Soluble parasite antigens from Babesia canis do not directly activate the kallikrein system in dogs infected with Babesia canis.

Soluble parasite antigens (SPA) from Babesia canis have been shown to induce protective immunity when used as vaccine. In order to explain the immune mechanisms of vaccination, the precise role of SPA in the pathogenesis of canine babesiosis is under investigation. Earlier studies suggested that the plasma kallikrein system is central in the pathogenesis of babesiosis, malaria and trypanosomosis, and significant plasma kallikrein activation during acute B. bovis and P. knowlesi infections has been described. In the studies presented here dogs were experimentally infected with B. canis to investigate whether the plasma kallikrein system is activated during babesiosis infection. Results showed that prekallikrein levels decreased during episodes of peak parasitaemia. No effect was found on the kallikrein levels. In order to determine whether B. canis SPA could activate plasma kallikrein, dogs were infused with variable amounts of B. canis SPA and plasma samples were taken for (pre-) kallikrein determination. The results indicated that B. canis SPA did not affect plasma (pre-) kallikrein levels. In addition, the effect of B. canis SPA on (pre-) kallikrein levels in normal dog plasma was determined in vitro. Again, no effect on (pre-) kallikrein levels was found. The results suggest that, although the kallikrein pathway may be involved in B. canis-associated pathology, the system is not directly activated by B. canis SPA. Furthermore, infusion of B. canis SPA as well as stroma of normal dog erythrocytes triggered the production of the acute phase reactant, C-reactive protein. This suggests that the inflammatory response that is triggered during B. canis infection could be in part due to the release and exposure of self molecules. The implications of these findings are discussed.