Interpretive Summary: Diurnal heat stress reduces pig intestinal integrity and increases endotoxin translocation.
By: Dr. Caitlin Vonderohe
Heat stress is a significant physiologic problem during the summer and in tropical climates for growing pigs. Pigs that are heat stressed grow slower, have reduced feed intake, and in some cases, the thermal challenge may be deadly. One of the primary ways that heat stress can cause physiologic upset in growing swine is by adversely affecting how well the intestine serves as a barrier between the bacterial populations of the gastrointestinal tract and the rest of the pig’s body. In the paper “Diurnal head stress reduces pig intestinal integrity and increases endotoxin translocation,” published in the February issue of Translational Animal Science, Gabler et al. used a diurnal heat stress model to determine how three days of cyclic heat stress affected pig growth performance, and the movement of bacteria and associated toxins from the gastrointestinal tract to the rest of the pig’s body.
Forty-eight crossbred gilts were used in this study and fed the same diet throughout the project. The pigs were placed in two rooms in thermoneutral conditions for 21 days. Half of the pigs were then exposed to three days of diurnal heat stress conditions (cyclic pattern of increased and decreased temperatures). Pig weights were measured before the pigs were placed on test, prior to the heat event and after the heat event. Blood samples were taken before pigs were harvested and additional samples of gastrointestinal contents, and gastrointestinal and muscle tissue were collected after harvest.
Blood was analyzed for measures related to the level of endotoxin in the blood (endotoxin, LPS-binding protein) and markers of the acute immune response (glucose, NEFA, BUN, insulin, TNF-α, IL1-β, and haptoglobin) to endotoxin and bacteria in the blood. Using chambers were also used to measure the permeability of ileal tissues and western blot analysis was performed to detect the abundance of heat shock and hypoxia-inducible proteins that have previously been shown to increase in heat stressed tissues.
There were no differences in growth performance between the two groups of pigs (thermoneutral and heat stressed) prior to the heat challenge. However, when the heat stressed pigs were subjected to the three day heat event, they had reduced feed intake, poorer average daily gain, and their body weights at the end of the study were 7% lighter than the pigs kept in thermoneutral conditions. Ex-vivo experiments with intestinal tissues mounted in using chambers showed a significant (200%) increase in small intestinal permeability to large molecules in the pigs that experienced a three day heat event compared to the thermoneutral pigs.
Three days of diurnal heat stress also increased the level of endotoxin in the blood by 150% and tended to reduce the concentration of LPS-binding protein compared to non-heat stressed pigs. There was also a reduction in circulating level of non-esterified fatty acids, insulin, and tumor necrosis factor-α, a higher circulating level of secretory phospholipase-2 activity, and tended to have a higher circulating level of haptoglobin in the blood of heat stressed pigs compared to pigs housed in thermoneutral conditions. Heat stressed animals also had 180% more heat shock protein-70 present in the ileum than pigs that were not heat stressed, however there were no differences in heat shock protein-70 or hypoxia-inducible proteins in muscle tissue.
Pigs that experienced three days of diurnal heat stress were unable to regulate their body temperatures, even when the ambient temperature was lowered overnight. Heat stressed pigs had greater respiratory rates, poorer average daily gain, and significantly reduced feed intake. Using chamber data, coupled with the changes in inflammatory proteins observed in the blood of heat stressed pigs, allowed the authors to conclude that three days of diurnal heat stress resulted in significant disruptions to the function of the small intestine and dysfunction of the intestine as a barrier to bacteria and toxins. Overall, the authors conclude that three days of diurnal heat stress is a suitable model for the physiologic effects of long term heat stress that can occur during summer months in growing swine.
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