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Effects of racing on lymphocyte proliferation in horses.

OBJECTIVE
To measure the lymphocyte proliferation response in horses 12 to 16 hours after completion of a race.


ANIMALS
8 Thoroughbreds that competed in 14 races and 3 control Thoroughbreds that did not race.


PROCEDURE
Horses participated in races during the late afternoon or evening. Venous blood samples were collected on a morning before a race (1 or 2 days before the race or on the day of the race), on the afternoon of a race (40 to 60 minutes after the race), and on the morning of the day after a race (12 to 16 hours after the race). Lymphocyte proliferation responses and WBC count were measured in samples obtained in the mornings. Plasma cortisol was measured in all samples.


RESULTS
Lymphocyte proliferation responses were significantly reduced and WBC counts significantly increased 12 to 16 hours after a race. Plasma cortisol concentrations were significantly increased 40 to 60 minutes after a race. In samples from the control horses, lymphocyte proliferation responses, WBC counts, or plasma cortisol concentrations did not differ significantly among time periods.


CONCLUSIONS AND CLINICAL RELEVANCE
A decrease in proliferative responses of circulating lymphocytes can be found as late as 12 to 16 hours after a horse participates in a race. Although the clinical consequences of these exercise-related alterations of the immune response are not yet known, managers of horses should take into account that the immune system of a horse may be affected by racing.