IgG(T) Antibodies Identify Foals at Risk for R Equi

Scientific Summary Rhodococcus equi is of worldwide importance as a cause of pneumonia and of a variety of extrapulmonary lesions (EPLs) in young foals. Control of R. equi pulmonary infections currently relies on early detection of disease using thoracic ultrasonography and initiation of treatment with antimicrobial agents prior to the development of clinical signs. Although this approach is highly sensitive, it lacks specificity. In particular, it fails to differentiate those foals with progressive disease from those that will regress their lesions spontaneously. While little remains known about the factors that determine the susceptibility of foals for developing disease, it is likely that protection against R. equi involves opsonizing antibodies. Specifically, IgG antibodies are thought to be important because entry of R. equi into macrophages mediated by Fc receptors leads to enhanced bacterial killing. Thus, an increase in IgG against R. equi is observed after natural exposure and oral or intra-tracheal inoculation. However, not all IgG isotypes are equally effective. While IgG(T) has been shown to be a potent complement activator in horses, it is typically associated with a non-protective immune responses in R. equi-infected foals. We have also demonstrated that IgG(T) responses had a strong positive correlation with rhodococcal pneumonia in a recent challenge study of neonatal foals using a low dose of R. equi.8,10 Our hypothesis is that rising VapA-specific IgG(T) antibodies is an early predictor of rhodococcal pneumonia and/or extrapulmonary lesions in young foals. To test this hypothesis we propose the following specific aims: (1) Collection of sera and data from foals on a rhodococcus-endemic farm, and (2) Characterization of the anti-VapA antibody profile of foals diagnosed with R. equi infection, as well as control animals. Lay Summary Pneumonia caused by Rhodococcus equi remains a significant financial burden and welfare concern for the equine industry. Young foals are uniquely susceptible to R. equi infections. Currently there are no effective vaccines or other prophylactic measures to prevent Rhodococcus infections in foals. Most foals are exposed early in life to this bacterium, as evidenced by thoracic ultrasound screening. While this screening can identify foals infected with this bacterium, it does not identify those at risk for developing disease. Recently we have shown that experimentally infected foals that develop disease show a characteristic antibody response prior to the onset of clinical signs. The goal of this project is to screen foals on an endemic farm to determine if this same response is seen in the field. If successful, this will provide an enhanced diagnostic capability for equine veterinarians.