Researching Laminitis Risks for Horses and Ponies
The RVC has been at the forefront of laminitis research for decades and continue to progress advances in the understanding of and treatment of the debilitating condition.
Laminitis is a common and extremely painful condition of the equine foot and characterised by failure of the attachment of the epidermal cells of the epidermal (insensitive) laminae to the underlying basement membrane of the dermal (sensitive) laminae. It can occur as a single episode of acute disease or, more commonly, as repeated bouts over a prolonged period.
There are three types of laminitis, namely sepsis-associated, endocrinopathic and supporting limb laminitis. Endocrinopathic laminitis is the commonest form of the disease in the UK, accounting for up to 90% of cases. It encompasses laminitis associated with the two common endocrine diseases equine metabolic syndrome (EMS) and pituitary pars intermedia dysfunction (PPID). The central feature of EMS is insulin dysregulation (ID) and additional features include adipose tissue dysregulation (resulting in altered production of adipose-tissue derived hormones known as adipokines, such as adiponectin) and obesity. The increased risk of laminitis in animals with PPID is also associated with ID.
Endocrinopathic laminitis has been associated with a range of risk factors including metabolic, morphometric and management risk factors. However, further study is required to investigate the relative strength of different risk factors and to precise quantification of risk more precisely requires further study. Identification of animals at an increased risk would allow targeted implementation of management changes that may reduce this risk and thus reduce the frequency of occurrence of this painful disease.
Endocrinopathic laminitis has been associated with a range of risk factors including metabolic, morphometric and management risk factors. However, further study is required to investigate the relative strength of different risk factors and to precise quantification of risk more precisely requires further study. Following a recent cohort study of non laminitic ponies in Southern England, we reported a one-year baseline risk for the development of laminitis of 4%. The estimated one-year risk increased to 9.3% in ponies with increased basal serum insulin concentration and to 13.9% in those with low plasma adiponectin concentrations. However, there was no benefit in combining these parameters into a clinical prediction model.
Ponies in that cohort study were only sampled on a single occasion once and management data were not collected. Thus, we undertook a second cohort study in which ponies were visited every six months for up to four years. Physical, metabolic and management data were collected, and ponies were monitored for the development of laminitis.
Metabolic data included basal circulating concentrations of ACTH, adiponectin, triglycerides, glucose and insulin and the insulin response to an oral sugar test (OST). Factors significantly associated with laminitis included basal serum insulin concentration, insulin concentration in response to the OST, basal plasma adiponectin concentration and the presence of divergent hoof growth rings on the feet. Low, medium and high laminitis risk groups were defined based on basal insulin or the insulin response to the OST.
Researchers at the RVC have been studying laminitis for more than 20 years and were one of the first groups to define endocrinopathic laminitis. They demonstrated that laminitis-prone animals had a metabolic phenotype which included insulin dysregulation, increased circulating triglyceride concentrations and hypertension that was only apparent in the spring/summer rather than in winter, coinciding both with the consumption of spring/summer grass and the time of year at which laminitis risk is increased.
Identification of animals at an increased risk of endocrinopathic laminitis relies on detection of insulin dysregulation. Research at the RVC has demonstrated that either the oral glucose test (OGT) or the oral sugar test (OST) can be used to detect ID manifesting as basal hyperinsulinaemia and/or an excessive insulin response to oral carbohydrate consumption. RVC researchers revealed that these two tests are comparable and measurement of peak circulating insulin concentration following a single feed of glucose or corn syrup provides a simple and practical way of identifying animals at increased risk of laminitis. Further research has shown that using a higher than previously recommended dose of commercially available corn syrup (Karo Light corn syrup) for the OST (0.45ml/Kg) is more reliable than a lower dose (0.15ml/Kg).
Adipose tissue dysregulation is an additional feature of equine metabolic syndrome and results in altered production of adipose tissue-derived hormones (adipokines). RVC research has demonstrated that circulating concentrations of the adipokine adiponectin are lower in animals with a history of endocrinopathic laminitis and in healthy animals that go on to develop laminitis in the future. Thus, measurement of adiponectin concentrations is recommended as an additional measure of laminitis risk.
The research has been supported by grants from a number of charitable sources including the Horse Trust, the Horse Race Betting Levy Board, BVA Animal Welfare Fund, Norman Hayward Fund, the Laminitis Trust, PetPlan Charitable Trust and RVC Mellon Fund. In addition, research has been undertaken with support from WALTHAM Petcare Science Institute (MARS).
Comparison of immunofluorescence and chemiluminescence assays for measuring ACTH in equine plasmaEVJ2020