The use of cheap and thermoresistant
vaccines in poor tropical countries for the control of
animal diseases is a key issue. Our work aimed at designing and validating a process for the large-scale production of a ready-to-use
inactivated vaccine for ruminants. Our model was heartwater caused by the obligate intracellular bacterium Ehrlichia ruminantium (ER). The conventional
inactivated vaccine against heartwater (based on whole bacteria inactivated with
sodium azide) is prepared immediately before injection, using a syringe-extrusion method with
Montanide ISA50. This is a fastidious time-consuming process and it limits the number of
vaccine doses available. To overcome these issues, we tested three different techniques (syringe, vortex and homogenizer) and three
Montanide ISA adjuvants (50, 70 and 70M). High-speed homogenizer was the optimal method to emulsify ER
antigens with both ISA70 and 70M adjuvants. The
emulsions displayed a good homogeneity (particle size below 1 μm and low phase separation), conductivity below 10 μS/cm and low
antigen degradation at 4 °C for up to 1 year. The efficacy of the different formulations was then evaluated during vaccination trials on goats. The inactivated ER
antigens emulsified with ISA70 and ISA70M in a homogenizer resulted in 80% and 100% survival rates, respectively. A cold-chain
rupture assay using ISA70M+ER was performed to mimic possible field conditions exposing the
vaccine at 37 °C for 4 days before delivery. Surprisingly, the animal survival rate was still high (80%). We also observed that the MAP-1B antibody response was very similar between animals vaccinated with ISA70+ER and ISA70M+ER
emulsions, suggesting a more homogenous
antigen distribution and presentation in these
emulsions. Our work demonstrated that the combination of ISA70 or ISA70M and homogenizer is optimal for the production of an effective ready-to-use
inactivated vaccine against heartwater, which could easily be produced on an industrial scale.