Equine obesity is an increasingly common problem associated with severe consequences. Research is developing quickly and data collected by Equine Veterinary Journal (EVJ) shows that work relating to equine obesity is widely read and cited, highlighting clinical and research interest in this area. This virtual issue brings together selected articles published recently in EVJ that have made important contributions to our understanding of this important welfare concern.

     Obesity in equids is considered to involve one or more of generalized or localized excessive adipose tissue and a predisposition to weight gain or resistance to weight loss; common criteria to define obesity has often been a body condition score (BCS) ≥7/9 to correspond with a total body fat percentage of around ≥20%. While obesity is common in equids but the prevalence varies in different populations. The results of a large owner questionnaire survey included in this virtual issue indicated that 31% of horses/ponies were obese based on owner assessment with a higher prevalence of obesity amongst native breeds and those used for pleasure riding or that were left unridden. Owners' recognition of obesity may however be limited, potentially underestimating the true prevalence.

     A range of clinical conditions may be associated with equine obesity but the most serious and best studied is the increased risk of endocrinopathic laminitis associated with insulin dysregulation (ID). Insulin dysregulation comprises hyperinsulinemia (basal or in response to carbohydrate challenge) and/or tissue insulin insensitivity and is identified relatively frequently with obesity. 

       Whilst the mechanical effects of excessive weight are undoubtedly unhelpful to the hoof in the presence of trigger factors for laminitis it is the direct effect of insulin on the laminae that appears fundamental to the pathogenesis of endocrinopathic laminitis. The pathogenesis of laminitis remains elusive. Rather than a ‘unifying theory’ by which diverse predisposing factors converge on a common pathway, there are considered to be different ‘forms’ of laminitis with different pathogeneses: endocrinopathic, mechanical, and inflammatory. The study by Burns identified the important differences between inflammatory laminitis that may be induced with massive carbohydrate overload and the more prolonged moderate carbohydrate exposure of pasture‐associated laminitis and supports the hypothesis that pasture‐associated laminitis is endocrinopathic rather than inflammatory.

     The question is pertinent in equids and particularly ponies in which weight gain during the summer is a normal and necessary adaptation to seasonal changes in nutrition and climate. The question is complex, how they can be assessed, and potential breed, seasonal, and age effects. Several publications indicate that obesity alone is not always associated with metabolic dysfunction nor laminitis. Counter‐intuitively with some previous studies, obesity was not associated with reduced insulin sensitivity and tissue insulin sensitivity increased in horses that received the high-glycemic meal. An association between high-glycemic diets and increased insulin sensitivity was also reported by other studies that are included in the current virtual issue. 

     The relevance to the development of laminitis of insulin dysregulation and dysfunctional fat rather than obesity per se was illustrated in a cohort study of ponies in Southern England. Metabolic and morphometric features of ponies were recorded and the ponies were followed for the development of laminitis over the following three years. There were no significant differences in indicators of adiposity such as BCS and crest thickness between ponies that did and did not develop laminitis, indeed median BCS was notably high (8/9) in ponies that remained free of laminitis for three years. However, metabolic markers associated with dysfunctional adipose tissue: basal insulin, insulin post dexamethasone, and total adiponectin were significant factors in a multivariable model to predict the development of clinical laminitis.

     Diagnostic testing for insulin dysregulation is an important challenge for practitioners. When undertaking such tests, consideration of the specific question each test is addressing, and the clinical value of the results are important.

     Insulin and glucose dynamics can be broken down into several distinct but related components with a plethora of available diagnostic tests. From a practical perspective, the two components of insulin dysregulation are hyperinsulinemia and tissue insulin resistance.  Whilst these biological features are related, their relationship may vary depending on factors including insulin clearance, gastric emptying, incretin hormones, and β‐cell function.

     Intravenous tests have been considered the ‘gold standard’ for assessment of both pancreatic response and tissue insulin sensitivity. This virtual issue includes an assessment of the repeatability of the hyperglycaemic clamp that will be of interest to researchers. However, from a clinical perspective, it is the interaction of the individual and its diet that appears crucial in pasture‐associated laminitis and particularly of hyperinsulinemia. Diagnostic tests that focus on this are the most practical and clinically applicable.

     The most straightforward diagnostic test is the assessment of basal hyperinsulinemia. Fasting in preparation for this test is no longer recommended, although high carbohydrate hard feed should be withheld. Whilst the measurement of basal insulin concentration is insensitive for the detection of insulin dysregulation it appeared relatively sensitive for the prediction of laminitis in ponies within the following year (78%) and recurrence of laminitis in a grazing season (100%). However, it should be noted that both of these studies used an insulin assay that is now discontinued and assay choice may affect results.

     The oral sugar test is growing in popularity and assesses the horse's response to an oral carbohydrate challenge to mimic the ingestion of high nonstructural carbohydrate pasture. Two studies that report on this test are included in this virtual issue. The oral sugar test had limited repeatability and further analysis of the data indicated that repeatability was lower in insulin dysregulated ponies. The clinical importance of these observations is that in clinical practice, small changes in insulin values should be interpreted with caution.

      Insulin concentration is a continuous variable but clinicians are tempted to seek specific diagnostic thresholds to divide the equine population into dichotomous groups. Under the experimental conditions of a dietary model of endocrinopathic laminitis, the risk of laminitis increased over a range of insulin concentrations following a glucose challenge rather than around a single specific threshold. Insulin dynamics may be influenced by a wide range of factors and recent research included in this virtual issue has focused on the effects of age, breed, and diet. Increased insulin secretory responses to glucose in aged horses may be a particularly important finding when considering the association between pituitary pars intermedia dysfunction (PPID) and laminitis.

     PPID has often been considered a risk factor for laminitis. In an informative study included in this virtual issue, Karikoski measured basal insulin concentrations and examined hoof lamellar histology in cases of PPID with and without clinical evidence of laminitis. The authors hypothesized that PPID would be associated with subclinical lamellar changes even in horses that had not shown clinical signs of laminitis. Histology of PPID cases without basal hyperinsulinemia and with no history of laminitis was normal whereas all cases of PPID with laminitis had basal hyperinsulinemia. The authors concluded that PPID alone may not cause laminitis but that hyperinsulinemia was the key factor. This conclusion is supported by other clinical studies; in one population study of PPID cases, only 13% had a known history of laminitis. In another case series, 32/36 (89%) of laminitis cases were considered endocrinopathic of which 11 had PPID but only one laminitis case had PPID without basal hyperinsulinemia. Any causal relationship between PPID and hyperinsulinemia is currently unclear and requires further research.

      Another area of this current discussion is nature or nurture? Veterinary surgeons may sometimes assume that the owner's management (nurture) alone causes obesity and is the primary driver of ID and associated metabolic traits. However, in a thought‐provoking study, Norton demonstrated high levels of heritability for many metabolic traits associated with equine metabolic syndrome (EMS). Basal insulin in Welsh ponies – a significant predictor of clinical laminitis had a particularly high heritability (h2SNP = 0.8). The importance of genetics has also been indicated in other studies including the heritability of laminitis in a pony herd. Differences in breed susceptibility to EMS are also well-described. These heritability data do not mean that diet and management are unimportant, on the contrary in high-risk individuals management is likely to be crucial. While it is too simplistic to assume EMS could be ‘bred out’, genetic testing could potentially identify individuals at risk of EMS and better inform some breeding decisions.

     The chronicity of the underlying disease in endocrinopathic laminitis is an important factor that may affect management, recurrence, and outcome. In all cases of owner recognized laminitis, it was confirmed by a veterinary surgeon however, the work showed that owners were less skilled in this area – recognizing just 55% of cases. The chronicity of the underlying disease and limited involvement of veterinary surgeons also places an emphasis on the role of the farrier in providing long‐term regular care, also explored in this virtual issue. The research highlighted the unique opportunity afforded to farriers in laminitis prevention and contrasted the holistic approach taken by some farriers with a more task‐based approach of others in which that opportunity could be lost.

      In summary, this collection of papers illustrates that equine obesity is a common and complex problem. From a pathological perspective it is important to look beyond obesity as simply an excess of fat but instead to consider the associated metabolic changes. From a practical perspective, the need for long‐term care is crucial, in part due to genetic predispositions. Many questions remain nor least the elusive pathogenesis of endocrinopathic laminitis and the most effective laminitis treatment, management, and prevention strategies.

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Materials provided by the British Equine Veterinary Association.  Note: Content edited for style and length.

Journal Reference:

1. "Clinical insights: Equine obesity" (04 August 2020) by Edward J. Knowles & Lucy Grieve. Retrieved from https://beva.onlinelibrary.wiley.com/doi/full/10.1111/evj.13307