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. equi strain 103+. Eleven foals were not treated prior to infection, and 3 control foals were neither treated nor infected. Heart rate, respiratory rate, and rectal temperature were recorded twice daily, and serum fibrinogen concentration and WBC count were determined every other day following infection. Foals were euthanatized 14 days following infection, and lung lesions and 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 lungs of treated foals. CONCLUSIONS AND CLINICAL RELEVANCE: Degree of protection against R. equi-induced pneumonia provided by purified immunoglobulin specific for VapA and VapC was similar to that provided by commercially available HIP. Results not only suggest that immunoglobulin is the primary component of HIP that confers protection against R. equi-induced pneumonia in foals but also indicate that antibodies against R. equi VapA and VapC are protective.