Heat Stress Responses in Birds: A Review of the Neural Components

SIMPLE SUMMARY: Heat stress is a major environmental condition negatively impacting the wellbeing of various avian species. In chickens, heat exposure is associated with disruption of metabolic and immune system function, and an increased risk of mortality. This has a negative impact on the food economy, as chicken products make up roughly 34% of the world’s protein. Techniques to mitigate exposure to high temperatures have been discussed in depth, and most research suggests that the root cause of heat stress-induced physiological aberrations is alterations in the stress response and reduced food intake. Unfortunately, little is known about thermoregulation in birds. That thermoregulation, food intake, and the stress response are all mediated by the hypothalamus make it tempting to speculate that it is the central hub at which these systems interact and signals from diverse pathways are integrated. Thus, this review discusses the neural circuitry in birds associated with thermoregulation, food intake, and stress response at the level of the hypothalamus, with a focus on how these systems might interact in the presence of heat exposure. ABSTRACT: Heat stress is one of the major environmental conditions causing significant losses in the poultry industry and having negative impacts on the world’s food economy. Heat exposure causes several physiological impairments in birds, including oxidative stress, weight loss, immunosuppression, and dysregulated metabolism. Collectively, these lead not only to decreased production in the meat industry, but also decreases in the number of eggs laid by 20%, and overall loss due to mortality during housing and transit. Mitigation techniques have been discussed in depth, and include changes in air flow and dietary composition, improved building insulation, use of air cooling in livestock buildings (fogging systems, evaporation panels), and genetic alterations. Most commonly observed during heat exposure are reduced food intake and an increase in the stress response. However, very little has been explored regarding heat exposure, food intake and stress, and how the neural circuitry responsible for sensing temperatures mediate these responses. That thermoregulation, food intake, and the stress response are primarily mediated by the hypothalamus make it reasonable to assume that it is the central hub at which these systems interact and coordinately regulate downstream changes in metabolism. Thus, this review discusses the neural circuitry in birds associated with thermoregulation, food intake, and stress response at the level of the hypothalamus, with a focus on how these systems might interact in the presence of heat exposure.