Is Blastocystis Hominis A Parasite?

The case of Blastocystis hominis is very interesting, but controversial. Over the years, this microorganism has been classified and reclassified many times. In turn, it has been considered as a fungus, more precisely a kind of yeasts, an amoeba, a flagellated and a sporozoan protozoan parasite. At this stage, it was very clear that this organism was a protozoan parasite, but the scientists were not able to really assess in which group of protozoan parasite it belongs. Now, based on the newest taxonomic molecular methods involving DNA and RNA sequencing, Blastocystis hominis is now considered as a protozoan parasite belonging the Stramenopiles group along with algae and some fungi. This group usually includes organisms with two flagella with different shapes. It is a very unusual classification for a protozoan parasite and it is still a matter of controversy. As such, it is possible that this classification might be refined in the future.

The denomination Blastocystis hominis is also not very conventional. In fact, it exists many different species of Blastocystis able to infect a wide range of organisms. It is very unconventional, because parasites are usually known to be specific for a limited number of hosts. Furthermore, many different species are able to infect human and it is common to call it then Blastocystis hominis, whatever the species involved. This common usage is explained by the great difficulties to determine exactly what species is involved.

Species of Blastocystis are present worldwide and infections in human are very common. However, it is still debated if this parasite is able to cause a symptomatic infection in human. If it really occurs, this parasite appears to cause gastrointestinal symptoms. The disease caused by this parasite is known as blastocystosis. Some scientists suggest that the fact that all species infecting human are called Blastocystis hominis is the cause of this controversy. Actually, they made the hypothesis that some of these species remain asymptomatic while others are able to provoke a symptomatic infection. However, as mentioned earlier, the formal identification of the specific species of Blastocystis involved remains tricky. Furthermore, some studies suggest a link between Blastocystis and the irritable bowel syndrome as well as the inflammatory bowel diseases, such as ulcerative colitis and Crohn’s disease. However, more research will be needed to confirm or infirm this hypothesis.

Finally, mainly because of the controversy around the occurrence or not of a symptomatic infection, there is no real scientific evidence to support the effective use of a specific treatment. However, some antiprotozoal drugs appear to have a certain efficacy in order to improve the patient’s symptoms, but the real efficacy of these antiprotozoal drugs is still a matter of controversy in the scientific community.

Source: Centers for Disease Control and Prevention

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