Exercise alters the immune response to equine influenza virus and increases susceptibility to infection

Equine influenza virus remains a major health concern for the equine indust ry in spite of ongoing vaccination programmes. Previous work has shown that the immune system of horses can be affected by strenuous exercise. The pos sible adverse consequence of exercise-induced alterations in lymphocyte res ponses measured in vitro was unknown. Here we demonstrate that subjecting v accinated ponies to a 5 day strenuous exercise programme results in a signi ficant suppression of their T cell-mediated immune response to equine influ enza virus as measured by decreased lymphoproliferation and gamma interfero n production measured in vitro. These same ponies also demonstrated increas ed susceptibility to influenza disease following a challenge exposure to th e same strain of virus. Rested ponies that bad received the same vaccine an d challenge were completely protected from disease. Our results demonstrate that exercise-induced suppression of the equine immune response to influen za virus can be associated with an increased susceptibility to disease.