Page 965 - The Veterinary Care of the Horse
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Figure 20.1 The horse’s brain and the adrenal glands; it is the overproduction of cortisol by the adrenal glands as a result of
pituitary gland enlargement or a tumour that is responsible for many of the symptoms which typify PPID
In a normal horse, secretion of hormones from the pars intermedia is controlled by a
neurotransmitter called dopamine which is released by nerve endings in the hypothalamus.
The disease is usually seen in older animals, over 15 years of age but young animals can be
affected. For reasons that are currently unknown, aged horses and ponies affected by PPID
have less dopamine production, allowing overactivity of the pars intermedia of the pituitary
gland. The pars intermedia enlarges and occasionally an adenoma forms. Ponies appear to be
more susceptible than horses and the disease also occurs in donkeys.
