Influence of heat stress on the immunity in growing beef cattle

Introduction: adverse environmental conditions, such as high temperature and humidity, seem to affect the immune system in beef cattle, as already demonstrated in dairy cattle. Aim: the aim of this study was to evaluate the haematological changes (complete blood count), and the serum levels of 4 major cytokines (IL-4, IL-6, TNF-α, IFN-γ) in intensively farmed bull cattle (A) and white veal calves (B) after exposure to a heat stress condition. Materials and methods: our observations were conducted during the summer of 2009, collecting blood samples, in 10 bull calves (A) and in 11 white veal calves (B), prior to and 3-4 days after a heat wave (defined by a mean daily Temperature-Humidity Index value higher than 72 for three consecutive days), predicted by Regional Agency for the Environment. Blood cells value and serum cytokines concentration (by using a multiplex sandwich enzyme-linked immunosorbent assay based on chemioluminescent detection of the analytes), were determined. Results and discussion: mean daily Temperature-Humidity Index in both farms moved, during the test, from 68.3 to 75.6 with a difference of +7.3. A significant increase in white blood cells, neutrophils and lymphocytes, after the heat wave was recorded. In particular, mean white blood cells count increased from 6438.1±1289.8 /mm3 to 9061.9±2300.5 /mm3 (p<0.01), neutrophils from 1618.6±728.1 /mm3 to 2394.9±1551.1 /mm3 (p<0.05) and lymphocytes from 4389±951.4 /mm3 to 5849.5±1696.9 /mm3 (p<0.05). In the same time, in response to the heat wave, serum levels of IL-4 significantly increased from 3.3±3.9 pg/ml to 7.4±7.6 pg/ml (p<0.01), whilst serum levels of IFN-γ significantly decreased from 73.9±155.4 pg/ml to 28.1±31.8 pg/ml (p<0.01). No significant change of serum concentrations of pro-inflammatory cytokines such as TNF-α and IL-6 in both farms was found. Conclusions: our results suggest that a heat wave of at least 3-4 days of length is effective to impact on the immune system function of intensively farmed bull cattle and white veal calves, shifting the immune response from a Th1 type (IFN-γ driven) to Th2 type (IL-4 driven). These immunological changes could reflect a depression of the cell-mediated immunity in spite of increase of circulating leukocytes in the blood stream.