PathogensCanine babesiosis is caused by the intraerythrocytic protozoan parasite complex Babesia canis (B.canis canis, B. canis vogeli, B. canis rossi), and increasingly by B. gibsoni, which is extending its range in the USA and Europe. B. canis and B. gibsoni are traditionally distinguished on the basis of their size within parasitized erythrocytes. B. canis is the larger organism, classified as large Babesia (2 x 5 µm in the intraerythrocyte stages), and occurs single or paired within the cells, whereas B. gibsoni are considered small babesians (1 x 3 µm; mostly less than 1/8 of the cell diameter) and usually appear singly as round or oval forms in parasitized cells.In very acute forms of the disease multiplication of parasites is rapid and individual red blood cells may contain multiple Babesia organisms.
It is generally considered that B. canis comprises three distinct subspecies, which have been characterized at the molecular level and are grouped within the Babesia canis (sensu lato) clade. Pathogenicity varies among the different strains: B. canis rossi, the prevalent strain in South Africa, causes severe clinical disease; B. canis canis, the principle cause of babesiosis in Europe, is less pathogenic, although severe forms commonly occur. B. canis vogeli infection causes relatively mild disease worldwide. The relative importance of tick species in the transmission of canine babesiosis varies with geographical location. B. gibsoni is considered to be a highly pathogenic agent. Molecular studies have recently identified novel small form babesians that infect dogs. One isolate discovered in the Iberian peninsular is closely related to B. microti (a rodent piroplasm) and sometimes referred to asTheileria annae, and another small piroplasm has been named recently as B. conradae and is found in dogs in California. The canine Babesia species do not cause disease in humans. In the USA, B. microti is the most common cause of a mild human babesiosis; and the bovine pathogen B. divergens, transmitted by Ixodes ticks, causes severe clinical disease in humans in some parts of Europe.
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Definitions of parasitologic terms