Exploring the links between stress and innate immune responses in cattle
Cattle morbidity and mortality are observed more frequently during potential stressful situations, such as weaning, transportation, and commingling. The objectives of this dissertation were to identify intrinsic factors, such as sex-classification and temperament and extrinsic factors such as transportation, feeding strategies and weaning that might influence the stress and innate immune responses of cattle. A sexually dimorphic immune response to endotoxin or corticotropin-releasing hormone (CRH) was observed among Brahman heifers and bull calves. Heifers displayed less sickness behaviors but had greater rectal temperature and circulating tumor-necrosis-factor alpha (TNF-α) response to an in vivo endotoxin challenge than bulls. Heifers also had less of a circulating corticoid response than bulls after CRH and had an acute increase in circulating cytokines that was greater than bulls. Brahman bulls classified as temperamental had less neutrophil activity after transportation than bulls classified as calm. Weaning is a potentially stressful event for calves.. Many innate immune responses in Holstein calves including neutrophilia and decreased phagocytic and oxidative burst capacities were suppressed transiently following weaning. Early weaning calves at 24 d of age rather than at 45 d of age also suppressed neutrophil L-selectin expression. Furthermore, early weaning lightweight calves also suppressed the secretion of TNF-α from whole blood cultures stimulated with LPS. Switching Holstein calves from twice daily to once a daily feeding during the 4th week of life was a mild stressor. There was only a tendency for neutrophilia among the once-fed calves. No other aspects of innate immune responses were acutely suppressed by feeding frequency. These studies indicate that the stress effect of stress on immune responses of cattle is complex. Sex classification, age, and temperament of cattle influence how an animal responds to stress and subsequently, how stress will impact the innate immune system. These studies all indicate that stress can cause an initial suppression of the immune system, but in several instances, the innate immune system had compensatory response, where increased activity after stress is observed.