Optimisation of the inactivated vaccine dose against heartwater and in vitro quantification of Ehrlichia ruminantium challenge material

The aim of this work was to evaluate the minimal protective dose of the inactivated vaccine against heartwater. In order to conduct a reliable vaccination trial, an in vitro quantification method of Ehrlichia ruminantium (ER) challenge doses was developed. In experimental conditions, homologous Gardel challenges with 2.1 × 104 to 9.4 × 104 live ER elementary bodies were reproducible and mimicked a natural challenge. Similar results were obtained when animals were challenged with 3 × 104 live elementary bodies from five different ER strains. A 28-fold reduction of the vaccine dose did not decrease protection when compared to the conventional dose of inactivated vaccine. Two injections of 35 μg of ER antigen induced good protection against heartwater. Moreover, we found that the amount of IFNγ secreted in blood from vaccinated animals after in vitro stimulation with ER antigen was not a reliable predictor of survival and could not be used to test the potency of the inactivated vaccine.