| Heat stress (HS) is a physiological condition when animals or humans can no longer regulate their internal euthermic temperature. When livestock and humans are subjected to this environmental stress, it can be detrimental to health, well-being and performance, and if severe enough even death. Swine are particularly susceptible to HS, costing the industry millions of dollars annually in lost production and carcass quality. One of the major organs affected by HS is the gastrointestinal tract. Heat stress reduces intestinal function and integrity, increases risk of endotoxemia and causes metabolic dysfunction. Together, these issues antagonize pig performance and health. Therefore, the overall objective of this dissertation was to chronologically characterize how a growing pig first perceives and initially copes with a severe heat-load. Additionally, we also determined the involvement of reduced feed intake utilizing a thermal neutral pair-fed to HS feed intake model. Knowing that HS alters intestinal integrity and function, this dissertation also assess the ability of an organic zinc source (ZnAA) as a nutritional mitigation strategy for HS in pigs. To address these objectives, the thesis has been organized into four research chapters.;In the first study, (Chapter 2, 24 h of HS), pigs experienced changes in intestinal integrity, altered expression of tight junction proteins, increased circulating endotoxin concentrations and markers of cellular stress (heat shock and hypoxia response). Interestingly, under HS conditions, glucose transport machinery was also upregulated. In Chapter 3 (0-6 h of HS), pigs exposed to a short duration of HS had increased body temperatures, reduced feed intake, changes in neuropeptide hormones, and lighter body weights compared to controls. These pigs also had compromised intestinal integrity, which was evident as early on as 2 h HS. By 6 h HS, the ileum was more severely affected compared to the colon. At 12 h HS (Chapter 4), the zinc-amino acid complex was able to lower core temperatures but had no other phenotypic effects compared to a control diet. The ZnAA also ameliorated some negative effects of HS on the intestine by reducing circulating endotoxin, increasing LPS-binding protein and improved metabolic markers (lower plasma urea nitrogen). Interestingly, many of the negative effects on intestinal integrity appear to be similar to that seen due to reduced nutrient intake as pair-fed and HS pigs at 12 h had a similar intestinal integrity profile. On the metabolic and oxidative stress side, however, feed intake appears to play less of a role as the intestinal proteomic profile appeared to be mainly impacted by HS alone (Chapter 5). Many of the proteins identified were involved in the cellular stress response, metabolism, and oxidative stress. In conclusion, heat stress directly and indirectly (via reduced feed intake) affects post-absorptive metabolism and intestinal integrity and both variables probably contribute to decreased growth parameters in young pigs. |
