Acute stress effects on pig performance, immunity, and behavior

Stress activates the hypothalamic-pituitary-adrenal axis (HPA). Corticotropin-releasing hormone (CRH) is the primary initiator of the stress response. The hypothalamus releases CRH which in turn stimulates the anterior pituitary to secrete adrenocorticotropin hormone (ACTH) and other peptides. Increased ACTH levels stimulate the adrenal cortex to release glucocorticoids. Stress not only effects the neuroendocrine and endocrine systems, but also alters behavior, immunity and overall physiology. The objectives of these studies were to (1) determine if physiological concentrations of various hormones had an effect on in -vitro NK cytotoxicity in pigs, and (2) to determine the effects of three common stressors (heat, cold and shipping) on pig performance, physiology and behavior. In experiment 1, peripheral blood was collected from 59 pigs to determine the effects of physiological concentrations of stress hormones on porcine natural killer cell (NK) activity. A standard cytotoxicity assay was utilized with peripheral blood mononuclear cells and 51Cr-labeled K-562 target cells. Hormones and concentrations were: porcine corticotropin releasing hormone (pCRH) at 0, 0.47, 4.7, 47 and 467 ng/mL; P-endorphin at 0, 0.001, 0.01, 0.1, 1, 10 and 100 ng/mL, Cortisol at 0, 3.6, 36 and 360 ng/mL, adrenocorticotropic hormone (ACTH) at 0, 0.045, 0.45, 4.5 and 45 ng/mL, epinephrine at 0, 0.022, 0.22, 2.2 and 21.9 ng/mL and norepinephrine at 0, 0.021, 0.21, 2.1, 20.6 ng/mL. Over the range of doses evaluated, pCRH, ACTH, Cortisol and norepinephrine had no significant effect on porcine NK activity. P-endorphin and epinephrine had significant (P < .01) effects on porcine NK activity. All doses of â-endorphin suppressed (P < .05) porcine NK activity. Porcine NK activity was greater (P < .05) when incubated with 0.22 ng/mL epinephrine than with 0.022 or 21.9 ng/mL epinephrine. These results suggest that physiological concentrations of CRH, ACTFI, Cortisol, and norepinephrine alone do not modulate pig NK activity. â-endorphin seems to be a potent suppressor of porcine NK activity. Epinephrine influenced pig NK activity in a dose-sensitive manner. In the second study, 132 weanling pigs were used to investigate the effects of three common stressors (and a control) and differing social status on behavior, immunity, plasma Cortisol and blood chemical measures. Eleven blocks of 12 pigs each were evaluated. Each block contained four pens, established at weaning, and examined for outcome of agonistic interactions. Pigs were labeled socially Dominant (DOM), Intermediate (INT) or Submissive (SUB). Two weeks later, random pens of pigs experienced either a control treatment (CON) or for 4 hours they were either shipped (SHIP), heat stressed (HEAT) with overhead heat lamps in their home pens, or cold stressed (COLD) by direct application of water and an air current. A battery of physiological and behavioral measures were collected. Treatments did not influence body weights, however, percentage weight change during SHIP was greater (P < .001) than for other treatments (.23, -2.9, .72, .91% ± .96 for CON, SHIP, COLD, HEAT, respectively). Serum glucose was elevated (P = .02) for SHIP pig compared with other treatments. Serum Cortisol was decreased (P = .03) for HEAT compared with COLD and SHIP pigs. Social status influenced (P < .01) body weights at weaning and throughout the study with body weights heaviest for DOM pigs than INT and SUB pigs. Social status also influenced serum globulin, albumin/globulin (A/G) ratio, and Cortisol concentrations. Globulin decreased, while A/G increased in DOM pigs. SUB pigs showed an increase in Cortisol compared to DOM and INT pigs. An interaction between social status and treatment occurred for NK cytotoxicity at 50:1 E:T (P = .06). Socially DOM and INT pigs had higher NK cytotoxicity than SUB, COLD pigs. An interaction was also observed between social status and stress treatment for lymphocyte blastogenesis under Phytohemagglutinin mitogen. Among HEAT pens, the INT pigs had lower lymphocyte proliferation than SUB pigs, but were not different from the DOM pigs. Among SHIP pens, the INT pigs had higher lymphocyte proliferation than either the DOM or SUB pigs. In conclusion, social status had large effects on plasma Cortisol, globulin, A/G ratio, body weight and weight changes, although only acute shipping stress resulted in weight loss. Many immune and blood measures were not changed among acutely stressed pigs, however, the relationship between social status and LTA and NK cytotoxicity was disrupted during acute stress. Pig behavior was largely influenced across all stress treatments. COLD pigs had greater overall activity, including, feeding, standing and attacking and they had less lying behavior. The HEAT and SHIP pigs had depressed activity levels and spent more time lying. During acute stress, behavioral symptoms appears to be the most accurate and reliable indicator of stress.