Transportation Stress and Adaptation in Sheep: Physiological to Molecular Responses

Abstract

Transportation exposes sheep to physical, physiological, and psychological stressors—such as loading, vehicle movement, confinement, social mixing, feed/water deprivation, and thermal extremes. These factors disrupt homeostasis, triggering the sympathetic–adrenomedullary system and the hypothalamic–pituitary–adrenal axis. This neuroendocrine cascade increases heart rate, respiration, cortisol, glucose, lactate, and muscle enzymes, while altering thyroid hormones and hydration status. Prolonged transport induces fatigue, oxidative stress, tissue damage, behavioral changes, and impaired meat quality. Cell-level adaptation relies heavily on heat shock proteins, particularly HSP70 and HSP90, to protect proteins and preserve cellular integrity. Beyond basic physiological metrics, emerging molecular tools—including heat-shock signaling, antioxidant pathways, circulating microRNAs, metabolomics, and biologgers—offer deeper insight into stress dynamics. A study on Avishaan sheep after a 20-hour, 750-km journey revealed a 7.6% body weight loss along with significant metabolic and biomarker alterations (GPT, cholesterol, triglycerides, GGT, CK-NAC, and LDH), despite minimal changes in routine vital signs. Mitigating these impacts requires an integrated assessment of physiological, biochemical, and molecular markers alongside climate-resilient transport management. Keywords: sheep, stress, adaptation, HSPs, biomarkers, animal welfare Transportation is a routine but potentially stressful component of animal production system. Animals may be transported for marketing, slaughter, breeding, restocking, seasonal grazing, drought or flood management and changes in ownership. Road transport is particularly important because it is widely used, economical however many welfare problems arise from loading procedures, vehicle design, journey conditions and unloading practices (Knowles, 1998). Transportation should therefore be viewed as a multidimensional biological challenge, rather than as a single physiological event (EFSA Panel on Animal Health and Welfare, 2022). The severity of transportation stress is influenced by journey duration, stocking density, road and driving conditions, vehicle design, ventilation, weather, altitude, animal age, breed, sex, fleece condition, nutritional status and pre-transport health. Psychological stress occurs when animals perceive handling, confinement, noise, unfamiliar surroundings, predators or unfamiliar sheep as threats (European Commission, 2018; EFSA Panel on Animal Health and Welfare, 2022). Consequently, understanding transport stress requires simultaneous consideration of environmental, behavioural, physiological, biochemical and molecular responses. a) Major Stressors During Transportation Handling, loading and unloading: - Gathering, chasing, restraint, loading and unloading can cause substantial physical and psychological stimulation. Rough handling may result in fear, excessive movement, slipping, falling, bruising and skeletal injuries. The stress response may therefore begin before the vehicle starts moving. Vehicle movement and vibration: - During transportation, sheep continuously adjust their posture to compensate for acceleration, braking, turning and road vibration. Rough roads and poor driving increase muscular effort and the risk of falls and trauma. Excessive stocking density restricts movement and lying, whereas excessive space may allow animals to be thrown against partitions or other animals. Thus, road quality and driving behaviour are not merely logistical factors; they directly influence physiological load. Thermal stress: - Thermal conditions are particularly important during transportation in hot and semi-arid environments. High temperature, humidity, solar radiation and inadequate ventilation may increase respiratory rate and body temperature and promote dehydration and oxidative stress. In densely stocked vehicles, accumulation of heat and moisture may further compromise thermal balance. Conversely, cold, rain and direct exposure can increase the energetic cost of thermoregulation. Feed and water restriction: - Fasting initially stimulates hepatic glycogen utilization and gluconeogenesis. With increasing duration, adipose reserves are mobilized, resulting in increased free fatty acids and ketone bodies such as β-hydroxybutyrate. Protein catabolism may also increase. Water deprivation can cause haemoconcentration and alterations in packed cell volume, total protein, albumin and plasma osmolality. Body-weight loss is therefore a useful practical indicator, although it represents both tissue and gastrointestinal-content changes. Psychological and social stress: - Sheep are social animals and may respond to unfamiliar handlers, separation, confinement, loud noise and mixing with unfamiliar animals through agitation, vocalization, escape attempts and excessive movement. Conversely, severe fatigue may result in immobility, prolonged standing, recumbency or reduced responsiveness. b) Neuroendocrine and Physiological Responses Transportation activates two major stress pathways: the sympathetic–adrenomedullary (SAM) system and the hypothalamic–pituitary–adrenal (HPA) axis. The SAM pathway provides a rapid response: Stress perception → sympathetic activation → adrenal medulla → adrenaline/noradrenaline → increased heart rate, respiration and energy mobilization. The HPA pathway produces a comparatively sustained endocrine response: Hypothalamus → CRH → pituitary → ACTH → adrenal cortex → cortisol. c) Physiological and Biochemical Indicators Cardiovascular, respiratory responses and rectal temperature: - Heart rate is a sensitive indicator of acute sympathetic activation and may increase during handling, loading, vehicle movement and unloading. Respiratory rate reflects both stress and thermoregulation and becomes particularly important under hot conditions. Rectal or core temperature provides information about the balance between heat production and heat dissipation. Body weight, packed cell volume and hydration-related variables provide additional information about the consequences of prolonged journeys. Metabolic responses: - Glucose and lactate reflect immediate energy demand and muscular activity. Free fatty acids and β-hydroxybutyrate indicate mobilization of lipid reserves, while urea may increase with protein catabolism or dehydration. Creatine kinase (CK), lactate dehydrogenase (LDH), aspartate aminotransferase and gamma-glutamyl transferase may indicate muscular effort, cellular leakage or tissue disturbance. Importantly, research from ICAR-CSWRI in Avishaan sheep following a 20-h, 750-km road journey recorded a 7.6% reduction in body weight and significant changes in GPT, cholesterol, triglycerides, GGT, CK-NAC and LDH after transportation (Dangi et al., 2025). Routine physiological parameters did not necessarily show corresponding significant changes, indicating that biochemical variables can reveal transport-associated effects not captured by conventional measurements. These observations support the concept that transport assessment should incorporate both systemic physiological responses and biochemical evidence of metabolic and muscular stress. d) Oxidative Stress and Antioxidant Defense Oxidative stress represents an important link between systemic transportation stress and cellular damage. Physical exertion, heat exposure, fasting and possible hypoxia can increase reactive oxygen species (ROS). Oxidative biomarkers should be interpreted together with physiological, endocrine, metabolic and behavioural measurements. The general sequence is: Transportation → increased metabolic demand → ROS generation → oxidative challenge → antioxidant activation. e) Heat Shock Proteins and Molecular Adaptation At the cellular level, transportation-related heat, oxidative stress, physical exertion and metabolic disturbance may activate the heat-shock response. Important HSP families include HSP27, HSP60, HSP70, HSP90 and HSP110. Among them, HSP70 is particularly important as a molecular chaperone that assists protein folding, prevents aggregation of damaged proteins and facilitates cellular repair. A simplified pathway is: Stress → protein unfolding/cellular disturbance → HSF1 activation → heat-shock element binding → HSP transcription → HSP production → protein stabilization and cellular protection. HSP70 should not be regarded as a simple linear stress meter. Its expression depends on the type, severity and duration of stress, tissue, genotype, environmental conditions and previous exposure. Heat-adapted sheep may possess stronger cellular defense mechanisms involving HSPs and antioxidant pathways. This raises an important possibility for transportation research: breed-specific HSP and antioxidant responses may contribute to individual differences in transport resilience. f) Adaptation: From Acute Stress to Recovery Transportation responses change with time and can be broadly divided into four stages: Initial response: Handling and loading produce rapid cardiovascular and endocrine activation. Acute transport: Vehicle movement, vibration, thermal conditions, social disturbance and restricted movement maintain physiological demand. Adaptation: Some physiological variables may decline as animals reduce activity and adjust to the transport environment. Recovery: After unloading, animals restore energy and fluid balance through rest, feeding and drinking, while endocrine, cardiovascular and cellular systems gradually return toward baseline. Importantly, adaptation does not necessarily mean absence of stress. A declining cortisol response may reflect HPA negative feedback or habituation while other physiological or molecular disturbances continue. This emphasizes the need to monitor both the journey and the post-transport recovery period. g) Welfare and Meat-Quality Consequences Poor transportation can result in deh