PPID, in full Pituary Pars Intermedia Disease, is a common hormone disorder in (mostly older) horses. This condition used to be known primarily as "Cushing's disease" by analogy to similar conditions in humans and other animal species. Yet there are also substantial differences. In horses, the cause is located in the brain (specifically the hypophysis). This in contrast to e.g. the dog (adrenal glands). Given the fact that this is a general disorder, effects are possible throughout the body. Also in the mouth and more specifically in the oral mucosa we often see changes. 

The "peripheral resistance" of your horse has decreased, which can lead to a higher incidence of all kinds of diseases. 

A periodic dental examination and monitoring of blood levels (ACTH) is recommended as well as ( medical ) treatment if possible.

From a dental point of view, PPID increases a horse's risk of periodontitis, increased mobility of the teeth and an increased risk of sinusitis.