Immunogenicity of an Electron Beam Inactivated <i>Rhodococcus equi</i> Vaccine in Neonatal Foals

<div><p><i>Rhodococcus equi</i> is an important pathogen of foals that causes severe pneumonia. To date, there is no licensed vaccine effective against <i>R. equi</i> pneumonia of foals. The objectives of our study were to develop an electron beam (eBeam) inactivated vaccine against <i>R. equi</i> and evaluate its immunogenicity. A dose of eBeam irradiation that inactivated replication of <i>R. equi</i> while maintaining outer cell wall integrity was identified. Enteral administration of eBeam inactivated <i>R. equi</i> increased interferon-γ production by peripheral blood mononuclear cells in response to stimulation with virulent <i>R. equi</i> and generated naso-pharyngeal <i>R. equi</i>-specific IgA in newborn foals. Our results indicate that eBeam irradiated <i>R. equi</i> administered enterally produce cell-mediated and upper respiratory mucosal immune responses, in the face of passively transferred maternal antibodies, similar to those produced in response to enteral administration of live organisms (a strategy which previously has been documented to protect foals against intrabronchial infection with virulent <i>R. equi</i>). No evidence of adverse effects was noted among vaccinated foals.</p></div