Year of Publication


Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation




Veterinary Science

First Advisor

Dr. David W. Horohov


Rhodococcus equi (R. equi) is one of the most serious causes of pneumonia in young foals. The clinical disease is of great concern to breeding farms worldwide due to the impact of mortality on economic losses. While adult horses are resistant to R. equi, foals exhibit a distinct age-associated susceptibility. The mechanism underlying this susceptibility in foals is not well understood. Interferon-gamma (IFNg) plays an important role in the clearance of R. equi, but its expression is impaired in neonatal foals. Moreover, the regulation of this age-related IFNg expression in foals remains unknown. In humans, IFNg expression has been shown to be regulated by DNA methylation, lymphoproliferation, and influenced by environmental exposure. Therefore, we hypothesized that environmental exposure promotes IFNg expression through regulation of DNA methylation and lymphoproliferation. The objectives were: (1) to estimate the relevance of IFN-g production and R. equi infection in foals; (2) investigate the role of lymphoproliferation and DNA methylation in the regulation of IFN-g expression in foals; (3) to evaluate the effect of environmental exposure on IFN-g expression by housing foals in a barn environment verses pasture.; (4) to investigate the effect of environment exposure on antigen-presenting cells (APC), which sensor the environmental antigens and modulate IFN-g production by T cells. The results demonstrated that the IFN-g expression was inversely correlated with the age-related susceptibility to R. equi infection. lymphoproliferation promoted IFN-g expression in foals, whereas, DNA methylation repressed IFN-g expression. The IFN-g expression was augmented in foals exposed to the barn air which contained higher numbers of aerosol miroorganisms. DNA on the IFN-g promoter was demethylated and the lymphoproliferative activity was elevated in foals with barn-air exposure. The barn-air exposure also promoted the maturation and activation of APC to prime IFN-g expression by T cells in foals. Overall, this body of work demenstrated a relationship between IFN-g expression and R. equi infection, provided novel information on mechanisms that regulate IFN-g expression, and identified the effect of environment on mechanisms responsible for IFN-g expression.