ISSUE FOCUS FEED & ADDITIVE MAGAZINE July 2025 59 neutral zone – is between 18 and 22°C. When birds are kept within this temperature range, they do not have to spend energy on maintaining a constant body temperature.6 Heat stress is the result of unsuccessful thermoregulation in the animals, as they absorb or produce a higher quantity of heat than they can lose. It means that there is a negative balance between the net amount of energy flowing from the animal to the environment and the energy it produces.7 CONTRIBUTING FACTORS TO HEAT STRESS IN POULTRY This energy imbalance is influenced by environmental factors such as sunlight, thermal irradiation, air temperature, humidity, and stocking density, but also by animal-related factors such as body weight, feather coverage and distribution, dehydration status, metabolic rate, and thermoregulatory mechanisms.7, 8 When the environmental temperature is above the thermoneutral zone, the animals activate thermoregulation mechanisms to loose heat through behavioral, biochemical, and physiological changes and responses.9-12 Heat stress can be classified into two main categories: Acute and chronic. Acute heat stress refers to a short and fast increase in environmental temperature (a few hours), whereas under chronic heat stress the high temperatures persist for more extended periods (several days). Some studies suggest that, in some circumstances, poultry animals show a degree of resilience to acute heat stress.7, 9, 10 However, in the long-run, their compensatory mechanisms are not sufficient to maintain tissue integrity and thus health and performance.11 THE ANIMAL’S RESPONSE TO HEAT STRESS The exposure of poultry to heat stress changes the gene expression of cytokines, upregulates heat shock proteins (HSP), and reduces the concentration of thyroid hormones.10, 12 When heat stress persists, these cascades of cellular reactions result in tissue damage and malfunction. The animals exposed to heat stress suffer adverse effects in terms of performance, which are widely known and include high mortality, lower growth and production (Figure 1), and a decline in meat and egg quality.13, 14 160 140 120 100 80 60 40 20 0 900 800 700 600 500 400 300 200 100 0 Day 1-10 Day 10-22 Day 22-28 Day 28-35 Day 35-42 FITC-d (% of control) Body weight gain in heat stressed broilers Control HS FITC-d BWG (grams) Figure 1. Body weight gain of broilers exposed to chronic heat stress (35°C continuously from day 21). A marker for tight junction permeability was added to feed (FITC-d - fluorescein isothiocyanate dextran); its fluorescence (in serum) increased with heat stress exposure time, showing higher intestinal permeability. (Adapted from Ruff et al., 2020)
RkJQdWJsaXNoZXIy MTUxNjkxNQ==