Blastocystis - Taxonomy

Taxonomy

For many years, scientists believed one species of Blastocystis infected humans, while different species of Blastocystis infected other animals. So they called Blastocystis from humans Blastocystis hominis and gave different species names to Blastocystis from other animals, for example Blastocystis ratti from rats. In recent years, various genetic analysis have shown that Blastocystis hominis as a unique entity does not exist, i.e. there is no single species of Blastocystis that infects humans.

In fact, a number of distinct genetic types of Blastocystis can infect humans, including those previously called Blastocystis ratti and the differences are so great that they could be considered separate species. Because of this, in 2007 scientists proposed discontinuing the use of the term Blastocystis hominis. Their proposal was to refer to Blastocystis from humans and animals as Blastocystis sp. subtype nn where nn is a number assigned to each group according to the degree of genetic identity of the Blastocystis organisms, based on gene sequences, rather than the host that was infected. At that time nine subtypes were known to infect mammals and birds, all of which had been found in humans.

A tenth group was reported in China in 2007, but a full analysis of its relationships has not yet been performed and it is not yet clear whether it is a group within a described subtype or a new subtype. A definite tenth subtype was then found in a variety of other mammals, including primates, but it has not as yet ben found in humans.

There are now at least 13 genetically distinct small subunit ribosomal RNA lineages. These additional subtypes were found in a variety of mammalian hosts (including elephants and giraffes) and it is very likely that more subtypes will be found as more hosts are surveyed.

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