Peter Irwin

The classification of Babesia spp. places them in order Piroplasmida, within the phylum Apicomplexa. Babesia spp. are often referred to as piroplasms, a collective term for morphologically similar protozoan parasites that utilize mammalian erythrocytes in their life cycle. Piroplasms encompass two main genera, Babesia and Theileria; which are currently the subject of intense research interest and molecular-based (re-)characterization. Since babesiosis is an emerging disease in many parts of the world, it is very important to determine the precise species of the parasite causing the clinical illness and the isolate(s) normally present in any given geographical location. The success of treatment may depend on such information since most anti-babesial drugs have limited efficacy against different Babesia spp. Similarly, inclusion of appropriate antigens into serologic tests is necessary to reduce the risk of inaccurate results during screening for diagnostic or epidemiological purposes. The 18S rRNA gene has been favoured for molecular phylogenetic studies to date, but others regions of the small subunit ribosomal RNA gene, including the first and second transcribed spacers (ITS1 & ITS2) and 5.8S rRNA gene, and the heat shock protein genes (HSP 70 and HSP 90) have also been used, or show promise for future taxonomic studies. Until recently only two Babesia parasites were thought to occur in dogs; the relatively large intra-erythrocytic piroplasm referred to as Babesia canis and a smaller parasite, known predominantly as the cause of canine babesiosis in Asia, Babesia gibsoni. Since the late 1980's "B. canis" has been reclassified into three separate species (B. canis, B. rossi and B. vogeli) on the basis of cross-immunity, serological testing, vector specificity and molecular phylogeny and a new 'large' Babesia sp. has been recently described in a dog in North Carolina (1). Molecular characterization of small canine piroplasms has also shed light on the classification of these parasites. Studies, predominantly utilizing the 18S rRNA gene locus have revealed that, in general, small canine piroplasms are more closely related to Theileria than to Babesia. These two genera were previously separated on the basis of certain life cycle features and transovarial passage within the tick vector. However such historical definitions need revisiting in the light of these molecular studies. To date at least 4 genetically and clinically distinct small piroplasms affect dogs which include: Babesia gibsoni--originally described in India nearly a century ago (2) and now occurring sporadically in other parts of the world including the Australia; Babesia conradae, a piroplasm that occasionally infects dogs in California (3); Theileria annae, a Babesia microti-like parasite that has so far been reported only in northwest Spain, transmitted by Ixodes hexagonus (4,5); and a fourth small piroplasm, B. (=T.) equi has also been reported in dogs in Spain (6).