Citation

Rhodococcus equi is a bacterium commonly isolated from soil that primarily causes pneumonia in foals and immunocompromised adult horses. Many vaccines were designed and tested to protect foals from developing pneumonia; however, to date, there is no vaccine that will protect foals from intrabronchial challenge with R. equi, except live, virulent R. equi. To evaluate electron-beam (e-beam) irradiation as a method of inactivation of R. equi, 2 concentrations (Concentration 1, 1 × 10^(8) colony-forming units/ml [CFU/ml] or Concentration 2, 1 × 10^(9) CFU/ml) of R. equi were submitted to a range of e-beam radiation doses, ranging from 0 to 7 kGy. All microorganisms of Concentrations 1 and 2 were adequately inactivated by 4 and 5 kGy, respectively, and the bacterial cell wall remained intact, whereas heat-inactivated samples indicated a compromised cell wall. Both concentrations were tested for immunogenicity and effects on fecal microbiome in neonatal foals. Mucosal and serum antibody responses were studied, as well as cell-mediated immune responses. Enteral administration of e-beam inactivated R. equi increased IFN-γ production and generated naso-pharyngeal R. equi-specific IgA in newborn foals. The inactivated vaccine appeared safe and immunogenic in neonatal foals in the presence of maternal antibody. No impact of treatment on fecal microbiome composition or diversity was observed among vaccinated foals; however, marked and significant differences in microbial communities and diversity were observed between foals at 32 days of age relative to 2 days of age regardless of treatment. In conclusion, electron-beam irradiation is an appropriate method for inactivation of R. equi, and e-beam irradiated R. equi vaccine is immunogenic in neonatal foals. Also, age-related changes in immune responses and the fecal microbial population occurred in healthy foals vaccinated enterally with e-beam inactivated R. equi. Mucosal vaccination does not result in major changes of the fecal microbiome in foals.