Babesia divergens experimental infection of spleen-intact sheep results in long-lasting parasitemia despite a strong humoral response: preliminary results.

Babesia divergens is an intraerythrocytic Apicomplexa and the main agent of bovine babesiosis in Europe. The infection in cattle develops in 2 phases: an acute phase with hemolytic anemia and a chronic phase with asymptomatic persistence of the parasite for several years. The acute phase of B. divergens infection can be studied using the gerbil (Meriones unguiculatus) as a laboratory model but unlike cattle, this animal rapidly eliminates the parasite. An experimental model to study the chronic phase of infection was therefore developed by our laboratory. Spleen-intact sheep, with a potential full immune response, were inoculated with infected red blood cells (iRBC) or with free merozoites, by several routes (intraperitoneal, intravenous or subcutaneously). No clinical signs were ever observed but the installation of a persistent low level infection was shown in sheep with susceptible erythrocytes (able to sustain B. divergens growth in vitro). Neither feature was observed in sheep with non-susceptible erythrocytes. IgG production, involving both IgG1 and IgG2, was mainly directed against the major merozoite surface antigen Bd37, similar to the humoral immune response described in naturally infected cattle. The use of spleen-intact sheep to study the immune response to B. divergens is discussed.