Seasonal heat stress effects on the physiological, hematological, hormonal and biochemical responses of Tunisian sheep breeds.

Abstract

Heat stress represents a major environmental constraint affecting sheep production and welfare in Mediterranean and arid regions. This study assessed thermoregulatory, hematological, endocrine and metabolic responses to increasing thermal load in three indigenous Tunisian breeds reared under natural environmental conditions in northern Tunisia. Thirty multiparous ewes belonging to Barbarine (BTR), Queue Fine de l'Ouest (QFO) and Noire de Thibar (NTB) breeds (10 animals/breed) were monitored during three thermal phases corresponding to thermoneutral, mild heat stress and moderate heat stress conditions, defined according to temperature-humidity index (THI) classes. Physiological variables were recorded throughout all thermal phases, whereas hematological, hormonal and biochemical parameters were assessed during thermoneutral and moderate heat stress conditions. Data were analyzed using a repeated-measures mixed linear model including breed, THI class and their interaction as fixed effects and ewe as a random effect, with significance set at P < 0.05. Heat stress significantly increased rectal temperature, respiratory rate, heart rate and skin temperature in all breeds (P < 0.05). NTB ewes exhibited the highest rectal temperature under moderate heat stress, suggesting lower thermotolerance. Moderate heat stress triggered elevations in red blood cell counts, hemoglobin, total protein and cholesterol, whereas glucose, mean corpuscular hemoglobin concentration and alkaline phosphatase decreased (P < 0.05). Plasma thyroxine and cortisol declined in all breeds (P < 0.05). Interestingly, thyroid-stimulating hormone (TSH) was significantly reduced only in NTB ewes (P < 0.05), possibly reflecting a lower threshold or more pronounced down-regulation of the hypothalamic-pituitary-thyroid axis under severe metabolic adjustments compared to lighter-fleeced local breeds. These findings demonstrate breed-specific adaptive variations essential for optimizing sheep selection and management under Mediterranean climatic shifts.