Effect of exercise on the immune response of young and old horses

Objective - To compare exercise-induced immune modulation in young and older horses. Animals - 6 young and 6 aged horses that were vaccinated against equine influenza virus. Procedure - Venous blood samples were collected for immunologic assessment before and immediately after exercise at targeted heart rates and after exercise for determination of plasma lactate and cortisol concentrations. Mononuclear cells were assayed for lymphoproliferative responses and incubated with interleukin-2 (IL-2) to induce lymphokine-activated killer (LAK) cells. Antibodies to equine influenza virus were measured. Results - Older horses had significantly lower proliferative responses to mitogens than younger horses prior to exercise. Exercise caused a significant decrease in lymphoproliferative response of younger horses, but not of older horses. Activity of LAK cells increased slightly with exercise intensity in younger horses. Cortisol concentrations increased in both groups after exercise; younger horses had higher concentrations after exercise at heart rates of 180 and 200 beats/min than those of older horses. Plasma lactate concentrations increased with exercise intensity but there were no differences between older and younger horses. Older horses had lower antibody titers to equine influenza virus than younger horses. Exercise did not affect antibody titers. Conclusion - Although lymphoproliferative responses and antibody titers of older horses were less than those of younger horses, older horses were more resistant to exercise-induced changes in immune function, possibly because of lower cortisol concentrations. Clinical Relevance - Stress and aging are known to affect immune function. Older horses had reduced immune function, but were more resistant to exercise-induced immune suppression than younger horses.