Equine Cushing’s Disease
Also known as Pituitary Pars Intermedia Dysfunction (PPID), Equine Cushing’s is one of the more common diseases that we see in older horses. In the past this condition was probably under recognised with most just drawing the conclusion that their horse was getting older. Currently about 30% of aged horses are diagnosed with the condition.
Usually seen in horses over 15 years of age, the likelihood of developing this condition increases with age. Many horses in their late 20s and early 30s are therefore likely to be living with undiagnosed PPID.
This condition is associated with the abnormal function of the pituitary gland. This is a hormone-producing organ located at the base of the brain. In a healthy animal the pituitary gland receives stimulation from other areas of the brain by nerves, preventing it from releasing too much hormone. However as horses age, these nerves begin to degrade meaning the pituitary gland receives less signals from the rest of the brain causing it to release excessive amounts of hormones such as ACTH (adrenocorticotrophic hormone).
These hormones lead to the development of symptoms that we commonly associate with PPID.
- Hirsutism; the most commonly recognised symptom of PPID this relates to the excessively long and curly coat that sufferers commonly develop, whilst some animals will struggle to lose their winter coats.
- Increased appetite, drinking and urination; occuring in up to 30% of horses with PPID these symptoms are fairly non-specific and may also indicate that there are some other internal problems.
- Unusual fat distribution and insulin resistance; 60% of PPID horses present with fatty deposits (particularly around the eyes). Insulin resistance is also associated with equine metabolic syndrome.
- Laminitis; the most serious side effect of PPID, often associated with considerable amounts of pain.
- Recurrent infections; PPID horses suffer from infections more readily than normal horses as the excess hormones supress the immune system. For example it is not uncommon to find PPID horses suffering with successive foot abscesses or parasite associated infections.
- Lethargy and increased sweating; owners report that their horse is ‘slowing down with age’ or ‘sweating up more easily.’ These symptoms usually resolve well with treatment.
- Infertility; occurs in older mares due to altered hormone production.
Not all horses with PPID will demonstrate all these symptoms, however usually one or two of the above are enough to raise our suspicions, particularly when examining an older horse.
PPID is diagnosed by a blood test, with a turnaround time of <72 hours. This test measures levels of ACTH in the blood plasma. Whilst this test can be run at any time of the year, the biggest difference between normal and abnormal horses is best observed in the autumn. In some uncommon occasions when results are inconclusive there are further hormone tests that may be run.
Although the eventual prognosis depends upon the severity of clinical signs, PPID can be managed very effectively via treatment with Pergolide (Prascend). This replicates the signals in the brain that would normally be passed by the degenerating nerves thus preventing the pituitary gland from producing excessive hormones. As hormone levels return to normal, clinical signs of the disease improve.
Unfortunately if pergolide treatment is stopped it does not take long for clinical signs to return, so most patients are on the tablet for life. Fortunately the tablet is small and easy to give to most horses.
Often your vet will start with a low dose (half a tablet once daily) and retest the blood in three months’ time. If ACTH levels have returned to normal then we continue with the same dose, but if there is still further improvement to be made then the dose can be increased further.
There are a number of natural remedies that have been used to treat PPID; however none have been scientifically proven to be effective.