Pituitary pars intermedia dysfunction (Equine Cushing's disease) in an onager (Equus hemionus onager)

Pituitary pars intermedia dysfunction (PPID), also known as Equine Cushing's disease, is most often diagnosed in older horses and ponies. To the authors' knowledge, there have been no reports of its diagnosis in captive nondomestic equids. A 13-yr old onager (Equus hemionus onager) at the Zoological Society of London (ZSL) Whipsnade Zoo showed clinical signs suggestive of PPID, including hirsutism, fat redistribution, weight loss, laminitis, and chronic infections. A dexamethasone suppression test was performed to confirm PPID. Subsequently, adenomatous hyperplasia and microadenoma of the pars intermedia were identified postmortem. Four months later, this onager's dam died suddenly, and adenomatous hyperplasia of the pars intermedia was diagnosed following necropsy. The dam had shown no clinical signs of PPID. Examination of archives identified eight other adult onagers that died or were euthanized between 1993 and 2007. The brain was not examined in four of these, but pituitary glands were described as enlarged during necropsy in three animals based on the subjective assessment of an experienced zoo and wildlife pathologist, making an overall prevalence of enlargement of 83.3%. Hyperplastic pituitary changes are positively correlated with age in domestic equids, and this may also be the case in onagers. Alternative etiologies are also discussed.