Equine endocrinology.

Abstract

The two common endocrine disorders of the horse are pituitary pars intermedia dysfunction (PPID) and equine metabolic syndrome (EMS). PPID is a neurodegenerative disorder of the pituitary pars intermedia characterized by loss of dopaminergic (inhibitory) input. The exact cause remains unknown, but the pathogenesis is associated with oxidative damage, mitochondrial dysfunction and protein misfolding. PPID is more common in ponies compared with horses and the risk increases with age such that the mean age of affected animals in retrospective case series is 18-23 years. Common clinical signs include hypertrichosis, muscle atrophy, recurrent laminitis, polyuria and polydipsia, hyperhidrosis, lethargy and secondary infections. Diagnosis is based on the signalment, clinical signs and further diagnostic test results. The currently recommended further diagnostic tests are basal adrenocorticotropic hormone (ACTH) concentration and ACTH response to thyrotropin-releasing hormone stimulation. Not all owners will elect to treat the disease specifically and instead manage the individual clinical signs, but the dopamine agonist pergolide is the pharmacologic treatment of choice. If the animal fails to respond, the serotonin antagonist cyproheptadine or the Cortisol synthesis inhibitor trilostane can be considered. Either the clinical response or repeating the further diagnostic tests can be used to monitor the response to therapy. EMS is a collection of endocrine and metabolic abnormalities, including insulin dysregulation, hyperleptinaemia, increased adiposity and hypertriglyceridaemia, which are associated with an increased susceptibility to laminitis in affected individual animals. EMS first develops in horses <15 years of age and diagnosis is based on clinical signs including obesity or regional adiposity and recurrent laminitis; demonstration of fasting hyperinsulinaemia; demonstration of insulin dysregulation; and demonstration of peripheral insulin resistance. EMS should be managed with exercise and diet changes to induce weight loss and improve insulin sensitivity and pharmacological intervention should be used only in the short term and reserved for cases refractory to management changes alone. It should be noted that PPID and EMS can occur concurrently and a subset of animals with PPID also have insulin dysregulation.

Pituitary Pars Intermedia Dysfunction (PPID) in Horses

“…Other medical therapies of PPID include the serotonin antagonist cyproheptadine [ 80 , 109 , 146 , 151 , 152 ], the 3-hydroxysteroid dehydrogenase inhibitor trilostane [ 64 , 153 , 154 ], and adrenolytic mitotane [ 155 , 156 ]. However, there is currently limited evidence to suggest their efficacy in the treatment of disease [ 6 , 64 , 146 , 152 , 154 , 155 , 157 ].…”