Canine babesiosis is caused by intraerythrocytic protozoan parasites of the genus Babesia including the large species B. canis, B. vogeli, B. rossi and the small species B. gibsoni, which is extending its range in the USA and Europe. Traditionally differentiation of the species was performed on the basis of their size within parasitized erythrocytes: The larger organisms, classified as large Babesia (2 x 5 µm in the intraerythrocyte stages), occuring single or paired within the cells and the small babesians (1 x 3 µm; mostly less than 1/8 of the cell diameter), usually appearing singly as round or oval forms in parasitized cells. Originally the large Babesia pathogens were summarised into the Babesia canis complex including the three subspecies B. canis canis, B. canis vogeli and B. canis rossi, which are nowadays considered to be separate species in their own right: B.canis, B. vogeli, B. rossi.
Molecular studies have recently identified novel small form babesians that infect dogs. One isolate discovered in the Iberian peninsular is closely related to the rodent piroplasm B. microti (B. microti-like piroplasm/Spanish isolate) and sometimes referred to asTheileria annae, and another small piroplasm has been named recently as B. conradae and is found in dogs in California. A fourth ‘large’ Babesia sp. has been described recently in a number of dogs in North Carolina (yet unnamed or referred to as Babesia sp. (Coco)). Additionally three Theileria species have been isolated in a small number of dogs’ blood in Europe (Theileria (Babesia) equi and Theileria annulata) and from dogs in South Africa (unnamed Theileria sp.), but further information on these pathogens has to be assembled.
Pathogenicity varies among the different species: B. rossi, the prevalent strain in South Africa, causes severe clinical disease; B. canis, the principle cause of babesiosis in Europe, is less pathogenic, although severe forms commonly occur. B. 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.
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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