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Topic:Physiology
The physiology of horses encompasses the study of the biological functions and processes that occur within the equine body. This includes the examination of various systems such as the cardiovascular, respiratory, musculoskeletal, digestive, and nervous systems. Understanding equine physiology is essential for comprehending how horses adapt to different environmental conditions, perform physical activities, and respond to health challenges. Research in this field often focuses on the mechanisms of energy metabolism, thermoregulation, and muscle function during exercise, as well as the physiological responses to stress and disease. This page compiles peer-reviewed research studies and scholarly articles that explore the diverse aspects of equine physiology, providing insights into the biological processes that support the health and performance of horses.
Effects of Exercise on Ocular Parameters and Oxidative Stress Biomarkers in Saliva and Tear Fluid of Horses. Physical exercise is a physiological stressor in horses and may induce oxidative stress through increased production of reactive oxygen species. Saliva and tear fluid are accessible non-invasive matrices that may reflect oxidative balance, while ocular parameters including intraocular pressure (IOP), tear secretion (TS) and central corneal thickness (CCT) are key components of ocular homeostasis. Objective: To evaluate the effects of a standardised submaximal exercise session on IOP, TS and CCT, and on oxidative stress biomarkers (CAT, SOD, GSH, MDA, IMA) measured in saliva and tear fluid. Met...
Cycle length variation assists in differentiation of atrial, ventricular, and sinus arrhythmias in horses at rest. Criteria for arrhythmia classification in horses are insufficient, particularly when differentiating atrial premature complexes (APCs) from sinus arrhythmia (SA). This study investigated RR and PP interval changes between SA, APCs, and ventricular premature complexes (VPCs). Unassigned: 30 resting ECGs were reviewed: 10 each containing SA, APC, and VPC. Sequences of 10 RR and PP intervals were recorded for each arrhythmia (20 events/ECG). Percent RR and PP deviations were calculated (1) between sequential intervals (%ΔSEQPP and %ΔSEQRR, respectively) and (2) compared to averaged baseline int...
Horses undergoing supramaximal fixed-speed treadmill exercise after administration of furosemide experience less severe acidosis and slower onset of fatigue. To document changes in acid-base and ion balance during exercise following furosemide administration and to relate them to performance. Unassigned: 6 fit Thoroughbred horses received 0.5 mg/kg IV furosemide (F) or a placebo (C) in randomized crossover order 4 hours before galloping to fatigue on a treadmill at the speed equated with an oxygen demand of 1.15 X maximum oxygen consumption (V˙o2max). Arterial and mixed venous blood was sampled before treatment, preexercise, after 2-minute warm-up, at fatigue, and 1 minute postexercise. The pH, temperature-corrected Pco2, total CO2, and sodium, po...
Pulsed wave tissue Doppler imaging of the left ventricle in foals: Feasibility, inter- and intra-observer variability and wave pattern over the first days of life. Tissue Doppler imaging (TDI) may improve the assessment of myocardial function in neonatal foals. Objective: To document the feasibility of left ventricular pulsed wave (PW) TDI, inter- and intra-observer variability and wave pattern over the first 5 days of life. Methods: Prospective observational study. Methods: Transthoracic echocardiography was performed in 25 healthy Standardbred foals within the first 24 h after birth (T1), between 60-84 h (T2), and 108-132 h (T3) after birth using right parasternal short-axis (R-LV SAX) and subcostal long-axis views at the mitral annulus, both a...
Seasonal modulation of platelet indices in mares. Platelet indices provide valuable information on platelet activation, heterogeneity, and production kinetics, yet their physiological variability remains poorly characterized in horses. Although seasonal changes in platelet count have been described, the seasonal modulation of derived platelet indices in mares remains poorly understood. The aim of this study was to evaluate the seasonal variation of platelet count and derived platelet indices in clinically healthy mares from the Valencia region (Spain), and to assess their interrelationships. Forty clinically healthy Spanish Purebred mares (6-...
Species-specific effects of spinal stiffness on gait and actuation-cost proxy in simulated cheetah and horse galloping. Cursorial quadrupeds employ distinct mechanical strategies for high-speed locomotion, with felids relying more strongly on axial motion than equids. The biomechanical role of spinal stiffness, however, is difficult to isolate experimentally because it is coupled to limb mechanics, morphology, and coordination. Here, we used comparative three-dimensional trajectory optimization to test how spinal stiffness influences gait mechanics and actuation-demand proxies in simulated cheetah-like and horse-like galloping. Homologous cheetah and horse models were evaluated across spinal-stiffness values, p...
Species-Specific Mechanisms of Sperm Transport: Predominant Roles of Motility in Ewes and Uterine Contractility in Mares. The factors controlling sperm transport within the female genital tract vary among species. The aim of this study was to quantify the respective roles of sperm motility and uterine contractions in passage of sperm within the reproductive tract of the mare and ewe. Sperm imaging was performed in vivo within the female genital tract of mares and ewes using probe-based confocal laser endomicroscopy (pCLE) after intrauterine insemination during two stages of the cycle characterized by high (oestrus) or low (luteal phase) uterine contractility. Fluorescent microbeads (2 µm in diameter) were insemi...
In-Hand Training of Piaffe and Passage with Leg Stimulation: A Literature-Based Biomechanical Argument for Why This Method Fails to Produce True Collection. In-hand training of piaffe and passage via tactile leg stimulation (Handarbeit mit Touchieren) is widely practised in dressage, yet consistently produces horses that elevate their legs without achieving collection - a pattern termed Schenkelgänger (leg-movers) in classical German equitation. Based on published kinetic, kinematic, and electromyographic literature, this review argues that tactile leg stimulation activates the spinal nociceptive withdrawal reflex, an unloading movement mechanistically opposed to the elastic tendon rebound and ventral muscular chain activation that define true co...
Recognition of unfamiliar predators in domestic horses through only visual predator cues. Acoustic, olfactory, and visual predator cues trigger various adaptive responses among a variety of prey species even when they are under human protection since birth. Previous studies have found that domesticated animals show increased vigilance, stress, flight, and aggregation behaviors in response to predation threats. These investigations, however, did not directly test visual threatening cues alone in animals that communicate more with body language than vocalizations while considering individual temperament and social status. The current study aims to address this issue. Eighteen horses ...
Circadian rhythm of heart rate variability in tropical horses: age-associated autonomic alterations and environmental air pollution effects in an urban field setting. Heart rate variability (HRV) reflects autonomic nervous system activity and is influenced by age, environmental conditions, and circadian rhythms. Information regarding these interactions in horses maintained under tropical conditions remains limited. This study aimed to characterize 24-h heart rate (HR) and HRV patterns in horses raised under tropical conditions and to investigate the effects of aging and environmental factors, including air pollution, on autonomic regulation. Unassigned: Fifteen clinically healthy horses aged 4-20 years were allocated to three age groups: 4-7 years, 8-14 yea...
Preliminary investigation of ghrelin in horses and ponies: receptor expression and associations with prandial state, morphometry and signalment. The hormone ghrelin has had limited attention in Equidae, despite connections with appetite and metabolic health in other species. Ghrelin influences hunger and food intake, energy expenditure and storage, glucose metabolism, and thermoregulation, through its receptor-mediated effects on tissues and via interaction with other hormones. There are two circulating forms, acylated and des-acylated ghrelin, considered to have distinct effects on metabolism in other species. The aims of this study were to confirm the presence of ghrelin receptors in equine tissues, to validate an assay for the measu...
Hair steroid hormones concentrations in jennies and their donkey foals from parturition to 6 months postpartum. The period after parturition/birth represents a delicate time for the dam and the offspring, during which several physiologic changes occur under finely tuned hormonal control, with the hypothalamic-pituitary- adrenal (HPA) axis playing a key role. This period, well studied in horses, received little attention in donkeys, despite the importance of species-specific knowledge for population preservation programs in many endangered donkey breeds. The present study aimed to investigate cortisol (C), DHEA(S), 17-β-estradiol (E2), progesterone (P4), testosterone (T), and allopregnanolone (AlloP) ha...
Platelet indices’ dynamic in broodmares: Effects of parity and lactation. Lactation and parity are physiological factors influencing platelet (PLT) indices, yet their combined effects in mares remain poorly characterized. As a dynamic process, lactation involves metabolic and endocrine changes that may influence thrombopoiesis; therefore, reproductive history and lactation stage may modulate PLT indices. Objective: To characterize longitudinal platelet (PLT) indices during early lactation in mares and evaluate the influence of parity on thrombopoietic responses. Methods: Fifty-five clinically healthy Spanish Purebred mares were included (20 primiparous and 25 multip...
Horses without ocular disease maintain similar electroretinographic responses and peripapillary retinal thicknesses across age groups. To determine the impact of age on equine electroretinography (ERG) parameters and retinal thickness in horses with healthy eyes. We hypothesized that ERG b-wave amplitudes would be significantly lower in older horses and that total retinal thickness would not vary significantly with age. Unassigned: Horses with clinically normal eyes comprising 3 age groups (≤ 10, 11 to 20, and ≥ 21 years) underwent scotopic and photopic ERGs, high-resolution fundus photography, and spectral-domain optical coherence tomography. Electroretinography results were compared between age groups using mean implici...
Sixty minutes of daily solitary turnout improved physiological and behavioral indicators of stress and welfare in stall-housed horses. Housing horses in stalls restricts natural roaming behaviors, which in turn increases stress and stereotypic behaviors. Daily turnout for voluntary exercise is common for stress mitigation, but there is no consensus for optimal duration of turnout sessions. Objective: We sought to understand how minimal and moderate-duration daily turnout affects behavioral and physiological stress indicators. Methods: In a Latin-square design, adult Quarter horses (N=11) were allowed 0, 15, or 60 minutes/day of solitary turnout for 12-day periods. In-stall behaviors and 24-hour time budgets were assessed on d...
Repeatability and test-retest reliability of thermal and pressure pain threshold testing in healthy thoroughbred horses. Quantitative sensory testing (QST) has been used in horses to assess nociceptive responsiveness but its broader clinical application is limited by variability across anatomical sites, environmental conditions, operator technique, and behavioral endpoints. This study aimed to evaluate the feasibility, within-session repeatability, and test-retest reliability of thermal threshold (TT) and pressure pain threshold (PPT) testing in healthy Thoroughbred horses. Unassigned: Eleven adult Thoroughbred horses (4 mares, 7 geldings; median [IQR] age 6 [5-6] years; body weight 558 ± 47 kg) underwent...
Using a wearable electrocardiogram (ECG) patch adapted for 7-day monitoring in the horse. Arrhythmias can cause poor performance and sudden death in horses. Long-term (>24 h) electrocardiogram (ECG) monitoring has been used to identify arrhythmias in humans and is superior to short-term monitoring for transient arrhythmia detection. Televet is the current equine gold-standard ECG device. The Cardea SOLO™ (CS) is a single-use monitor with automated ECG analysis software allowing ECG monitoring for 7 days. Objective: Our objective was to validate the performance of the CS compared to the Televet. Methods: Method validation. Methods: ECGs were performed on 12 healthy horses s...
Determining optimal practices for foal weaning – A protocol for a systematic review and network meta-analysis. Weaning is a stressful time in a foal's (Equus caballus) life. As humans artificially manage weaning in domestic horses, it provides an opportunity to safeguard horse welfare early in life, with potential long-term benefits. While numerous studies have examined the effects of different weaning and management interventions on foal stress, optimal weaning practices have yet to be identified. Overall purpose of the study: To identify which weaning interventions and management practices have the greatest potential to improve foal welfare based on physiological and behavioural outcomes. Research qu...
Dental disorders are associated with minimal synovial changes in the temporomandibular joint in horses. Dental disorders have been proposed as factors contributing to temporomandibular joint (TMJ) discomfort in horses, but objective evidence linking these conditions to synovial fluid alterations remains limited. This study evaluated whether the severity of dental abnormalities influences the physicochemical and cytological characteristics of TMJ synovial fluid and whether occlusal adjustment induces measurable synovial changes over time. Unassigned: Thirty-three horses were assigned to mild, moderate, or severe dental abnormality groups. Synovial fluid was collected bilaterally from the discotem...
Physiological Evaluation of Quarter Horses Undergoing Five-Month Taming Focused on Hippotherapy. Preparing horses for hippotherapy programs requires management approaches that prioritize animal welfare and facilitate adaptation to novel stimuli. This study monitored physiological responses during the implementation of a progressive taming protocol for Quarter Horses intended for hippotherapy. Seven animals were monitored over a five-month period, including an acclimation phase and three taming stages. Continuous heart rate, plasma cortisol concentration, and ocular infrared thermography were assessed before and after exercise. Baseline ( = 0.0466) and minimum ( = 0.0095) heart rate values...
The progression values of heart rate and biochemical plasma parameters during training of 3-years old warmblood stallions for breeding licensing. Physiological adaptation to training in young sport horses remains incompletely characterized, particularly during standardized preparation for breeding licensing. Objective: The aim of this study was to evaluate cardiovascular and plasma biochemical responses to acute exercise and long-term training in three-year-old Czech Warmblood stallions during standardized free jumping exercise. Methods: Thirteen Czech Warmblood stallions were monitored over a 112-day training period. Heart rate parameters (minimum, maximum, and mean) were recorded during standardized free jumping tests. Plasma biochemi...
Feasibility and safety of repeated 23% hypertonic saline boluses for electrical impedance tomography contrast enhancement in anaesthetised horses. Hypertonic saline (HS) has been used to enhance electrical impedance tomography (EIT) cardiac-related signals associated with pulmonary perfusion in other species, but its suitability and physiological effects in horses are unknown. This exploratory study aimed to determine whether intravenous administration of 100 mL of 23% HS produces a detectable change in thoracic impedance in anaesthetised horses, and to evaluate the physiological effects of repeated bolus administration. Six adult horses were anaesthetised twice using total intravenous anaesthesia. Hypertonic saline 23% was administered ...
Exploration of the relationship between laboratory stress parameters and leukocyte coping capacity in ponies exposed to repetitive handling during a research project. Stress is an umbrella term indicating increased allostatic load with the potential to reach a pathological state of the organism. In this context, cortisol concentration and neutrophil granulocytes-to-lymphocyte ratio (NEU: LYM) are expected to increase. On the functional pathway, stress reduces the oxidative burst capacity of neutrophils and therefore, the leucocyte coping capacity (LCC) quantifying this oxygen radical generation of neutrophils. Therefore, we hypothesized that in ponies subjected to repetitive handling, LCC will decrease and will be negatively correlated with cortisol concent...
Horsepower: equine-assisted interventions are associated with reduced PTSD symptoms, improved sleep quality, and quality of life in military veterans with treatment-resistant PTSD. Military veterans are at an elevated risk of developing posttraumatic stress disorder (PTSD), while first-line trauma-focused treatments are less effective in this population. This highlights the need for novel, evidence-based complementary interventions for military veterans with treatment-resistant PTSD. Although equine-assisted interventions (EAIs) are increasingly popular and appear promising, many studies suffer from methodological issues. Therefore, further research incorporating both psychometric and physiological parameters is warranted. Unassigned: Nine consecutive groups of eight vet...
Estrous cycle-dependent changes in platelet indices in healthy mares. Platelets (PLTs) are key regulators of hemostasis, inflammation, and tissue remodeling, and their phenotype is known to fluctuate with sex steroid concentrations in several species. However, the influence of the estrous cycle on PLT characteristics in mares has not been investigated. We hypothesized that PLT indices vary between reproductive phases in association with changes in estradiol-17β (E2) and progesterone (P4). The aim of this study was to characterize PLT indices in estrus and diestrus and to determine how these hematological parameters relate to circulating E2 and P4 concentrations...
Evidence of Physiological Comodulation During Human-Animal Interaction: A Systematic Review. This review examines the evidence in the literature for physiological co-modulation during human-animal interaction, identifying studies that assessed comodulation via simultaneous measurement of physiological signals in both species, performing quantitative comparisons. We searched (last search: August 5, 2025) PubMed, EMBASE, Scopus, Google Scholar, Animal Studies Repository, Cochrane, and the Consensus App academic search engine. Risk of bias was assessed using an adapted version of the ROBINS-I V2 tool. The results, grouped by data analysis method, interaction context, and physiological pa...
Physiological determinants of plasma biochemical parameters in horses: a structured review. The biochemical profile of horses is a cornerstone in equine clinical diagnostics, offering valuable insights into health status, disease progression, and physiological adaptation to internal and external stimuli. However, the interpretation of these biochemical parameters is far from straightforward, as they are subject to significant variation influenced by a range of physiological factors. Age, sex, breed, level of physical activity, nutritional condition, and reproductive status can all modulate baseline biochemical values, potentially leading to diagnostic inaccuracies if not properly con...
Annual Variation in Stallion Testicular Measurements and Its Association With Hormonal Profiles and Semen Quality. In a stallion breeding-soundness examination, testicular volume may be estimated either from total scrotal width or by measuring the height, width and length of both testes. This study aimed to determine whether testicular morphometric parameters obtained by ultrasonography fluctuate throughout the year, and whether these variations affect semen quality and hormone production in stallions located at low latitudes in the Southern Hemisphere. Eight healthy light-breed stallions were evaluated every 15 days for 1 year, encompassing the breeding (October-April) and non-breeding(May-September) ...
Renal Perfusion in Healthy Horses: A Feasibility Study With Contrast-Enhanced Ultrasound. This study assessed the feasibility of contrast-enhanced ultrasound (CEUS) for evaluating renal perfusion in horses and compared perfusion parameters between age groups. In a prospective study, eight healthy horses were divided into young (3-5 years, n = 4) and old (16-17 years, n = 4) groups. All animals underwent B-mode, Doppler ultrasonography, and CEUS of the right kidney after an intravenous bolus of SonoVue (0.02 mL/kg). Qualitative analysis assessed contrast kinetics, whereas quantitative parameters (peak intensity, time to peak [TP], area under the curve, and wash-out time) were o...
Thoracic electrical impedance tomography using a two-plane electrode configuration in horses. Traditional one-plane (oneP) electrical impedance tomography (EIT) electrode configurations limit evaluation to a convex lens-shaped lung slice, prone to overrepresentation of central areas. Two-plane (twoP) EIT is expected to refine this technique and result in more representative lung imaging. Objective: To compare ventilation distribution using single slice reconstructions, obtained with twoP or oneP. Methods: Computational modelling and in vivo exploratory randomised experimental trial. Methods: A finite element simulation model of oneP and twoP was calculated to estimate captured lung fie...
