ISSUE FOCUS FEED & ADDITIVE MAGAZINE July 2024 37 stress often have increased water consumption and reduced feed intake, resulting in reduced productivity. Increased dust bathing may help cool them down, and spreading or drooping their wings can increase airflow to their bodies. As birds become increasingly uncomfortable, they may become more restless, change their nesting or roosting behavior to seek cooler locations, or even become more aggressive with increased pecking (Figure 1). THERMAL STRESS’ IMPACT ON PHYSIOLOGIC SYSTEMS Thermal stress affects entire systems. One important system is the immune system. As the body focuses more on temperature regulation, it diverts energy away from immune functions, leading to immune suppression. Thermal stress also triggers the release of stress hormones such as cortisol, which further suppresses the immune response. This immunosuppression increases susceptibility to disease, ultimately reducing productivity and increasing mortality. In addition, immunosuppression reduces the effectiveness of vaccinations. Thermal stress increases the production of reactive oxygen species. This oxidative stress damages immune cell components, impairing their function and ability to eliminate pathogens. On top of this thermal stress also perturbs the gut microbiota. As the microbiota plays a critical role in modulating the immune system and protecting against pathogen invasion, thermal stress events often result in increased susceptibility to infections through the gut as well. Thermally stressed birds have caloric deficits due to increased metabolism during cold shocks or decreased feed intake during hot periods. This energy deficit ultimately impairs growth, reproduction, immune function, and performance. In addition, extreme heat can cause heat stroke, and severe cold can cause frostbite and tissue damage. Moreover, thermal stress can disrupt the reproRELATIVITY HUMIDITY (%) <50 little to none 50 - 57 mild 58 - 66 moderate 67 - 72 severe > 72 extreme TEMPERATURE (°C) Figure 2. Temperature–Humidity Index: A measure indicating the level of discomfort poultry is thought to experience because of the combined effect of temperature and humidity. Numerical values < 50 correlate with little to no heat stress; 50 – 57 correlates with mild heat stress; 58 – 66 correlates with moderate heat stress; 67 – 72 correlates with severe heat stress, and values > 72 correlate with extreme heat stress.
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