Rhodococcus equi is an important equine respiratory pathogen which causes pyogranulomatous bronchopneumonia 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 an abundance of 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. Many different candidates have showed success in the mouse model, however, the translation of the vaccine candidates from the mouse model to foals is often unsuccessful. Recently, an adenoviral vector vaccine for R. equi based on the VapA antigen, was tested in the mouse model and showed strong immunogenicity, with a strong antibody response, both IgG and IgG subclasses, and a mixed cytokine Th1/Th2 bias response, favouring the desired Th1 response necessary for the foal immune system to be capable of generating an effective immune response. Upon challenge with aerosolised R. equi, bacterial clearance was observed. The efficacy of this vaccine however, cannot be confirmed until it is tested in the foal.
Mouse model data must be considered with caution, although mice are considered an acceptable model for testing R. equi vaccines, it cannot be used to replicate the rhodococcal pneumonia infection that occurs in the foal, herein lies one of the difficulties in translating the vaccine from the mouse model to the foal. Coupled with R. equi intracellular survival and the lack of information on equine cytokine profiles in R. equi disease, the testing of R. equi vaccine candidates in the foal presents many challenges.