Recognising PPID — what stands behind Equine Cushing's Syndrome

Pituitary Pars Intermedia Dysfunction (PPID), colloquially Equine Cushing's Syndrome, is the most common hormonal disease of the older horse. The older name is somewhat misleading: unlike in the classic Cushing's of humans, the cause does not lie primarily in the adrenal gland, but in a loss of control at the level of the brain.

What happens in the pituitary gland

PPID is a slowly progressing, degenerative change of dopaminergic nerve cells in the hypothalamus. These neurons normally inhibit the pars intermedia, a part of the pituitary gland. If they are lost through oxidative damage, this dopaminergic brake falls away — the pars intermedia proliferates and produces hormones excessively, among them increasingly precursors that are processed to ACTH. Important: the ACTH formed by the pars intermedia is largely biologically of little activity, which is why the disease is correctly designated as a dysfunction of the pars intermedia and not as a classic Cushing's.

From this mechanism the leading signs explain themselves. They develop insidiously over years and are initially easily attributed to age — which makes early recognition harder.

The leading signs — and why the coat is so telling

The most striking and most specific sign is hypertrichosis: a delayed, incomplete or absent coat shedding, often with a long, curly coat that stays put even in summer. Technically, hypertrichosis is the more apt term than the formerly common hirsutism, because the hair follicles abnormally persist in the growth phase for long. The presence of a regional or generalised hypertrichosis justifies a high clinical suspicion.

Further signs are muscle loss, especially along the topline, a sagging belly, increased drinking and urinating, an increased susceptibility to infection with poorly healing wounds — and, clinically especially relevant, an increased tendency to laminitis. It is precisely the endocrinopathic laminitis that is often the first occasion to think of PPID at all; frequently an insulin dysregulation exists in parallel.

The diagnostic path

The suspicion from history and inspection is subdivided veterinarily via the laboratory. Established is the determination of the basal ACTH level in the blood; with an early or unclear manifestation a TRH stimulation test improves the informative value. An important pitfall is the seasonality: ACTH rises physiologically in late summer and autumn, which is why the values must be interpreted against seasonal reference ranges. This classification belongs in veterinary hands — no lay eye makes a secure diagnosis from the coat alone.

On treatment, only the frame shall be named: PPID is a veterinarily steered disease. The active substance approved for it is a prescription-only veterinary medicine and belongs exclusively in veterinary prescription and course control — it is here expressly not advertised and not dosed. In addition, a consistent management contributes: needs-based, often NSC-low feeding with a view to the frequently accompanying insulin dysregulation, careful coat care and clipping as well as close hoof control.

Where compounds can sensibly flank, it is about the general metabolism, not about the disease itself. Milk thistle is traditionally used for the nutritional accompaniment of the liver metabolism — as a supplementary feed, not as an intervention in the pituitary gland function. That changes nothing about the core: PPID is recognised and treated by the veterinarian. The earlier the suspicion is clarified, the better the consequential phenomena, above all the laminitis, can be steered.