Pathogenesis and Transmission
The protozoans Babesia spp. are generally transmitted by ticks and reach the blood stream while the ticks are feeding. Once inside the host, the parasite attaches to an erythrocyte, is engulfed via endocytosis, matures, and then starts asexual reproduction, producing merozoites. Infected erythrocytes eventually rupture and released merozoites invade other erythrocytes. The main pathogenesis associated with babesiosis is hemolytic anemia. Hemolytic anemia is the result of direct erythrocyte injury caused by the parasites and also by immune-mediated mechanisms. In addition, most dogs with babesiosis have thrombocytopenia. Puppies are generally more susceptible to babesiosis and are at greatest risk of serious illness and death. Low blood oxygen (hypoxaemia) as a result of the anaemia contributes to morbidity and the most pathogenic strains cause kidney and liver injury, and systemic inflammatory disease.
Rhipicephalus sanguineus is the primary vector for Babesia in warmer regions worldwide, like in Southern Europe, Southern USA, Australia and Latin America. In Western and Central Europe, the main vectors for Babesia are Dermacentor ticks, esp. Dermacentor reticulatus. Transmission within the tick is both transstadial (infection at any stage for Rhipicephalus and the next stage being infectious) and transovarial (females of Rhipicephalus and Dermacentor may transfer infection to the next generation through eggs). As a consequence, nymphs and adults of Rhipicephalus can be infectious when larvae or nymphs have fed on an infected dog (transstadial), whereas adults of Dermacentor will only be infectious from the previous infected tick generation (transovarial), because larvae and nymphs of Dermacentor do not feed on dogs. In Southern Africa B. rossi is transmitted by Haemaphysalis elliptica (formerly H. leachi). The small piroplasm Babesia gibsoni is transmitted by Haemaphysalis longicornis in eastern Asia but the vector competence of Rhipicephalus sanguineus for Babesia gibsoni is a disputed matter. In the case of B. gibsoni outside Asia transmission is also caused by blood exchange in fighting dogs. In the case of the other described Babesia and Theileria species only for T. annae Ixodes hexagonus has been mentioned as putative vector, for the rest the vectors are unknown.
Babesiosis occurs sporadically and local outbreaks may occur in kennels with severe tick infestations. Prevalence in endemic areas is reported to range from 3.8% to 59%.
Transmission of all Babesia spp. is possible by needles or blood transfusion (iatrogenic).
- Irwin PJ: Canine babesiosis: From molecular taxonomy to control. Parasit Vectors 2009, 2 (Suppl. 1), S4
- Shaw SE, Day MJ, Birtles RJ, et al.: Tick-borne infectious diseases of dogs. Trends Parasitol. 2001, 17, 74-80