Taxonomic Nomenclature Changes


“What’s in a name? That which we call a rose by any other name would smell as sweet.”  It might smell as sweet, yes, but it would be much harder to remember.  In our lab we have experienced some confusion resulting from recent nomenclature changes.  Hopefully the following digest of some recent name changes relevant to veterinary diagnostics will reduce confusion for other labs as well as for our own.

Perhaps the most baffling changes in nomenclature have been in the order Rickettsiales.  The taxa designations in this order were previously based on morphological, ecological, epidemiological and clinical characteristics.    

A 2001 publication by Dumler et al.5 proposed re-classification based on genetic analysis of 16S rRNA, groESL, and surface protein genes.  A number of species formerly classified in the genera AnaplasmaEhrlichiaCowdriaNeorickettsia andWolbachia have been re-arranged into the genera AnaplasmaEhrlichiaWolbachia, and Neorickettsia.  Ehrlichia equi has been determined to be insufficiently distinct from either Ehrlichia phagocytophila or the human granulocytic ehrlichiosis (HGE) agent to justify separate species designations for any of these pathogens.  Moreover these three agents have been—or rather, this agent has been—determined to belong in theAnaplasma genus.  Thus Ehrlichia equiEhrlichia phagocytophila, and HGE are all now designated Anaplasma phagocytophila.  Another former EhrlichiaEhrlichia risticii, now belongs to the Neorickettsia genera; it is now known as Neorickettsia risticii.  Other Rickettsiales changes include:  Ehrlichia bovis is now Anaplasma bovis;Cowdria ruminatum is now Ehrlichia ruminatumEhrlichia platys is now Anaplasma platys; and Ehrlichia sennetsu is now Neorickettsia sennetsu.  Further changes have been made higher up the taxonomic tree that I will not treat here; they are detailed extensively in the Dumler publication.

In a 1998 publication by Mehlhorn and Schein6 it was argued that Babesia equi should be re-designated Theileria equi.  Among other reasons cited for the reclassification was the fact that equi, in common with Theileria spp. first infects lymphocytes wherein it inititates a schizogonic phase resulting in the production of motile merozoites.  By contrast, other Babesia species first enter erythrocytes.  Also in common with Theileriaspp. is equi’s development in the salivary glands of its vector ticks.  Typical ofTheileria spp., equi is not present in tick organs other than salivary glands and is not transmitted transstadially from egg to larva (though it can be transmitted transstadially from nymph to adult stages).  Babesia spp. generally infect other tick organs in addition to the saliva glands and transstadial transmission from egg to larva is typical. The morphology of sexual stages of equi differs from that of typical Babesia spp. 

Moreover, equi shares a surface protein common to Theileria spp.  Furthermore, drug susceptibility of equi is similar to Theileria spp. and differs from Babesia spp.  The membership of equi in the genus Theileria is also supported by ssRNA analysis. 

Thus there is compelling developmental, morphologic, biochemical, and genetic evidence supporting equi’s classification as a Theileria spp. and Theileria equi is beginning to be the accepted designation, though Babesia equi is still widely used.

Oh, you were hoping for interesting reading?  May I suggest Romeo and Juliet by William Shakespeare?