Rhodococcus equi is an equine respiratory pathogen which causes pyogranulomatous bronchopneumonia primarily in foals. This ubiquitous gram positive coccobacillus has a mortality rate of 50% in infected foals. It causes an estimated 3% of all Thoroughbred foal deaths, has a worldwide distribution and is endemic on farms in Australia. R. equi virulence is associated with an 80-90Kb plasmid which harbours 10 virulence associated proteins; Virulence associated protein A (VapA) is the primary cause of virulence in horses and aids bacterial survival within the host macrophage.
Despite decades of research into vaccine development (a number of these utilising VapA as the primary target), there is no commercially available vaccine for R. equi. In recent years adenoviral vectors have shown significant promise as a vaccine vector candidate for other actinomycetes including Mycobacterium tuberculosis. In our study, we developed an adenoviral vector based vaccine containing the vapA gene (hAdV-vapA). This vaccine was tested in the murine model and under challenge with aerosolised R. equi, demonstrated bacterial clearance in a prime/ boost regime, with the hAdV alone inducing immunostimulation. In all studies a significant total IgG response was exhibited with the prime/boost regime, where IgG subclasses indicate a mixed Th1/Th2 response. Cytokine profile of stimulated murine lung and spleen cells (including IFN-γ, TNF-α, IL-2, IL-4, IL-10 & IL-6) also demonstrated a Th1 and Th2 response. The results of this study have shown that the hAdV-vapA vaccine is capable of inducing a significant R. equi specific immune response in the murine model. However, further testing of this vaccine candidate in the foal model is required to determine suitability as a possible commercial vaccine candidate.