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Compendium March 2013 (Vol 35, No 3)

Clinical Snapshot: Hirsutism in a Pony

by Adam Stern, DVM, CMI-IV, CFC, DACVP

    Case Presentation

    A euthanized, 30-year-old, castrated male pony presented to the University of Illinois Veterinary Diagnostic Laboratory for necropsy. The pony had a >1-year history of increased appetite, polyuria, somnolence, and pronounced hirsutism, with the hair over most of the trunk and legs being up to 9 cm in length (FIGURE A).

    1. What is the most likely cause of the clinical abnormalities in this patient?

    2. What is the proposed pathogenesis?

    3. Are there breed or sex predilections for this disease? What are some of the clinical signs?

    Answers and Explanations

    1. On gross necropsy, a large mass extended from within the sella turcica and compressed the overlying hypothalamus (FIGURE B). The mass was well-demarcated and yellow to white. Histologically, the mass was composed of neoplastic cells that were polygonal, had eosinophilic and granular cytoplasm, and were compressing the adjacent and atrophic pars distalis. Based on the gross and histopathologic examination findings, a diagnosis of pituitary adenoma of the pars intermedia was made, which is consistent with equine Cushing disease.

    2. Pituitary adenoma of the pars intermedia can compress the overlying hypothalamus, resulting in deranged hypothalamic function.1,2 In turn, this can result in disturbances in carbohydrate metabolism, a ravenous appetite, hypertrichosis, and hyperhidrosis.2

    In addition to the space-occupying effects of the lesion, some adenomas of the pars intermedia are endocrinologically active, resulting in increased concentrations of corticotropin-like intermediate peptide, α- and β-melanocyte-stimulating hormone, and β-endorphin, which are derived from proopiomelanocortin (POMC).1,2 POMC is metabolized into these hormones within the pars intermedia; within the pars distalis, POMC is processed into corticotropin (also known as adrenocorticotropin hormone), β-lipotropin, and γ-lipotropin. Docility and diminished responsiveness to painful stimuli may be related to an increase in the β-endorphin level.1 Mild to moderate increases in plasma cortisol and immunoreactive corticotropin levels can also occur, as horses with pituitary adenoma of the pars intermedia have selective posttransitional processing of POMC.

    The pathogenesis of this hirsutism has not been determined.3

    3. There is no sex predilection for the development of equine Cushing disease.3 No specific equine breeds appear to be predisposed to the disease; however, the incidence is greater in ponies than in horses.3

    Common clinical signs of equine Cushing disease include hirsutism (i.e., a thick and long haircoat), acute and chronic laminitis, polyuria, polydipsia, hyperglycemia, hyperhidrosis, and muscle wasting. Up to 85% of affected horses have haircoat abnormalities.3

    1. Capen CC. Endocrine glands. In: Pathology of Domestic Animals. Vol 3. 5th ed. Philadelphia, PA: Saunders; 2007:340-343.

    2. La Perle K. Endocrine system. In: Pathologic Basis of Veterinary Disease. 5th ed. St. Louis, MO: Elsevier; 2012:672-673.

    3. Dybdal NO. Endocrine and metabolic diseases. In: Large Animal Internal Medicine. 3rd ed. St. Louis, MO: Mosby; 2002:1233-1236.

    References »

    NEXT: Focus on Nutrition: Home-Prepared Diets for Dogs and Cats


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