The Harry M. Zweig Memorial Fund for Equine Research

Exercise-Induced Immunosuppression:
A Cause of Illness and Vaccine Failures in Racehorses?

Dr. Dorothy M. Ainsworth

Horses which race or perform high intensity exercise are at increased risk for the development of lung infections and abscesses because their immune system is transiently depressed during and immediately after exercise. Thus, infectious lung diseases develop in racehorses when aspirated bacteria, which normally reside in the pharyngeal cavity, take advantage of the depressed respiratory tract defense mechanisms, rapidly proliferate and establish foci of infections. Exercise-associated immunosuppression also enables viral infections to become established in the respiratory tract, which in turn predispose to the development of secondary bacterial infections. Theoretically, if this immune suppression could be reversed or attenuated, the prevalence of infectious pneumonia, bronchitis or lung abscessation should dramatically decrease in the equine athlete. One possible therapeutic approach entails the use of immunomodulators or agents which re-direct ore enhance the activity of the immune system.

Currently there are many commercially-available immunomodulators that enjoy widespread clinical use. Unfortunately, no objective studies demonstrating the eeficacy of these agents have been performed. Thus, despite their widespread use on the racetrack or show circuit, the question of whether these agents actually enhance the immune response of the athletic horse or reduce its chance for development of respiratory tract infections remains unknown.

In this proposal, we will characterize the magnitude and the nature of the depressive effect of high intensity exercise on the function of the immune cells (lymphocytes) in the lungs and blood of horses. To do so, we will determine how well horses undergoing intense treadmill exercise (simulating race conditions), are able to respond to a nebulized vaccine, by analyzing their antibody and cytokine response to this vaccine. [Cytokines are proteins produced by the immune cells which are crucial for directing the magnitude and the nature of the immune response.] Using molecular biology techniques, we will quantitate the amount and types of cytokines produced using the polymerase chain reaction (PCR). Using a sensitive colorometric assay, we will also quantify the amount and type of antibody produced to this nebulized vaccine. The responses of the exercise horses will be compared to the responses of sedentary horses to document the degree of exercise-induced immunosuppression. This study will establish the foundation work required to determine if pre-treatment of exercising horses with a commercially used immunomodulator (EqStim) enhances the pulmonary immune responses.