Seasonal Heat Stress Effects on the Physiological, Hematological, Hormonal, and Biochemical Responses of Tunisian Sheep Breeds

Heat stress is a major environmental challenge for small ruminant production in Mediterranean and arid areas. This study investigated the effects of seasonal heat stress on physiological, hematological, hormonal, and biochemical parameters in Tunisian sheep raised in northern Tunisia. Thirty multiparous ewes (10 per breed) from White Fat-tailed Tunisian Barbarin (BTR), Fine-tailed Tunisian Western (QFO), and Fine-tailed Thibar Black (NTB) breeds were evaluated during a thermo-neutral period and a heat stress period characterized by elevated temperature–humidity index (THI) values. Rectal temperature, respiration rate, heart rate, and skin temperature were recorded, with skin temperature measured at selected anatomical sites. Blood samples collected during both periods were analyzed for hematological indices, plasma concentrations of thyroxine, triiodothyronine, cortisol, and thyroid-stimulating hormone, as well as biochemical parameters including glucose, cholesterol, total proteins, and alkaline phosphatase. Data were analyzed using analysis of variance. Heat stress significantly increased rectal temperature, respiration rate, and heart rate in all breeds, indicating activation of thermoregulatory mechanisms. The NTB breed exhibited the greatest increase in rectal temperature, suggesting higher sensitivity to heat stress. Hematological and biochemical responses to increasing THI included elevated red blood cell count, hemoglobin, cholesterol, and total protein concentrations, along with decreases in mean corpuscular hemoglobin, mean corpuscular hemoglobin concentration, glucose, and alkaline phosphatase. Heat stress significantly reduced thyroxine and cortisol concentrations, whereas triiodothyronine remained unaffected. Thyroid-stimulating hormone was significantly altered only in NTB ewes. These findings demonstrate breed-specific adaptive responses to heat stress and provide relevant information for sheep management and selection under Mediterranean climatic conditions.