Equine Pituitary Adenoma, What is PPID? By Shelley Newman DVM

Autopsy/necropsy, diagnosis/detection . . . what’s in a word? (sagepub.com). There are various expected pathologies in different species. In rats, pituitary adenomas are common, and often functional, producing the hormone prolactin. In dogs, pituitary adenocarcinomas can infiltrate the adjacent brain, and rarely secrete excess Adrenal Corticotrophic Hormone (ACTH) to induce changes in the adrenal glands, that may make the animal Cushingoid. In cats, there can be acromegaly (large facial bone structure) with pituitary adenomas (benign pituitary tumor). The presentation of pituitary neoplasia in the horse is unique, to all others. Yet, removal of the brain and sampling of the pituitary in a horse, takes special equipment and is not always included in a routine
necropsy. The condition is most common in older horses, with an average age of 20 years. There is no breed or gender predilection.
How would you suspect your horse has pituitary disease?
The most common symptom is a “shaggy hair coat” in a horse that would not otherwise have such a coat, even in cold weather. https://www.merckvetmanual.com/endocrine-system/the-pituitary-gland/hypertrichosis-associated-with-adenomas-of-the-pars-intermedia This is known as hirsutism.

The presence of nodules in the equine pituitary produce one or more “bumps” on its surface. We typically section it sagittally (https://pubmed.ncbi.nlm.nih.gov/18192571/) in an effort to look at all three anatomic components, the pars distalis, pars intermedia and pars nervosa.

The unique aspect of the horse’s lesion is that the proliferative lesions tend to occur in the pars intermedia, unlike in other species like the dog, where the cells of the pars distalis are most commonly affected, hence producing a different clinical presentation. This helps explain why the horse’s clinical
disease syndrome is so different from small companion animals.

The different anatomic regions of the pituitary are affected disproportionately across the species but in the horse, unlike other species, it is the pars intermedia that proliferates to form hyperplastic nodules or benign adenomas. It is only when the cells of these nodules, are able to secrete their normal hormonal products in excess, that systemic disease occurs.

In the horse, the syndrome created by excess hormone secretion from a hyperplastic pituitary is called PPID, or Pituitary Pars Intermedia Dysfunction. It tends to dysregulate the production of hormones from that anatomic location of the pituitary, and affects hair maturation such that a “shaggy” hair coat, even in summer, and a re-distribution of adipose tissue, such that fat deposits are noted on the top of the neck, tailhead and above the eyes, are consistently noted. There is often a loss of musculature along the topline. Additional signs may include a rounded abdomen (potbelly), exercise intolerance, delayed shedding of the hair coat, laminitis (foot inflammation) and abnormal sweating. The prognosis for horses diagnosed with PPID is variable. Some horses respond well to low levels of medication, while others need a much higher level. Earlier diagnosis results in a better quality of life, especially if laminitis (the scourge) can be avoided (https://www.merckvetmanual.com/musculoskeletal-system/lameness-in-horses/laminitis-in-horses ).

Now “shaggy hair coat” in a horse has additional meaning. To view examples of the pathologic change noted by pathologists, view www.vetartwork.com horse gallery where there are two examples of pituitary adenomas, and one of equine laminitis.