Antibodies, not vaccination, protect newborn foals from Rhodococcus

Abstract

Rhodococcus equi is a prevalent cause of pneumonia in foals worldwide. Our laboratory has demonstrated that vaccination against the surface polysaccharide &#x3b2;-1&#x2192;6-poly--acetylglucosamine (PNAG) protects foals against intrabronchial infection withwhen challenged at age 28&#x2009;days. However, it is important that the efficacy of this vaccine be evaluated in foals when they are infected at an earlier age, because foals are naturally exposed to virulentin their environment from birth and because susceptibility is inversely related to age in foals. Using a randomized, blind experimental design, we evaluated whether maternal vaccination against PNAG protected foals against intrabronchial infection with6&#x2009;days after birth. Vaccination of maresdid not significantly reduce the incidence of pneumonia in foals; however, activities of antibody against PNAG or for deposition of complement component 1q onto PNAG was significantly (&#x2009;<&#x2009;0.05) higher among foals that did not develop pneumonia than among foals that developed pneumonia. Results differed between years, with evidence of protection during 2018 but not 2020. In the absence of a licensed vaccine, further evaluation of the PNAG vaccine is warranted, including efforts to optimize the formulation and dose of this vaccine.Pneumonia caused byis an important cause of disease and death in foals worldwide for which a licensed vaccine is lacking. Foals are exposed toin their environment from birth, and they appear to be infected soon after parturition at an age when innate and adaptive immune responses are diminished. Results of this study indicate that higher activity of antibodies recognizing PNAG was associated with protection againstpneumonia, indicating the need for further optimization of maternal vaccination against PNAG to protect foals againstpneumonia.