Body composition of the horse

Morphometric Measurements and Muscle Atrophy Scoring as a Tool to Predict Body Weight and Condition of Horses

Accurate estimation of body weight (BW) and condition (BCS) is important in the equine practice. The main goal of this research was to develop models for the prediction of BW and BCS of horses in the practice using both common morphometric measurements and measurements of Cresty Neck Score (CNS) and Muscle Atrophy Scoring System (MASS) as a measure of muscularity. Our model showed that the BW of horses could be predicted with high reproducibility (concordance correlation coefficient = 0.97), accuracy (0.99), and precision (0.97) using the morphometric measurements of the height at withers, circumference of the chest, cane circumference, body length, and body circumference as well as the BCS, CNS, and muscle atrophy score of the hindlimbs. The stepwise multiple regression analysis revealed that the BCS of horses can be predicted with the data of parameters such as age, body length and an index consisting of measurements of the body circumference to height of withers, and the atrophy of the neck. Future research should use larger cohorts of animals to validate the findings of this study.

Body Condition Score in Danish Horses Related to Type, Use, and Training Level: Patterns, Risk, and Protective Factors

Body condition in horses is a growing concern that has different patterns of development in horses that are above and below the ideal range. This study used professional and para-professional evaluators (veterinarians, farriers, trainers, Danish Equestrian Federation (DEF) officials) who were trained and certified in the use of a modified Henneke scoring system to assign a body condition score (BCS) on a scale of 1–9. Scores of 5–6 are regarded as ideal, and 78.6% of the evaluated horses were in these groups. Only 4.8% of horses were below ideal BCS but 16.5% were above ideal BCS, and this was influenced by type, age, and training. A significant protective effect towards above ideal BCS was shown for horses trained at higher intensities. Cold-blooded horses and traditional ponies had increased risk for being above ideal BCS. Although BCS increased with age, a large proportion of geriatric horses were both above and below ideal BCS. Discipline was not related to BCS. Patterns of BCS distribution for horses attended by different professionals were investigated. Veterinarians attended more horses with BCS above and below ideal values, farriers mostly saw horses that were above ideal BCS, and officials at competitions mainly saw horses with ideal BCS.

Impacts of Adiposity on Exercise Performance in Horses

There is ample research describing the increased risk of health concerns associated with equine obesity, including insulin dysregulation and laminitis. For athletes, the negative effect of weight carriage is well documented in racing thoroughbreds (i.e., handicapping with weight) and rider weight has been shown to impact the workload of ridden horses and to some degree their gait and movement. In many groups of competitive and athletic horses and ponies, obesity is still relatively common. Therefore, these animals not only are at risk of metabolic disease, but also must perform at a higher workload due to the weight of their adipose tissue. Excess body weight has been documented to affect gait quality, cause heat stress and is expected to hasten the incidence of arthritis development. Meanwhile, many equine event judges appear to favor the look of adiposity in competitive animals. This potentially rewards horses and ponies that are at higher risk of disease and reinforces the owner's decisions to keep their animals fat. This is a welfare concern for these animals and is of grave concern for the equine industry.

Adiposity, reproductive and metabolic health, and activity levels in zoo Asian elephant (Elephas maximus)

Many captive Asian elephant populations are not self-sustaining, possibly due in part to obesity-related health and reproductive issues. This study investigated relationships between estimated body composition and metabolic function, inflammatory markers, ovarian activity (females only) and physical activity levels in 44 Asian elephants (n=35 females, n=9 males). Deuterium dilution was used to measure total body water from which fat mass (FM) and fat-free mass (FFM) could be derived to estimate body composition. Serum was analyzed for progestagens and estradiol (females only), deuterium, glucose, insulin and amyloid A. Physical activity was assessed by an accelerometer placed on the elephant's front leg for at least 2 days. Relative fat mass (RFM) - the amount of fat relative to body mass - was calculated to take differences in body size between elephants into consideration. Body fat percentage ranged from 2.01% to 24.59%. Male elephants were heavier (P=0.043), with more FFM (P=0.049), but not FM (P>0.999), than females. For all elephants, estimated RFM (r=0.45, P=0.004) was positively correlated with insulin. Distance walked was negatively correlated with age (r=-0.46, P=0.007). When adjusted for FFM and age (P<0.001), non-cycling females had less fat compared with cycling females, such that for every 100 kg increase in FM, the odds of cycling were 3 times higher (P<0.001). More work is needed to determine what an unhealthy amount of fat is for elephants; however, our results suggest higher adiposity may contribute to metabolic perturbations.

Pharmacokinetics of a sulfadiazine and trimethoprim suspension in neonatal foals

There is limited investigation of neonatal foal pharmacokinetic parameters for the antimicrobial combination of sulfadiazine (SDZ) and trimethoprim (TMP). Neonatal pharmacokinetic investigation of the sulfadiazine-trimethoprim combination is required to ensure safe and effective utilization in this population. The purpose of this study was to determine the pharmacokinetics of sulfadiazine-trimethoprim in five healthy neonatal foals with oral administration at 24 mg/kg every 12 hr (hrs) for 10 days. Blood samples were collected at serial time points at approximately 72 hr of age (steady-state) and at days 5 and 10 to monitor the influence of age within the neonatal period. Pharmacokinetic parameters were determined using a one-compartment model analysis, and mean ± SD was calculated. Cmax was 37.8 ± 13.4 μg/ml (SDZ) and 1.92 ± 0.25 μg/ml (TMP). Tmax was 1.4 ± 0.6 hr (SDZ) and 1.4 ± 0.4 hr (TMP). Cmin for SDZ and TMP was 16.84 ± 8.46 μg/ml and 0.46 ± 0.24 μg/ml, respectively. Elimination half-life was 10.8 ± 6.1 hr (SDZ) and 6.5 ± 2 hr (TMP). AUC0 → ∞ was 667 ± 424 μg × hr/ml (SDZ) and 21.1 ± 5.3 μg × hr/ml (TMP). Foals remained healthy, and the plasma concentration of sulfadiazine-trimethoprim reached levels above MIC(90) for Streptococcus equi ssp. (SDZ/TMP): 9.5/0.5 μg/ml).

The heart remains the core: cardiac causes of poor performance in horses compared to human athletes

Cardiac remodelling occurs in response to exercise and is generally beneficial for athletic performance due to the increase in cardiac output. However, this remodelling also may lead to an increased prevalence of cardiac murmurs and arrhythmias. In most cases, these are not considered to be significant. However, in some cases, there may be potentially deleterious consequences. Whilst sudden cardiac death (SCD) is a rare occurrence, the consequences are catastrophic for both the horse and potentially the rider or driver. Furthermore, the sudden death of a horse in the public arena has negative connotations in regards to public perception of welfare during equestrian sports. Prediction of which individuals might be susceptible to potential deleterious effects of exercise is a focus of interest in both human and equine athletes but remains a challenge because many athletes experience cardiac murmurs and exercise-induced arrhythmias that are clinically irrelevant. This review summarises the effects of exercise on cardiac remodelling in the horse and the potential effects on athletic performance and SCD. The use of biomarkers and their future potential in the management of athletic horses is also reviewed.

Presumptive Iatrogenic Microcystin-Associated Liver Failure and Encephalopathy in a Holsteiner Gelding

An 8‐year‐old Holsteiner gelding was presented for evaluation of anorexia, obtundation, icterus, and mild colic signs of 48 hours duration. History, physical examination, and initial diagnostics were suggestive of hepatic disease and encephalopathy. Microcystin toxicosis was suspected based on historical administration of a cyanobacteria supplement, associated serum biochemistry abnormalities, and characteristic histopathological changes. Microcystin contamination was confirmed in both supplement containers fed to the horse. Fulminant hepatic failure and encephalopathy progressed resulting in euthanasia. Necropsy findings were consistent with microcystin induced liver failure.

Preliminary investigation into a potential role for myostatin and its receptor (ActRIIB) in lean and obese horses and ponies

Obesity is a widespread problem across the leisure population of horses and ponies in industrialised nations. Skeletal muscle is a major contributor to whole body resting energy requirements and communicates with other tissues through the secretion of myokines into the circulation. Myostatin, a myokine and negative regulator of skeletal muscle mass, has been implicated in obesity development in other species. This study evaluated gene and protein expression of myostatin and its receptor, ActRIIB in adipose tissues and skeletal muscles and serum myostatin concentrations in six lean and six obese animals to explore putative associations between these factors and obesity in horses and ponies. Myostatin mRNA expression was increased while ActRIIB mRNA was decreased in skeletal muscles of obese animals but these differences were absent at the protein level. Myostatin mRNA was increased in crest fat of obese animals but neither myostatin nor ActRIIB proteins were detected in this tissue. Mean circulating myostatin concentrations were significantly higher in obese than in lean groups; 4.98 ng/ml (±2.71) and 9.00 ng/ml (±2.04) for the lean and obese groups, respectively. In addition, there was a significant positive association between these levels and myostatin gene expression in skeletal muscles (average R2 = 0.58; p<0.05). Together, these results provide further basis for the speculation that myostatin and its receptor may play a role in obesity in horses and ponies.

Pharmacokinetics of metronidazole in foals: influence of age within the neonatal period

Neonatal foals have unique pharmacokinetics, which may lead to accumulation of certain drugs when adult horse dosage regimens are used. Given its lipophilic nature and requirement for hepatic metabolism, metronidazole may be one of these drugs. The purpose of this study was to determine the pharmacokinetic profiles of metronidazole in twelve healthy foals at 1-2.5 days of age when administered as a single intravenous (IV) and intragastric (IG) dose of 15 mg/kg. Foals in the intravenous group were studied a second time at 10-12 days of age to evaluate the influence of age on pharmacokinetics within the neonatal period. Blood samples were collected at serial time points after metronidazole administration. Metronidazole concentration in plasma was measured using LC-MS. Pharmacokinetic parameters were determined using noncompartmental analysis and compared between age groups. At 1-2.5 days of age, the mean peak plasma concentration after IV infusion was 18.79 ± 1.46 μg/mL, elimination half-life was 11.8 ± 1.77 h, clearance was 0.84 ± 0.13 mL/min/kg and the volume of distribution (steady-state) was 0.87 ± 0.07 L/kg. At 10-12 days of age, the mean peak plasma concentration after IV infusion was 18.17 ± 1.42 μg/mL, elimination half-life was 9.07 ± 2.84 h, clearance was 1.14 ± 0.21 mL/min/kg and the volume of distribution (steady-state) was 0.88 ± 0.06 L/kg. Oral approximated bioavailability was 100%. Cmax and Tmax after oral dosing were 14.85 ± 0.54 μg/mL and 1.75 (1-4) h, respectively. The elimination half-life was longer and clearance was reduced in neonatal foals at 1-2.5 days as compared to 10-12 days of age (P = 0.006, P = 0.001, respectively). This study warrants consideration for altered dosing recommendations in foals, especially a longer interval (12 h).

Obesity prevalence and associated risk factors in outdoor living domestic horses and ponies

Reasons for performing study. The prevalence of obesity in companion animals, including horses and ponies has risen drastically in recent years and risk factors have been little investigated. Horses are unique amongst companion animals in that many are outdoor-living and forage independently on pasture; they also have a dual utility and companionship role. The body condition of wild and free-living equines is known to vary seasonally, yet previous estimates of the prevalence of obesity and associated risk factors in domestic animals do not consider this. Most previous studies were conducted during the summer months when pasture quality is greater and obesity prevalence is likely to be highest. In addition, many previous estimates do not use validated body condition scoring methods and rely on owner reporting.

Objectives. To examine the prevalence and risk factors predictive of equine obesity at both the end of winter and the end of summer, in a domestic population of leisure horses with daily access to pasture. Using validated body condition scoring methods and a single, trained observer.

Methods. Body condition and belly girth measurements were taken at the end of winter and during the summer in a population of leisure horses (n = 96) with outdoor pasture access for ≥6 h per day. Risk factor information was obtained by two owner questionnaires and analysed statistically using a mixed effects logistic regression model. The dependent variable was obese (BCS ≥ 7/9) or non-obese (BCS < 7/9). Risk factors associated with seasonal change in belly girth were also explored using a mixed effects linear regression model.

Results. Obesity prevalence rose significantly from 27.08% at the end of winter to 35.41% during summer (p < 0.001). Breed was the risk factor most strongly associated with obesity (p < 0.001). Supplementary feed was not a strong predictor and there was no association with low intensity structured exercise. As winter BCS increased, the percentage seasonal change in belly girth decreased.

Conclusions. Obesity prevalence differed between winter and summer in domestic equines. Supplementary feed and low intensity structured exercise in equines living outdoors for ≥6 h per day had limited or no effect on obesity levels. Seasonal variation in body condition was lower in obese equines.

Potential relevance. It is important to consider season when studying equine obesity and obesity-associated disorders. Risk factor analysis suggests preventative measures may need to be breed specific. The metabolic implications of a lessened seasonal change in body condition in obese animals, warrants investigation.

Highly athletic terrestrial mammals: horses and dogs

Evolutionary forces drive beneficial adaptations in response to a complex array of environmental conditions. In contrast, over several millennia, humans have been so enamored by the running/athletic prowess of horses and dogs that they have sculpted their anatomy and physiology based solely upon running speed. Thus, through hundreds of generations, those structural and functional traits crucial for running fast have been optimized. Central among these traits is the capacity to uptake, transport and utilize oxygen at spectacular rates. Moreover, the coupling of the key systems--pulmonary-cardiovascular-muscular is so exquisitely tuned in horses and dogs that oxygen uptake response kinetics evidence little inertia as the animal transitions from rest to exercise. These fast oxygen uptake kinetics minimize Intramyocyte perturbations that can limit exercise tolerance. For the physiologist, study of horses and dogs allows investigation not only of a broader range of oxidative function than available in humans, but explores the very limits of mammalian biological adaptability. Specifically, the unparalleled equine cardiovascular and muscular systems can transport and utilize more oxygen than the lungs can supply. Two consequences of this situation, particularly in the horse, are profound exercise-induced arterial hypoxemia and hypercapnia as well as structural failure of the delicate blood-gas barrier causing pulmonary hemorrhage and, in the extreme, overt epistaxis. This chapter compares and contrasts horses and dogs with humans with respect to the structural and functional features that enable these extraordinary mammals to support their prodigious oxidative and therefore athletic capabilities.

Effects of hypercapnic hyperpnea on recovery from isoflurane or sevoflurane anesthesia in horses

OBJECTIVE

To test the hypothesis that hypercapnic hyperpnea produced using endotracheal insufflation with 5-10% CO(2) in oxygen could be used to shorten anesthetic recovery time in horses, and that recovery from sevoflurane would be faster than from isoflurane.

STUDY DESIGN

Randomized crossover study design.

ANIMALS

Eight healthy adult horses.

METHODS

After 2 hours' administration of constant 1.2 times MAC isoflurane or sevoflurane, horses were disconnected from the anesthetic circuit and administered 0, 5, or 10% CO(2) in balance O(2) via endotracheal tube insufflation. End-tidal gas samples were collected to measure anesthetic washout kinetics, and arterial and venous blood samples were collected to measure respiratory gas partial pressures. Horses recovered in padded stalls without assistance, and each recovery was videotaped and evaluated by reviewers who were blinded to the anesthetic agent and insufflation treatment used.

RESULTS

Compared to isoflurane, sevoflurane caused greater hypoventilation and was associated with longer times until standing recovery. CO(2) insufflation significantly decreased anesthetic recovery time compared to insufflation with O(2) alone without significantly increasing PaCO(2) . Pharmacokinetic parameters during recovery from isoflurane with CO(2) insufflation were statistically indistinguishable from sevoflurane recovery without CO(2). Neither anesthetic agent nor insufflation treatment affected recovery quality from anesthesia.

CONCLUSIONS AND CLINICAL RELEVANCE

Hypercapnic hyperpnea decreases time to standing without influencing anesthetic recovery quality. Although the lower blood gas solubility of sevoflurane should favor a shorter recovery time compared to isoflurane, this advantage is negated by the greater respiratory depression from sevoflurane in horses.

Intravenous technetium-99m labelled PEG-liposomes in horses: a safety and biodistribution study

REASONS FOR PERFORMING STUDY

Liposomes are phospholipid nanoparticles that extravasate at sites of increased vascular permeability. They have potential in equine medicine for targeted drug delivery and diagnostic imaging of infectious, inflammatory and neoplastic lesions.

OBJECTIVES

This study evaluates the safety and biodistribution of i.v. polyethyleneglycol(PEG) liposomes in normal horses.

METHODS

PEG-liposomes were prepared by the film hydration method and labelled using (99m) Tc-hexamethyl-propylene-amine-oxime. A single dose of 0.24 µmol/kg bwt (99m) Tc-PEG-liposomes and 2.4 µmol/kg bwt unlabelled PEG-liposomes was administered to 10 conscious horses via i.v. infusion at a rate of 6 µmol/min for the first 15 min and 60 µmol/min thereafter. Clinical parameters, haematology, plasma biochemistry and serum complement activity were monitored serially. Scintigraphic imaging was performed at 1, 12 and 21 h post infusion (PI). Six horses were subjected to euthanasia at 24 h PI. The percentage injected dose per kilogram of tissue was calculated for multiple organs. Results were analysed using repeated measures ANOVA.

RESULTS

Horses did not demonstrate adverse reactions during or after liposome infusion. There was a significant elevation in heart rate and respiratory rate at 20 and 25 min PI. No significant complement consumption was detected, although a trend for decreased total haemolytic complement values at 20 min PI was present. Scintigraphic studies revealed a prolonged vascular phase that lasted to 21 h PI, with a reproducible pattern of organ distribution. Biodistribution studies revealed the highest concentrations of radiopharmaceutical within the lung, kidney, liver and spleen.

CONCLUSIONS

Intravenous liposome administration appears to be safe in horses. When administered in combination with PEG-liposomes, (99m) Tc-PEG-liposomes have long circulating characteristics and a reproducible pattern of organ distribution in horses.

POTENTIAL RELEVANCE

Radiolabelled liposomes may be useful for detecting infection, inflammation and neoplasia in the horse. Liposomes have significant potential for targeted drug delivery in the horse. This study establishes the scintigraphic findings and tissue distribution of 99mTc-PEG-liposomes after i.v. administration in healthy horses.

Assessment of body fat in the pony: part I. Relationships between the anatomical distribution of adipose tissue, body composition and body condition

REASONS FOR PERFORMING STUDY

Evaluation of equine body fat content is important for nutritional and clinical purposes. However, our understanding of total body fat and its regional distribution in the body is sparse. Currently, body fat evaluation relies on the subjective assessment of body condition score (BCS), which has never been validated against 'gold standard' chemical analysis or dissection measurements in ponies.

OBJECTIVES

To define the relationships between subjective (BCS), objective (morphometric) indices of body fat and 'gold standard' measurements of actual body composition.

HYPOTHESES

BCS and morphometry offer valid, noninvasive methods for determination of body fat in equids.

METHODS

Seven mature (mean ± s.e. 13 ± 3 years, 212 ± 14 kg, BCS 1.25-7/9), Welsh Mountain pony mares, destined for euthanasia (for nonresearch purposes), were used. For all ponies, body mass (BM), BCS and various morphometric measurements were recorded. Following euthanasia, all ponies were systematically dissected. Discrete white adipose tissue (WAT) depots were independently described. Gross, body chemical composition was determined by proximate analyses.

RESULTS

Total somatic soft tissues increased linearly (r(2) = 1.00), whereas body WAT content (1-26% live BM) increased exponentially (r(2) = 0.96), with BCS. WAT was equally distributed between internal and external sites in all animals irrespective of BCS. Nuchal fat was a poor predictor of total WAT (r(2) = 0.66). Periorbital WAT did not alter with BCS (r(2) = 0.01). Heart girth:withers height and ultrasonic retroperitoneal fat depth were closely associated with total, chemically-extracted lipid which comprised 1-29% live BM (r(2) = 0.91 and 0.88, respectively).

CONCLUSIONS AND POTENTIAL RELEVANCE

The exponential relationship between BCS and total body WAT/lipid suggests that BCS is unlikely to be a sensitive index of body fat for animals in moderate-obese states. Morphometric measurements (body girths and retroperitonel fat depth) may be useful to augment subjective BCS systems.

Effects of season and body condition on appetite, body mass and body composition in ad libitum fed pony mares

Changes in appetite, body mass (BM), body condition score (BCS), direct (ultrasonographic) and indirect (deuterium oxide dilution technique) measures of body fat were monitored in Welsh Mountain pony mares (n=11, 5-19 years of age) offered ad libitum access to a complete diet (gross energy 16.9±0.07 MJ/kg dry matter) for 12 weeks during summer (n=6; 246±20 kg) and winter (n=5; 219±21 kg). At the outset, each group comprised two thin (BCS 1-3/9), moderate (BCS 4-6/9) and obese (BCS 7-9/9) animals. For ponies that were non-obese at the outset, BM was gained more rapidly (P=0.001) in summer (0.8±0.1 kg/day) than winter (0.6±0.0 kg/day). This was associated with a seasonal increase in dry matter intake (DMI) which became maximal (summer, 4.6±0.3% BM as DMI/day; winter, 3.5±0.1% BM as DMI/day) during the second month. The appetite of the obese ponies was half that reported for non-obese animals in the summer and BM remained constant irrespective of season. Body 'fatness' increased progressively for non-obese but not obese ponies. Body fat content was exponentially associated with increasing BCS but BCSs >6 were not useful indicators of actual body fat. Endogenous circannual mechanisms to suppress winter weight gain were insufficient to prevent the development of obesity in ad libitum fed ponies.

Effect of dietary restriction on body condition, composition and welfare of overweight and obese pony mares

REASONS FOR PERFORMING STUDY

Increased prevalence of obesity among UK horses and ponies demands evidence-based advice to promote weight loss.

HYPOTHESIS

Restriction of dry matter intake (DMI) to 1% of body mass (BM, 67% [corrected] of predicted maintenance digestible energy [DE] requirements) would promote weight loss without compromise to health.

METHODS

Five mature (mean ± s.e. 10 ± 2 years), overweight/obese pony mares (BM, 257 ± 20 kg: body condition score [BCS] 6.8/9 ± 0.5) were studied over 12 weeks. Animals were individually housed. Daily provision of a chaff-based, complete diet (measured DE, 8.5 MJ/kg DM) was restricted to 1% of actual BM as DMI daily. BCS, girth measurements and ultrasound-derived measures of subcutaneous fat depth overlying the gluteal region and 12th intercostal space (rib-eye) were recorded weekly. Body fat content was estimated at the beginning and end of the study by deuterium oxide dilution methods. Clinical biochemistry was monitored weekly. Behaviour was observed (24 h, 3/5 ponies) on 3 occasions.

RESULTS

BM decreased by 4.3 ± 1.1% during the first week and thereafter by 0.7 ± 0.1% of BM at end of Week 1 each week. BCS remained constant. Heart and belly girths, rump width and subcutaneous fat depth at rib-eye decreased significantly with time and BM. Fat comprised 45 ± 19% of BM loss. Fatter animals lost relatively more fat. With decreased feeding activity, time spent in 'play' and rest increased by 36 ± 11% and 438 ± 95%, respectively.

CONCLUSIONS

This plane of nutrition resulted in an overall rate of weight loss of 1% of outset BM weekly. BCS was not a useful index of early weight loss but heart and belly girths and subcutaneous rib-eye fat were identified as alternative markers.

POTENTIAL RELEVANCE

This study provides an evidence-base for the management of weight loss in obese animals, especially those for which exercise may be contra-indicated.

Body composition in young Standardbreds in training: relationships to body condition score, physiological and locomotor variables during exercise

REASONS FOR PERFORMING STUDY

Body composition is an essential factor in athletic performance of human sprinters and long distance runners. However, in horses, many questions remain concerning relationships between body composition and performance in the different equine activities.

OBJECTIVES

To determine relationships between body composition, body score, physiological and locomotor variables in a population of young Standardbreds in training.

METHODS

Twenty-four 2-year-old Standardbreds were studied, body condition on a scale 0-5 and bodyweight recorded, and height at withers measured. Percentage of fat (%F), fat mass (FM) and fat free mass (FFM) were estimated echographically. During a standardised exercise test on the track, velocity, heart rate, respiratory frequency and blood lactate concentrations were measured. V4 and V200 (velocity for a blood lactate concentration of 4 mmol/l and velocity of 200 beats/min) calculated. Basic gait variables were measured at 3 different speeds with an accelerometric device.

RESULTS

Body composition variables: %F and FM were significantly related to body condition score and physiological variables. Body score was highly correlated to %F (r = 0.64) and FM (r = 0.71). V4 was negatively correlated to %F (r = -0.59) and FM (r = -0.60), P<0.05. V200 was also negatively related to %F and FM, (r = -0.39 and r = -0.37, respectively, P<0.1). No relationships were found between body composition and gait characteristics.

CONCLUSIONS

Body composition was closely related to indirect measurements of aerobic capacity, which is a major factor of athletic performance in middle distance running horses.

POTENTIAL RELEVANCE

As in human athletes, trainers should take special note to evaluate optimal bodyweight and body composition of race horses to optimise performance.

Current concepts of oxygen transport during exercise

This brief review examines the athletic potential of mammals in general and the horse in particular as it relates to oxygen (O2) transport and utilization. The horse has been bred selectively for over six millennia based upon its ability to run fast. Whereas this has optimized cardiovascular and muscle function and the capacity to deliver and utilize O2, it has resulted in lung failure during intense exercise. Horses in their athletic prime are considered and attention is focused on their maximal capacities as related to O2transport, irrespective of ageper se. Following a few comments on the history of O2, this review moves from established principles of O2transport at the integrative organ level to the microcirculation and the processes and principles that govern O2offloading, where much remains to be discovered. Four principal questions are addressed: (1) as an athlete, what are the most outstanding physiological characteristics of the horse? (2) what anatomical and physiological capacities facilitate this superlative performance and such prodigious O2fluxes (i.e. maximal VO2)? (3) do cardiovascular dynamics or intramuscular energetic processes limit VO2kinetics (i.e. the speed at which VO2increases at the onset of exercise)? VO2kinetics determine the size of the O2deficit and as such represent an important determinant of muscle metabolism and fatigue; and (4) what determines the efficacy of muscle microcirculatory O2exchange?

Ingestion of starch-rich meals after exercise increases glucose kinetics but fails to enhance muscle glycogen replenishment in horses

Fatiguing exercise substantially decreases muscle glycogen concentration in horses, impairing athletic performance in subsequent exercise bouts. Our objective was to determine the effect of ingestion of starch-rich meals after exercise on whole body glucose kinetics and muscle glycogen replenishment. In a randomized, cross-over study seven horses with exercise-induced muscle glycogen depletion were either not fed for 8 h, fed half of the daily energy requirements ( approximately 15 Mcal DE) as hay, or fed an isocaloric amount of corn 15 min and 4 h after exercise. Starch-rich meals fed after exercise, when compared to feed withholding, resulted in mild to moderate hyperglycemia (5.7+/-0.3 vs. 4.7+/-0.3 mM, P<0.01) and hyperinsulinemia (79.9+/-9.3 vs. 39.0+/-1.9 pM, P<0.001), 3-fold greater whole body glucose kinetics (15.5+/-1.4 vs. 5.3+/-0.4 micromol kg(-1)min(-1), P<0.05), but these only minimally enhanced muscle glycogen replenishment (171+/-19 vs. 170+/-56 and 260+/-45 vs. 294+/-29 mmol/kg dry weight immediately and 24 h after exercise, P>0.05). It is concluded that after substantial exercise-induced muscle glycogen depletion, feeding status only minimally affects net muscle glycogen concentrations after exercise, despite marked differences in soluble carbohydrate ingestion and availability of glucose to skeletal muscle.

Overview of horse body composition and muscle architecture: implications for performance

Locomotion requires skeletal muscle to sustain and generate force. A muscle's force potential is proportional to its weight. Since the larger the muscle the larger its potential power output, a better understanding of the proportion of skeletal muscle a horse possesses may lead to a better understanding of horse performance. Several techniques exist to assess body composition, which include dual energy X-ray absorption, underwater (hydrostatic) weighing, derivation from total body water, bio-electric impedance, air displacement, body condition scoring, cadaver dissection and ultrasound. The relevance of each method to the equine industry will be discussed as will the practical information that the existing horse body composition studies have provided. Attention will be given to the data regarding the implications of body composition on the performance horse. The limited number of studies discussing different varieties of muscle architectures and the functional importance of these muscles will also be addressed. These body composition data may provide a better understanding of important issues in horse care that can lead to more optimal horse care techniques and a healthier and safer environment for horses.

Rate and composition of sweat fluid losses are unaltered by hypohydration during prolonged exercise in horses

Rate and ionic composition of sweat fluid losses and partitioning of evaporative heat loss into respiratory and cutaneous components were determined in six horses during three 15-km phases of exercise at approximately 40% of maximal O2 uptake. Pattern of change in sweat rate (SR) and composition was similar during each phase. SR increased rapidly for the first 20 min of exercise but remained at approximately 24-28 ml . m-2 . min-1 during the remainder of each phase. Similarly, the concentrations of Na and Cl in sweat increased until 30 min of exercise but were unchanged thereafter. Sweat osmolality and concentrations of Na and Cl were positively correlated with SR. Sweat K concentration decreased during exercise but was not correlated with SR. Fluid losses were 33.8 +/- 1.5 liters, resulting in decreases of approximately 21% in plasma volume and approximately 11% in total body water. The approximately 6% hypohydration was not associated with an alteration in SR, sweat composition, or heat storage. Respiratory and cutaneous evaporative heat loss represented approximately 23 and 70%, respectively, of the total heat dissipated, and the partitioning of heat loss was similar in each exercise phase. We conclude that SR and the relative proportions of respiratory and cutaneous evaporative heat loss are unchanged in horses during prolonged low-intensity exercise despite moderate hypohydration.

Age-related storage of iron in the liver of horses

The non-haem iron concentration was estimated in post-mortem liver samples from 51 horses (age range 1-25 years). Two were normal and 49 had been suffering from conditions that were not expected to have had long-term effects on iron metabolism. Muscle samples (splenius and biceps femoris) from 23 of these horses were also analysed. There was a highly significant age-related increase in the non-haem iron concentration in the liver (r = 0.635, p < 0.001), but not in the muscles, in which the iron concentration was much lower than in the liver.

Drug therapy in the neonatal foal

The neonatal period in foals refers to the first 7 days of postnatal life. The effects of drugs (pharmacologic agents) may be different in neonatal foals, particularly during the first 3 days of postnatal life, from those in older foals and adult horses. The changed drug effects decrease as the physiologic processes that affect absorption, distribution, and elimination (metabolism and excretion) of drugs mature. Dosage regimens should take into account the altered pharmacokinetic profiles of drugs, and because of wide individual variation, the response to therapy should be closely monitored for signs of toxicity. In conjunction with the prudent use of drugs, good nursing care and the provision of supportive therapy are critical in the management of neonatal foal diseases. Over-crowding imposes stress upon young foals and predisposes them to an increased incidence of bacterial and parasitic infections. The collection of specimens for precise microbiologic diagnosis and correction of deficits in serum immunoglobulins should precede antimicrobial therapy. Although E. coli is by far the most common cause of bacterial infections in neonatal foals, other bacterial pathogens of unpredictable susceptibility often cause infection. The selection of an antimicrobial drug for specific therapy should be based on both the microbiologic (quantitative susceptibility) and pharmacologic (pharmacokinetic) properties of the drug. The use of an antimicrobial drug or combination of drugs that will produce a bactericidal effect is highly desirable. Whenever possible, a parenteral preparation that can be administered intravenously should be chosen. The bioavailability and selectivity of action of pharmacologic agents are influenced by the dosage form and route of administration. Diazepam is the sedative drug of choice for neonatal foals. Cimetidine, an H2-receptor antagonist, may be indicated in foals diagnosed to have gastric ulcers; hepatic microsomal oxidative metabolism of drugs administered concurrently with cimetidine is decreased. Nonsteroidal anti-inflammatory drugs (flunixin, phenylbutazone) have a higher incidence of toxicity in foals and, when indicated, should be used at lower dosage than in adult horses. Even though it is highly important to maintain hydration status and electrolyte balance, intravenous infusion should always be performed slowly. Immature renal function decreases the ability of the neonatal animal to excrete excess fluid. The use of drugs in neonatal foals requires greater precision in dosage, more attention to the route and rate of administration, and close monitoring of pharmacologic effects.

Management of anesthesia in the foal

Several unique pharmacologic and physiologic factors must be considered when attempting to anesthetize premature or neonatal foals. Juvenile foals are similar to adults in their physiology and metabolism. Anesthetic drug and protocol selection should reflect the differences between these two age groups. Neonates are best anesthetized using an inhalation technique, whereas older foals can be safely anesthetized with either parenteral or inhalation anesthetic agents. Careful monitoring is absolutely essential when anesthetizing foals. The clinician should plan to routinely administer fluids and measure electrolyte levels. A basic plan and knowledge of the methods and agents used to treat hypovolemia, severe cardiopulmonary collapse, and life-threatening arrhythmias can result in survival of many critically ill foals.

Tissue perfusion in anaesthetised sheep

1. Tissue perfusion was measured in seven sheep by the radioactive microsphere method. 2. The sheep were anaesthetised and ventilated to a near normal arterial carbon dioxide tension. 3. Cardiac output was measured utilizing the Fick principle as applied to thermodilution and at the end of the investigation the sheep were killed by an overdose of anaesthetic. 4. The results were compared with previously published values for conscious sheep and anaesthetised horses and found to be similar when known variations in body composition and physiology were considered.

Tissue blood content in anaesthetised sheep and horses

The blood content of tissue in eight anaesthetized sheep and three anaesthetized horses was measured using chromium labelled autologous red blood cells. 2. Approximately half the total blood volume was found to be distributed within the body tissues and, with the exception of the spleen and fat, the distribution was similar in these two species.

External balance of water and electrolytes in the horse

The components of the daily external water and electrolyte balances of five normal Standardbred geldings with a mean body weight of 427 kg, were determined over 14 day periods. The mean value of daily total water intake (a summation of free, combined and metabolic water intake) for the group was 27.6 litres, that is, 64.4 ml/kg body weight, while mean values for ambient temperature and humidity ranged from 16 to 25 degrees C and 55.4 to 75.5 per cent, respectively. The mean daily urine volume of 9.9 litres (23.2 ml/kg) for the group, was found to exceed the mean faecal water output of 7.2 litres (16.9 ml/kg). The mean output of sodium for the five animals, in urine and faeces, was 780 mmol or 79 per cent of the intake. The mean output of potassium, chloride, calcium and magnesium were 3163, 2521, 1824 and 1448 mmol or 95, 84, 74 and 75 per cent, respectively of intake. The sodium and potassium not accounted for in urine and faeces may well have been lost in sweat. Results are discussed and compared with those of a previous external balance study in the horse.

Body condition scoring and weight estimation of horses

Three hundred and seventy two horses of varying breeds, height and fatness were weighed and measured for height at the withers. They were assessed for condition score by adaptation of a previously published method. The heart girth and length of 281 of the horses were also measured. Weight of horses was highly correlated (P less than 0.001) with height (r2 = 0.62), condition score (r2 = 0.22) and girth2 x length (r2 = 0.90). Nomograms were constructed to predict weight from height and condition score, and girth and length measurements. Weight can also be accurately estimated from the formula: (formula, see text) The average value of 'Y' in this experiment was 11900 and this estimated weight with more accuracy than some previously published values of 'Y'. Racing Thoroughbred horses were found to be significantly lighter than non-racing Thoroughbreds of the same height and condition score. The method of assessment of condition score was shown to be repeatable between different operators with varying degrees of experience.

Pharmacologic considerations in drug therapy in foals

Rational drug therapy in the foal requires a sound knowledge of the pharmacodynamics and pharmacokinetics of various drugs as well as a thorough understanding of the physiologic differences that exist between the neonate and the adult and that may serve to alter drug disposition and, therefore, drug response. A summary of these physiologic factors with emphasis on the foal is presented and is followed by recommendations regarding the applied therapeutics of various antimicrobial agents.

Muscular adaptations to exercise and training

This article provides an overview of the characteristics of skeletal muscle, with an emphasis on equine skeletal muscle. A discussion of many of the adaptive processes that can occur in this tissue in response to altered states of physical activity is also included.

Cardiovascular response to exercise and training in the horse

The quality of the overall response to exercise in the horse is very similar to that seen in man and laboratory animals; differences are mainly quantitative and persist when relative body weight is taken into account. The apparently greater flow capacity of the equine muscle bed during maximal whole-body exercise implicates the extent of central circulatory adaptations as the limiting factor in performance but implies a role for increase in arteriolar capacitance/muscle capillarity as an appropriate response to intense endurance training. The blood oxygen-carrying capacity of the horse is often quoted as the major component of the animal's superior aerobic work capacity, although the measured maximal a-vO2 for the horse is only 2 to 3 ml greater than that found in elite athletes. In fact, comparison of published performance data for man and the horse reveals that improved a-vO2 accounts for 23 per cent of the difference, and increased Qc accounts for the remaining 77 per cent of the superior oxygen consumption in the horse. The extent to which the horse can increase Qc and muscle blood flow appears to represent its major adaptations for maximal aerobic performance. It is frequently observed that there have been far greater improvements in human athletic performance than in that of the race-horse, and this difference is usually attributed to the application of scientific training methods to the athlete. It has also been suggested that the horse may have reached the limit of its adaptive ability. The horse has a maximal oxygen pulse of at least 0.6 ml per kg per beat compared with 0.35 for man, a 90 per cent whole body oxygen extraction, and an 80 to 90 per cent higher muscle blood flow, with an overall capability of increasing Vo2 max by 35 times. These represent levels that would appear to be difficult to improve upon. However, insufficient research has been performed to firmly state true maxima for the horse, and current research does not reveal to what extent the horse is capable of responding to even conventional training methods. The relative improvement that such research could reveal would provide some objective guidance to the extent to which further improvement could be anticipated. A consistent finding in the majority of studies reviewed is the tendency for results to show a lack of statistical significance, which is particularly frustrating for a researcher when the trends are consistent with the initial hypotheses. This tendency arises because of small group sizes and inherent variability in the test population.(ABSTRACT TRUNCATED AT 400 WORDS)

Drug disposition in the neonatal animal, with particular reference to the foal

Differences between neonatal and adult animals in their response to drugs can usually be attributed to altered disposition (ie, distribution, metabolism and excretion) processes during the neonatal period. These alterations affect the plasma concentrations as well as the concentrations of drug attained at the receptor site. Some characteristics of the neonatal period include greater absorption from the gastrointestinal tract, lower extent of plasma protein binding, increased apparent volume of distribution of drugs that distribute in extracellular fluid or total body water, increased permeability of the 'blood-brain' barrier and slower elimination of many drugs. The hepatic microsomal oxidative reactions and glucuronide conjugation are deficient metabolic pathways for a varying period of time, usually up to six weeks after birth or even longer in some species. Decreased metabolism can affect the duration of action of lipid-soluble drugs. Functional immaturity of the kidneys decreases the renal excretion of polar drugs and drug metabolites. Overall renal function appears to reach maturity within two weeks after birth in ruminant species and pigs, while maturation may take at least four weeks in other species of domestic animals. Considerable physiological and biochemical development takes place during the first five days after birth with maturation continuing more slowly over the succeeding five weeks. The time it takes for any process to reach functional maturity depends on the process in question and varies with the species of animal. The absorption, disposition and pharmacological response to drugs during the first 24 h after birth may be unique to that time and, because of lack of information, are impossible to predict.(ABSTRACT TRUNCATED AT 250 WORDS)

A "standard horse" for use in physiologically based mathematical modelling

Standard data for the horse which can be used in physiologically based mathematical computer modelling are presented. The data includes figures for tissue mass, density and perfusion, obtained by measurement mainly from horses weighing 200 to 300 kg. Other related parameters such as mean transit times and tissue blood volume have been calculated and included in the actual values listed for a 250 kg horse.

An energetic basis of equine performance

Although different physiological and behavioural attributes are needed for various types of equine competition, successful racing depends primarily on the animal's metabolic ability to convert chemical energy to mechanical energy--the function of muscle. Components of these energetic processes include the rate, efficiency and interaction of aerobic and anaerobic metabolism in muscle and the supply and utilisation of fuel. In anaerobic work like racing, fatigue processes may be largely regarded as a function of an intramuscular fuel (phosphogen) depletion, despite the fact that substrates are supplied via the circulation. Physical work capacity in the horse depends then mainly on the rate of aerobic metabolism and the capacity of the anaerobic processes to supply energy for continued muscle contraction. Underlying these processes are physiological limitations of the cardiovascular system and the ultrastructure and biochemistry of muscle. A model is proposed whereby prediction of equine performance is based entirely on parameters of energy metabolism.

Sir Frederick Smith Memorial Lecture. A superb transport system--the circulation

This paper discusses certain aspects of circulatory physiology, including foetal circulation and the role of the thorax, and gives parameters for total blood volume, cardiac output, stroke volume and the distribution of blood between the pulmonary and peripheral circulation within the horse. The reader is taken on a journey around the circulatory system, beginning in the left atrium, into which richly oxygenated blood runs from the pulmonary veins. From here the blood is squeezed via the mitral valve into the left ventricular pump, where it is forced under high pressure through the peripheral circulation, the arterioles and capillaries, to collect, deoxygenated and under much lower pressure, into the venules and veins. The pumping action of the muscles and the negative intrathoracic pressure help to draw the blood back up to the right atrium, itself exerting a suction effect on the great veins. The blood is then expelled from the right ventricle into the low resistance, high capacitance pulmonary circuit, where it picks up oxygen, releases carbon dioxide and returns to the left atrium.