9 Higher antibody concentrations in hyperimmune plasma against the virulence protein A and poly-N-acetyl glucosamine provide greater protection against rhodococcal foal pneumonia

Abstract

Pneumonia caused by the bacterium Rhodococcus equi (RE) is an important cause of disease and death in foals at breeding farms worldwide. The efficacy of transfusion with hyperimmune plasma (HIP) for preventing RE pneumonia remains ill-defined. The objective of this study was to determine whether the odds of pneumonia were decreased with higher antibody (Ab) concentrations against either the RE virulence-associated protein A (VapA) or the polysaccharide β-1→6- N -polyacetyl glucosamine (PNAG) among Quarter Horse foals from 2 large breeding farms transfused with either RE HIP or PNAG HIP, respectively. Foals were randomly and equally assigned to be transfused within 24 h of birth with 2 L of either PNAG HIP (n = 116) or RE HIP (n = 117). Serum was collected post-transfusion to test by ELISA for concentrations of Abs against PNAG and the RE virulence-associated protein A (VapA), and deposition of complement component 1q (C'1q) onto PNAG. Foals were then monitored through 6 mo of age. Foals at Farm A (n = 119) were monitored twice daily for clinical signs of pneumonia, and foals at Farm B (n = 114) were monitored by ultrasound for subclinical pneumonia. Study investigators were masked to the plasma identity until after analysis was completed. For data analysis, binary variables of Ab levels (higher or lower) were created using the median value of VapA Ab concentration for RE HIP-transfused foals and PNAG and C'1q Abs for PNAG HIP-transfused foals. Data were analyzed using logistic regression, with pneumonia as the binary outcome variable, and binary concentration for a given Ab level and farm as dependent variables. Our results indicated that among RE HIP-transfused foals, the odds of pneumonia were approximately 6-fold higher ( P = 0.0005) among foals with lower ( i.e. , ≤ median) VapA concentrations, whereas 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 lower concentrations for PNAG and C'1q deposition, respectively. These data indicated that Abs against VapA, and the PNAG antigen with C'1q deposition activity are important indicators of protection against both subclinical and clinical RE pneumonia. Additionally, concentrations of Abs with C'1q deposition activity were a stronger predictor of efficacy than PNAG Ab concentration alone among PNAG HIP-transfused foals.