Babesia divergens: sequential exposure to heterologous tick-borne challenge of cattle immunized with a fraction of parasitized erythrocytes.

Two groups of cows previously unexposed to ticks or tick-borne infections were immunized with an acidic fraction, isolated by isoelectric focusing, of a lysate of a laboratory strain of Babesia divergens infected erythrocytes 5 months and 1 month before grazing on a pasture harbouring a large population of ticks known to transmit babesiosis. Both treated groups and the untreated became infected with a field strain of B. divergens after 19 of the 21 cattle (3 groups of 7) had first developed tick-borne fever. All those immunized one month and 6 of 7 immunized 5 months previously resisted lethal multiplication of B. divergens whereas 4 of 7 controls required treatment to prevent their deaths. There were significant differences between the groups in the magnitude of parasitaemia; reduction of packed cell volume and antibody titres is an indication that the pre-treatment with the acidic fraction of the laboratory strain had induced significant protection against the heterologous field strain. Problems in the field use of a babesial vaccine include concomitant field infection with tick-borne fever, the need for a suitable adjuvant and a study of lysates of different strains of B. divergens.