The Impact of Inflammation in Heat-stressed Dairy Cattle

Despite modern infrastructure, summer heat stress continues to compromise dairy calf welfare and productivity. Extreme ambient temperature and humidity events during summer are expected to increase, which will surely exacerbate the heat stress impact on dairy calves. Heat stress-indirect effect can reduce feed intake and consequently the supply of energy and nutrients as part of the regulatory mechanism of thermal homeostasis. For example, during heat stress, lactating dairy cows with high metabolic demand (4X maintenance) suppress feed intake to decrease metabolic heat production. In pre-weaned dairy calves with low metabolic demand (2X maintenance), nutrient intake has little effect on metabolic heat production; therefore, the heat stress impact on feed intake is negligible. Surprisingly, dairy calves exposed to heat stress reduce ADG and the feed use-efficiency by up to 35 %. The productive losses in heat-stressed calves may be independent of nutrient intake. To this end, evaporative cooling through increased respiratory rate is the main pathway to maintain thermal homeostasis, and the associated increase in energy utilization might explain, in part, the stunted performance. Using production data from normothermic and hyperthermic calves, we simulated energy utilization and showed that heat stress increased requirements for thermoregulation (17 % of ME maintenance). In addition, thermoneutral calves allocated 56 % of total energy requirements to frame growth, but hyperthermic calves used 40 % of total energy requirements to support frame growth, an estimated 1.2 Mcal ME differential between the two groups. Our findings showed that a pharmacological intervention to block heat-stress immune activation prevented the heat stress decline in calf growth. Heat stress changes in splanchnic blood flow can lead to hypoxia and nutrient shortage in the intestinal mucosa, increasing gut permeability and translocation of lumen antigens. Our findings in dairy calves confirmed that heat stress increased 20 to 24 % intestinal permeability to lumen contents, flattened jejunal villi, and increased jejunal crypt depth. Jejunum mucosa analyses showed that concentrations of IL-6 increased 2-fold and IL-36RA increased 4-fold, confirming tissue-specific effects of heat stress. Heat stress increased 13 % plasma concentrations of Serum Amyloid A, but below the 10 mg/L cut-off for healthy calves. Collectively, our findings suggest that energy supply not used for maintenance, thermoregulation, and growth may be used by the immune system. A better understanding of the gut-immune axis would contribute to advancing our understanding of heat stress pathophysiology and assessing interventions to prevent and mitigate health issues and improve productivity in dairy cattle.