Efficacy of experimental inactivated and live Rhodococcus equi vaccines for thoroughbred Arabian mares in mice

The aim of this study was to determine the efficacy of inactive Rhodococcus equi vaccine candidates included that bacterin+aluminum hydroxide (Al(OH)<inf>3</inf>), bacterin+VapA+(Al(OH)<inf>3</inf>), bacterin+Montanide IMS 3012 (IMS), bacterin+VapA+IMS, and live vaccine using mice as a model. The efficacy of vaccine was evaluated according to clinical findings, humoral and cellular immunity (levels of INF-g and IL-4), and results of microbiological culture from internal organs in dead or sacrificed mice. Inactive R. equi vaccines were subcutaneously administered to mice three times at 15-day intervals and live vaccine was intraperitoneally injected once. Fifteen days after the last vaccination, aerosol challenges were carried out with the pathogenic R. equi VapA⁺K2002 strain in all groups. Two mice were sacrificed from each challenge groups on days 1, 3, 5, and 7. The antibody titers of vaccinated mice were found to be significantly higher than those of the controls. The largest number of INF-g positive samples were detected in the bacterin+VapA+IMS and bacterin+IMS groups. IL-4 positivity was determined only in live vaccine groups. The lowest reisolation rate of R. equi from internal organs was observed in the bacterin+VapA+IMS group. It was concluded that R. equi vaccines, and especially bacterin+VapA+IMS, are useful to protect mice against R. equi infection.