Pituitary pars intermedia dysfunction (PPID), commonly called Cushing's disease, is the most common endocrine disorder in older horses and ponies, causing a constellation of welfare-relevant signs including laminitis, immunosuppression, and chronic pain.
PPID causes multiple overlapping welfare harms in older horses. The hypertrichosis (failure to shed winter coat) causes significant heat stress in summer — horses struggle to regulate body temperature and sweat profusely under heavy coats that would normally be shed. Muscle wasting reduces functional ability and makes horses more susceptible to injury. Immunosuppression elevates infection risk, and PPID horses commonly develop sinusitis, dental disease, and parasitic infections at higher rates. Most significantly, PPID-associated laminitis causes severe, potentially chronic pain that substantially reduces quality of life. The condition is prevalent, progressive, and often underdiagnosed in older horses whose signs may be attributed to normal ageing rather than a treatable medical condition.