Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Biomedical and Veterinary Medical Sciences - Comparative Biomedical Sciences

First Advisor

David W. Horohov


The effect of exercise stress on immune modulation and the mechanisms involved were examined in ponies and horses subjected to a strenuous treadmill-based exercise program. The magnitude of changes in exercise-induced cortisol production, lactate, and immunomodulation increased with increases in exercise intensity and the highest levels of each were seen in animals that attained heart rates greater than 200 bpm. Influenza vaccinated ponies and horses were exercised at heart rates greater than 200 bpm for 5 consecutive days and stress tests were performed on days 1 and 5. Exercised animals exhibited significant increases in plasma lactate, plasma cortisol, LAK cell activity, and suppression of lymphoproliferative responses to ConA, PHA, PWM, equine influenza virus, and KLH. The effects of exercise were more severe following the fifth day of exercise. A significant decrease in CD4+ T-cells and a significant increase in CD8+ T-cells were noted following exercise for both stress tests. PHA-stimulated post-exercise PBMC exhibited an increase in IL-2 receptor expression. However, the percentage of dual positive CD4+IL-2R+ and CD8+IL-2R+ T-cells was significantly decreased suggesting that the post-exercise increase in IL-2R expression was due to non T-cells. PHA-stimulated PBMC from exercised animals exhibited decreased IL-2, IL-4, and gamma-IFN mRNA expression following exercise for both stress tests. Influenza virus-stimulated PBMC exhibited a slight increase in IL-2 on day 1 and a slight decrease on day 5. Virus-stimulated PBMC exhibited significant decreases in gamma-IFN production for both stress tests. The overall level of IL-2 and gamma-IFN expression was increased on day 5 while IL-4 levels declined significantly. Activation and expression of the signal transduction proteins MEK1, JAK3, and STAT3 declined following exercise for both stress tests suggesting that this may play a role in the exercise stress-induced suppression of lymphoproliferation. Exercised ponies exhibited an increased susceptibility to influenza virus infection following the fifth day of exercise. No difference in susceptibility was noted following a single episode of intense exercise.