A: This is a disorder that’s better referred to as pituitary pars intermedia dysfunction (PPID). The word dysfunction is the best place to start, because this is a progressive disorder that begins with the dysfunction of the pituitary gland. This gland at the base of the brain sends out a number of hormones, and as the horse gets older and develops this condition, we see region of the pituitary gland–the pars intermedia–starts to send out more hormones. And it’s really a collection of hormones that get sent out and affects the horse in many different ways.
We also call this Cushing’s disease because there’s an element of it that leads to an increase in cortisol and other stress hormones in the body. It’s not as straight forward as just elevated cortisol concentrations all the time, but it certainly has this underlying problem of an increase in stress hormones. So, it’s a dysfunction that develops overtime into a small tumor or multiple tumors in this particular region of the pituitary gland and the key part is that they are active tumors sending out too much hormone all the time, so that’s what’s causing the signs we’re seeing in the animals.
Why is it so common? Really because lots of this goes hand-in-hand with aging and this particular part of the pituitary gland is controlled by some nerves that basically degenerate over time. They degenerate in some horses faster than others, and those are the animals we see the disease in. As a horse gets older the risk of getting PPID increases, until we get to an age (in the 20s or 30s) where one out of three old horses will have Cushing’s.