unpopular area for research.
Evidence That Blastocystis Causes Disease
Genetic studies have revealed a possible explanation for the variation in symptoms observed with Blastocystis infection (blastocystosis): there are at least nine subtypes, all of which infect humans. Studies suggest that some subtypes are associated with long lasting illness, some may cause an acute illness that resolves, and some cause no symptoms.
Symptoms associated with blastocystosis include:
- diarrhea
- constipation
- abdominal pain
- fatigue
- skin rash
A number of researchers have recently suggested a link between chronic blastocystosis and irritable bowel syndrome (IBS).
Much more research is needed to clarify what sort of organism Blastocystis is, and what role it plays in intestinal diseases of humans.
Sources:
Boorom, Kenneth F., Huw Smith, Laila Nimri, et al. 2008. “Oh my aching gut: irritable bowel syndrome, Blastocystis, and asymptomatic infection.” Parasites & Vectors 1:40
Roberts, Larry S. and John Janovy Jr. Foundations of Parasitology 6th Ed. Boston: McGraw Hill, 2000.
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
Join the Conversation