Evaluation of equine immunoglobulin specific for Rhodococcus equi virulence-associated proteins A and C for use in protecting foals against Rhodococcus equi-induced pneumonia

Objective To determine whether purified equine immunoglobulin specific for Rhodococcus equi virulence-associated proteins A and C (VapA and VapC) can confer passive protection against R equi-induced pneumonia in foals. Animals-Twenty-eight 3-week-old mixed-breed pony foals. Procedure 7 foals received IV injections of equine hyperimmune plasma (HIP) against whole-cell R equi., and 7 received purified equine immunoglobulin specific for VapA and VapC 1 day prior to intrabronchial infection with R e qui strain 103+. Eleven foals were not treated prior to infection, and 3 co ntrol foals were neither treated nor infected. Heart rate, respiratory rate , and rectal temperature were recorded twice daily, and serum fibrinogen co ncentration and WBC count were determined every other day following infecti on. Foals were euthanatized 14 days following infection, and lung lesions a nd concentration of R equi in lungs were assessed. Results-The onset of clinical signs of pneumonia was significantly delayed in the HIP- and immunoglobulin-treated groups, compared with the untreated infected group. Moreover, pulmonary lesions were less severe in the treated groups, and significantly fewer R equi organisms were cultured from the lu ngs of treated foals. Conclusions and Clinical Relevance-Degree of protection against R equi-indu ced pneumonia provided by purified immunoglobulin specific for VapA and Vap C was similar to that provided by commercially available HIP, Results not o nly suggest that immunoglobulin is the primary component of HIP that confer s protection against R equi-induced pneumonia in foals but also indicate th at antibodies against R equi VapA and VapC are protective.