4 Evaluating the Use of Leukocyte Coping Capacity as a Measure of Acute Stress in Growing Pigs During a Cycling Heating Period

Abstract

Abstract The study objective was to evaluate leukocyte coping capacity (LCC) as a measure of acute heat stress in growing pigs. The LCC method utilizes an artificial challenge procedure on whole blood samples to mimic the leukocyte respiratory burst that is typically observed in response to pathogen exposure. A reduction in respiratory burst size, measured in relative light units (RLU) by chemiluminescence, indicates less LCC and greater physiological stress. Forty pigs (mean 66.7 kg BW; range: 54.9-87.5 kg) were assigned to 1 of 2 treatments, 1) thermoneutral (TN; n = 20), where pigs were maintained under thermoneutral conditions (range: 18.9-23.7 °C), or 2) heat stress (HS; n = 20), where pigs were exposed to a cycling heating period (range: 19.5-28.6 °C) for 48 h. Over each 24 h period, HS housing temperatures rose gradually from 0800-1200 h, were maintained at a maximal level until 1600 h, and gradually returned to thermoneutral temperatures by 1800 h. Blood (2 mL) was collected from each pig at 0 (0800 h), +4, +8, +24 and +48 h relative to the start of the experiment. Whole blood samples were challenged with phorbol 12-myristate 13-acetate to induce a respiratory burst, which was measured using a portable luminometer. Data were analyzed using the MIXED procedure in SAS 9.4. An hour effect was observed (P < 0.001), where RLU values increased linearly throughout the experimental period. However, RLU did not differ between TN and HS pigs (57.9 ± 4.1 vs. 55.4 ± 4.1 RLU, respectively; P = 0.67). Additionally, no treatment by hour interaction was observed at any timepoint (P = 0.48). Overall, a cycling heating period did not alter LCC in growing pigs. Future studies should evaluate whether LCC is effective for measuring additional properties of heat stress, such as greater temperatures or chronic periods of exposure.