7 Sept 2015
Nicola Menzies-Gow
Job Title
Figures 2 and 3. A 10-year-old Welsh pony mare with generalised obesity.
Obesity is a medical condition in which excess body fat has accumulated to the extent it may have a negative effect on health, leading to reduced life expectancy and/or increased health problems. Equine obesity is a growing epidemic in the UK and worldwide, and is associated with significant detrimental effects on health.
Conditions associated with obesity in the horse include exercise intolerance, reduced reproductive performance, mesenteric lipomas and hyperthermia. Rather than being an energy storage depot, adipose tissue is endocrinologically active producing a large range of mediators known as adipokines, which have physiological roles. In obesity, there is abnormal production and/or regulation of these adipokines, some of which antagonise the effects of insulin resulting in insulin dysfunction and some of which are pro-inflammatory.
Conditions associated with obesity, insulin resistance and/or inflammation in the horse include hyperlipaemia, osteochondrosis and laminitis. Thus, there is an urgent need to implement dietary and exercise management changes in obese animals to prevent these potentially life-threatening complications.
Obesity is a medical condition in which excess body fat has accumulated to the extent it may have a negative effect on health, leading to reduced life expectancy and/or increased health problems.
It is defined in people using body mass index (BMI) where BMI = weight/(height). A BMI of between 25 and 29.9 is indicative of being overweight; a BMI of 30 to 39.9 is considered obese; while a BMI of more than 40 is considered severely obese.
Obesity is also defined using waist circumference; a person is obese if the waist circumference is more than 94cm in men and more than 80cm in women.
BMI is not useful in horses as most owners do not have accurate weigh scales. Instead, obesity is recognised using body condition scoring (BCS) or crest neck scoring (CNS) systems. BCS can be quantified using one to five (Table 1) or one to nine scales, which emphasise general adiposity; while CNS uses a one to five scale and emphasises regional adiposity (Table 2; Figure 1). A BCS of greater than 4/5 or greater than 7/9, or a CNS of greater than 3/5, are considered obese1. However, both scoring systems rely on subjective assessments of adiposity.
A more objective method for assessing body fat known as the body condition index (BCI) based on neck circumference, heart girth and belly girth measurements is under development (S Bailey, personal communication).
Equine obesity is a growing epidemic in the UK and worldwide. In a study of Scottish riding establishments, 45% of animals were fat or very fat2 and in a study conducted in the south-east, 16% of horses and 83% of ponies were overweight (BCS greater than 4/5; Figures 2 and 3)3. Obesity is more common in certain breeds, including draught type, cob type, native and Welsh, in animals described as good doers, in animals used for pleasure or not ridden, and in summer compared to winter4. In addition, obesity is frequently under-recognised by owners5.
In humans, obesity is associated with an increased risk of a wide range of diseases, including cardiovascular diseases such as hypertension, stroke and coronary heart disease, type-two diabetes, gall bladder disease, cancer, gout, respiratory conditions, osteoarthritis and infertility6-8. In horses, obesity also has wide-ranging health implications. It is associated with reduced reproductive performance9, hyperlipaemia10, reduced athletic performance2, mesenteric strangulating lipomas and developmental orthopaedic disease11 and is a risk factor for the development of laminitis12.
Equine obesity is a widespread problem with significant health implications that needs urgent attention.
As a herbivorous species, horses have evolved to rely on grass. Therefore, during summer and autumn they naturally gain adiposity in response to increased proopiomelanocortin secretion by the pituitary pars intermedia, which stimulates appetite and adipogenesis. These changes represent a critical survival mechanism that ensures stored energy in the form of body fat is available throughout the winter months. Normally, the period of food scarcity is finite and the acquired fat stores are depleted just prior to the onset of spring and resumption of grass growth. Thus, in the wild, acquisition of adipose tissue in summer and autumn is essential for winter survival.
The modern husbandry practices of providing excessive calories, physical inactivity, stabling and the use of rugs to help maintain body temperature year round all promote fat storage and/or decrease the use of stored fat. Consequently, the acquisition of excessive adiposity and its chronic persistence have adverse effects on health.
A number of disorders are associated with obesity in the horse, including:
Rather than merely being an energy storage depot, adipose tissue (namely the adipocytes) plays a role in normal autocrine, paracrine and/or endocrine function, regulating metabolism by secreting various hormones and protein factors, collectively known as adipocytokines or adipokines13.
Obesity is associated with enlarged adipocytes and once these reach their maximal storage capacity, adipose dysregulation is initiated and events such as energy failure, inflammation and cellular stress occur14. Abnormal production and/or regulation of these adipokines is also seen, some of which antagonise the effects of insulin, alter insulin signalling, induce endothelial dysfunction and are pro-inflammatory, creating a state of mild chronic inflammation15.
In humans, these pathological changes are implicated in the development of a variety of pathological conditions including metabolic syndrome, atherosclerosis, heart disease and type-two diabetes mellitus.
In the horse, hyperlipaemia and osteochondrosis have been linked with obesity-associated insulin dysfunction, while obesity-associated insulin dysfunction, endothelial dysfunction and inflammation are implicated in the pathogenesis of laminitis (Figure 4).
Hyperlipaemia results from excessive mobilisation of lipid from adipose tissue in times of stress and increased energy demands, such as starvation, parturition, lactation and systemic illness.
The consequences of increased circulating plasma triglyceride concentrations include circulatory disturbances and organ failure secondary to fat infiltration, particularly affecting the liver and kidneys. The risk of developing hyperlipaemia is increased in ponies, donkeys and miniature horses and obese individuals10.
Large foals growing rapidly are considered to be at increased risk of developmental orthopaedic disease (DOD). A multifactorial problem, DOD includes osteochondritis dissecans (OCD), epiphysitis, flexural and angular limb deformities and, perhaps, wobbler syndrome. Of these, OCD is probably the biggest problem in the equine industry in terms of lost revenue. Genetics, nutrition and exercise all play a role in the incidence of DOD in horses. Trauma due to excessive concussion due to obesity may increase the incidence of OCD11. In addition, a correlation between reduced insulin sensitivity (high glucose and insulin concentrations after feeding a meal of concentrates) and OCD in foals that are genetically predisposed to the disease has been reported16.
An association between laminitis and obesity has been demonstrated in some studies. For example, in a UK study 83% of laminitis cases were overweight or obese17. Those animals at greatest risk of laminitis have a metabolic phenotype including obesity and insulin resistance (IR) and exacerbation of IR and hyperinsulinaemia may precede the onset of clinical laminitis in susceptible animals18. However, the insulin-dependent insulin transporter GLUT-4 was not detected in laminar tissue and glucose uptake in lamellar explants was independent of insulin19. Thus, instead of a direct toxic effect of insulin and/or glucose, it has been suggested the same pathologic mechanisms that underlie the cardiovascular diseases associated with obesity and metabolic syndrome in humans, including changes in insulin signalling, inflammatory cytokines and endothelial dysfunction, could contribute to laminitis.
Equine obesity is a growing epidemic and a number of potentially life-threatening conditions are associated with obesity in the horse. There is an urgent need for increased owner awareness of this problem and its potential consequences through client education by the veterinary profession.
