\u3cem\u3eRHODOCOCCUS EQUI\u3c/em\u3e IN THE FOAL – IMPROVING DIAGNOSTIC AND PREVENTION MEASURES

Although Rhodococcus equi (R. equi), previously known as Corynebacterium equi, was first isolated from pneumonic foals almost a century ago, it remains the most common cause of subacute or chronic granulomatous bronchopneumonia in foals. While the majority of foals exposed to R. equi develop a protective immune response (regressors), others exhibit a unique susceptibility to infection (progressors). The determinants for either outcome are not completely understood. Therefore, current diagnostic and preventive measures are suboptimal and require betterment. In light of this current need, we hypothesized that immunoglobulin G subisotype T [IgG(T)] against the virulence-associated protein A (VapA) of R. equi, and whole blood cytokine expression profile of foals predict the outcome of infection and can be used as diagnostic markers of clinical disease. Further, we hypothesized that the use of R. equi hyperimmune plasma (HIP) decreases severity of disease in naturally infected foals, playing an important role in disease prevention in the field. Lastly, we hypothesized that specific anti-Rhodococcus equi pili antibodies passively acquired by foals via colostrum after immunization of pregnant mares with a Rhodococcus equi pili-based candidate vaccine will confer protection against induced disease, and therefore have an immediate impact on R. equi pneumonia prophylaxis. The objectives of this study were: (1) to describe the humoral immune response of progressor and regressor foals to R. equi following experimental challenge and natural infection, (2) to compare the cytokine and cell-marker expression profile in whole blood of progressor and regressor foals after challenge, (3) to evaluate the Vap-A specific IgG profile of a commercially available HIP product and its value as a prophylactic tool on an endemic farm, and (4) to evaluate the efficacy of a vaccine based on the Rhodococcus equi pili (Rpl). Although the IgG(T) response of progressor foals after challenge or following natural infection tended to be more pronounced than that observed in regressor foals, its performance as a diagnostic test for predicting disease outcome was poor. Likewise, whole blood cell-marker and cytokine expression profiles of progressor and regressor foals were not significantly different, undermining its reliability as a diagnostic tool. Evaluation of the association of HIP VapA specific IgG profile and rhodococcal disease outcome in the field resulted in the conclusion that progressor foals received significantly less VapA specific IgG, suggesting that HIP may have provided some protection to regressor foals. Although HIP appeared to have provided some protection against clinical pneumonia, Rpl maternally-derived IgG failed to confer any advantage to foals born from vaccinated mares. The Rpl candidate vaccine failed to confer protection to foals after challenge, and did not decrease disease severity in comparison to a control group. In summary, the results of this study do not support the use of VapA specific IgG(T) or whole blood cytokine expression profile as predictors of disease outcome. Further, our results suggest a positive effect of HIP on disease outcome. Lastly, the presence of systemic and local Rpl antibodies was not protective in foals