EFFECTS OF HEAT STRESS ON INFLAMMATION, INTESTINAL INTEGRITY, MICROBIOTA, AND PRODUCTIVITY IN DAIRY CALVES

Two studies were conducted to evaluate the effects of heat stress on growth performance, inflammation, intestinal integrity and microbiota changes, furthermore dexamethasone was tested to alleviate heat stress in dairy calves. In study one, we hypothesized that cyclical heat stress for 7 days alters pro- and anti-inflammatory signals, gut microbiota community composition, and intestinal barrier in dairy calves. Heat stress had a section-specific impact on intestinal integrity. Jejunal mRNA expression of TJP1 and expression of heat shock transcription factor-1 decreased in heat stress calves. Heat stress decreased interleukin-1 alpha and tended to decrease interleukin-17A. Conversely, heat stress increased jejunal concentration of interleukin-36 receptor antagonist. Heat stress reduced concentrations of interleukin-6 in plasma. Alteration of the large intestine bacterial communities was characterized by increased butyrate-producing bacteria genus Butyrivibrio_3 in heat stress calves. In study two, we hypothesized that a modulation of the immune response improves nutrient metabolism, intestinal morphology, barrier function, and gut microbial community composition in Holstein bull calves exposed to cyclical heat stress for 5 days. Treatments (saline vs dexamethasone), environments (thermoneutral vs heat stress), and time (study day or hour) were included in the statistical analysis. Feed intake did not change but a treatment by environment interaction showed that dexamethasone increased average daily gain and feed efficiency in heat stress calves. A treatment by environment by time interaction on study day 1 showed that the post prandial plasma glucose concentration declined in thermoneutral but increased in heat stress calves treated with dexamethasone. A treatment by environment by study day interaction showed that white blood cells and neutrophil concentrations increased more in heat stress than in thermoneutral calves treated with dexamethasone on study day 1. A treatment by environment by study day interaction showed that blood lymphocyte concentration decreased in heat stress calves treated with dexamethasone on study day 1. Treatment by environment interaction showed that jejunal crypt depth was reduced in heat stress calves treated with dexamethasone only. A treatment by environment interaction showed that jejunal IL-6 concentration was increased in thermoneutral dexamethasone but reduced in heat stress dexamethasone calves. Collectively, heat stress triggered inflammation, intestinal barrier dysfunction, and jejunal microbial community imbalance and dexamethasone improved productivity, intestinal morphology and reduced the pro-inflammatory tone in dairy calves.