Equine endocrine disorders, such as Pituitary pars intermedia dysfunction (PPID), are becoming more promptly identified and treated, thanks to ongoing research and scientific advances, including methods of diagnosis, pharmacokinetics and management protocols. Equine Veterinary Education (EVE) and Equine Veterinary Journal (EVJ) have combined forces to create a free online collection of their recent articles on Equine Endocrinology to raise awareness of these common disorders and to provide vets with a definitive source of references all in one place.

The collection encompasses a comprehensive range of topics within the field of endocrinology and includes authoritative review articles on insulin dysregulation [2], glucocorticoids and laminitis [3] and paraneoplastic syndromes [4]. The initiative has been made possible with the support of British Equine Veterinary Association Trust and has been compiled by leading international authorities on equine endocrinology, Professor Philip Johnson, of University of Missouri and Professor Nicholas Frank of Tufts University, Boston and the University of Nottingham.

 

PPID is a progressive neurodegenerative condition that is more easily recognised in its advanced form in older horses. Sometimes referred to as Cushing’s Syndrome, it is one of the most common equine endocrine disorders. Surveys show a PPID prevalence rate of up to 22% in horses over the age of 15 [5], with the odds of developing clinical signs associated with PPID increasing by approximately 20% per year after this age.

 

Insulin dysregulation (ID), characterised by increased insulin response to oral sugars, hyperinsulinaemia and insulin resistance, is identified in both PPID and equine metabolic syndrome (EMS)[2]. Both PPID and EMS have been linked to the development of recurrent laminitis with subsequent high risk of mortality [6]. The exact mechanisms which link insulin to laminitis remain to be clarified [1] but this online collection highlights several studies, which partially address this important jigsaw -5.

 

Best practice for the management of EMS is outlined by Professors Nick Frank and Ray Geor in the December issue of EVE [13]. The first goal is to induce weight loss, and helpful and explicit guidelines for designing diets for obese equids are provided. Exercise and dietary management [14] have been shown to reduce the clinical signs and indices of inflammation in equine metabolic syndrome -2, however, further studies are needed to quantify the efficacy of these management changes on long-term outcomes.

The online collection also includes several studies addressing the diagnosis of PPID. The value of plasma adrenocorticotropin hormone (ACTH), when interpreted with seasonally adjusted reference ranges, is discussed by several authors -2. Professors Frank and Geor advocate it as the most readily accessible test for monitoring PPID cases in a field setting, although they maintain that the thyrotropin- releasing hormone (TRH) stimulation test is more sensitive and therefore has advantages when diagnosing PPID in its early stages [13].

 

Pergolide has emerged as the drug of first choice for treatment of PPID [13]. While many vets recommend the early introduction of medication in the hope that it may avert the specter of laminitis, at present, there are no trials comparing pergolide with alternatives. Furthermore, there have been no long-term studies to show that horses treated with pergolide before the onset of clinical signs of PPID have better outcomes, including a reduced incidence of developing laminitis, than horses in which treatment is delayed until clinical signs become apparent.

 

“Better control of PPID and insulin dysregulation should lower the risk of laminitis to help improve the health and longevity of many predisposed horses and ponies,” said Professor Celia Marr, Editor of Equine Veterinary Journal. “Raising awareness of the benefits of early diagnosis and sharing the latest research on these and other associated conditions are imperative to provide horse owners with the best diagnostic and treatment programmes for their horses. However, it is easy to under-estimate obesity: horse owners need to think about improving management of their horses’ feeding and exercise in order to avert problems before they become a veterinary issue.”

References:

 

.     [1]  Kritchevsky, S.E. and Johnson, P.J.  (2014) Current Status and Future Directions: Equine pituitary pars intermedia dysfunction and Equine Metabolic Syndrome. Equine Veterinary Journal 46, to add.

.    [2]  Frank, N. and Tadros, E.M. (2014) Insulin Dysregulation. Equine Veterinary Journal 46, to add.

.    [3]  Cornelisse, C.J. and Robinson, N.E. (2013) Glucocortioid therapy and the risk of equine laminitis. Equine Veterinary Education 25, 39-46.

.    [4]  Axiak, S. and Johnson, P.J. (2012) Paraneoplastic manifestations of cancer in horses. Equine Veterinary Education 24, 367-376.

.    [5]  McGowan, T.W., Pinchbeck, G.P. and McGowan, C.M. (2013) Prevalence, risk factors and clinical signs predictive for equine pituitary pars intermedia dysfunction in aged horses. Equine Veterinary Journal 45, 74-79.

.    [6]  Tadros, E.M. and Frank, N. (2011) Endocrine disorders and laminitis. Equine Veterinary Education 25, 152-162.

.    [7]  TÓth, F., Frank, N., Martin-JimÉNez, T., Elliott, S.B., Geor, R.J. and Boston, R.C. (2010) Measurement of C-peptide concentrations and responses to somatostatin, glucose infusion, and insulin resistance in horses. Equine Veterinary Journal 42, 149- 155.

.    [8]  Venugopal, C.S., Eades, S., Holmes, E.P. and Beadle, R.E. (2011) Insulin resistance in equine digital vessel rings: An in vitro model to study vascular dysfunction in equine laminitis. Equine Veterinary Journal 43, 744-749.

.    [9]  Borer-Weir, K.E., Menzies-Gow, N.J., Bailey, S.R., Harris, P.A. and Elliott, J. (2013) Seasonal and annual influence on insulin and cortisol results from overnight dexamethasone suppression tests in normal ponies and ponies predisposed to laminitis. Equine Veterinary Journal 45, 688-693.

.    [10]  Burns, T.A., Watts, M.R., Weber, P.S., McCutcheon, L.J., Geor, R.J. and Belknap, J.K. (2013) Distribution of insulin receptor and insulin-like growth factor-1 receptor in the digital laminae of mixed-breed ponies: An immunohistochemical study. Equine Veterinary Journal 45, 326-332.

.    [11]  Gauff, F., Patan-Zugaj, B. and Licka, T.F. (2013) Hyperinsulinaemia increases vascular resistance and endothelin-1 expression in the equine digit. Equine Veterinary Journal 45, 613-618.

.    [12]  Dunkel, B., Wilford, S.A., Parkinson, N.J., Ward, C., Smith, P., Grahame, L., Brazil, T. and Schott II, H.C. (2014) Severe hypertriglyceridaemia in horses and ponies with endocrine disorders. Equine Veterinary Journal 46, to add.

.    [13]  Frank, N. and Geor, R. (2014) Current best practice in clinical management of equine endocrine patients. Equine Veterinary Education to add.

.    [14]  Hudson, A.B., McGowan, C.M. and Morgan, R. (2012) The clinical management of EMS at the Philip Leverhulme equine hospital. Equine Veterinary Journal 44, 6.

.    [15]  Menzies-Gow, N.J., Wray, H., Bailey, S.R., Harris, P.A. and Elliott, J. (2014) The effect of exercise on plasma concentrations of inflammatory markers in normal and previously laminitic ponies. Equine Veterinary Journal 46.

.    [16]  Rendle, D.I., Rutledge, F., Hughes, K.J., Heller, J. and Durham, A.E. (2013) Effects of metformin hydrochloride on blood glucose and insulin responses to oral dextrose in horses. Equine Veterinary Journal 45, 751-754.

.    [17]  Owers, R. and Chubbock, S. (2013) Fight the fat! Equine Veterinary Journal 45, 5-5.

.    [18]  Copas, V.E.N. and Durham, A.E. (2012) Circannual variation in plasma adrenocorticotropic hormone concentrations in the UK in normal horses and ponies, and those with pituitary pars intermedia dysfunction. Equine Veterinary Journal 44, 440-443.

.    [19]  McGowan, T.W., Pinchbeck, G.P. and Mc Gowan, C.M. (2013) Evaluation of basal plasma α-melanocyte-stimulating hormone and adrenocorticotrophic hormone concentrations for the diagnosis of pituitary pars intermedia dysfunction from a population of aged horses. Equine Veterinary Journal 45, 66-73.

.    [20]  Rendle, D.I., Litchfield, E., Heller, J. and Hughes, K.J. (2014) Investigation of rhythms of secretion and repeatability of plasma adrenocorticotropic hormone concentrations in healthy horses and horses with pituitary pars intermedia dysfunction. Equine Veterinary Journal 46, to add.