Experimental infection with Babesia divergens in cattle persistently infected with bovine virus diarrhoea virus

Nine Norwegian Red cattle, aged 7-14 months, persistently infected with bovine virus diarrhoea virus (BVDV) were inoculated with a Swedish strain of Babesia divergens. Six persistently infected cattle of the same age and breed were kept as controls. Blood and serum samples were collected regularly during the observation period. Rectal temperatures were recorded every morning for 25 days post infection, and the animals were examined clinically on a daily basis. Sera were examined for antibodies to B. divergens by indirect immunofluorescence antibody test (IFAT). Eight of the infected animals developed fever of 2-5 days duration. Babesia divergens organisms appeared in the erythrocytes of all infected animals on the day after inoculation. The parasitaemia lasted for 4-11 days. One animal had a transient haemoglobinuria. Compared with the control group, there was a 20% decrease in the haematocrit. There was a transient lymphopenia and thrombocytopenia during the period of fever. There were no differences in mean numbers of neutrophils between the two persistently infected groups. Compared with cattle free of BVDV, the persistently infected cattle had consistently lower total leucocyte count that was mainly due to decreased mean numbers of neutrophils and monocytes. All infected animals develop antibodies > or = 1:1280 between day 7 and 10 post infection. The magnitude of the antibody response was considerably lower than that of BVDV-free animals inoculated with the same strain and dosage of B.divergens.