The efficacy of transfusion with hyperimmune plasma (HIP) for preventing pneumonia caused by
Rhodococcus equiremains ill-defined. Quarter Horse foals at 2 large breeding farms were randomly assigned to be transfused with 2 L of HIP from adult donors hyperimmunized either with R. equi(RE HIP) or a conjugate vaccine eliciting antibody to the surface polysaccharide β-1→6-poly- N-acetyl glucosamine (PNAG HIP) within 24 hours of birth. Antibody activities against PNAG and the rhodococcal virulence-associated protein A (VapA), and to deposition of complement component 1q (C՛1q) onto PNAG were determined by ELISA, and then associated with either clinical pneumonia at Farm A (n = 119) or subclinical pneumonia at Farm B (n = 114). Data were analyzed using multivariable logistic regression. Among RE HIP-transfused foals, the odds of pneumonia were approximately 6-fold higher (P = 0.0005) among foals with VapA antibody activity ≤ the population median. Among PNAG HIP-transfused foals, the odds of pneumonia were approximately 3-fold (P = 0.0347) and 11-fold (P = 0.0034) higher for foals with antibody activities ≤ the population median for PNAG or C՛1q deposition, respectively. Results indicated that levels of activity of antibodies against R. equiantigens are correlates of protection against both subclinical and clinical R. equipneumonia in field settings. Among PNAG HIP-transfused foals, activity of antibodies with C՛1q deposition (an indicator of functional antibodies) were a stronger predictor of protection than was PNAG antibody activity alone. Collectively, these findings suggest that the amount and activity of antibodies in HIP ( i. e., plasma volume and/or antibody activity) is positively associated with protection against R. equipneumonia in foals.