Why is Southern African canine babesiosis so virulent? An evolutionary perspective

Babesiosis is one of the most important canine diseases in South Africa. Countrywide, babesiosis is diagnosed in around 10% of dogs presented to veterinary practices [1]. At the Onderstepoort Veterinary Academic Hospital on the outskirts of Pretoria, South Africa, around 12% of sick dogs presented are diagnosed with babesiosis, and around 31% of these are admitted for more intensive treatment [2]. Canine babesiosis, referred to as "malignant jaundice or bilious fever", was first reported from the Cape Colony in 1893 [3]. Early reports alluded to the virulent nature of the disease in South Africa [4-6], which differed from the manifestation in other parts of the world. Working with a South African isolate, Nutall and Hadwen (1909) commented that the parasites used in experiments in Italy must have been much less virulent than theirs [7,8]. Furthermore, dogs that survived infection with a French isolate were fully susceptible to the South African one, prompting Laveran & Nattan-Larrier to conclude in 1913 that the African babesia of dogs, if not a separate species, was at least a variety distinct from the French one [9,10].It was soon evident that isolates from different geographic regions were vector-specific. South African isolates were transmitted by Haemaphysalis elliptica, previously misidentified as Haemaphysalis leachi [4,11]. Isolates from North Africa, the Middle East and India were transmitted by Rhipicephalus sanguineus, while those from Southern Europe were transmitted by Dermacentor reticulatus [12-14]. Later research confirmed this vector-specificity [15,16].It is rather curious, therefore, that generally no notice was taken of this clear evidence of distinct biological differences between region-specific isolates. Until 20-odd years ago, it was generally accepted that babesiosis in dogs was caused by two species: a large piroplasm, Babesia canis, and a small one, Babesia gibsoni. Although the parasites had some characteristics in common, the disease man