Q: I have a question for your vet about Cushing’s disease. I know about some of the symptoms, such as a curly coat that doesn’t shed out. And I believe that it is not curable. What I would like to know is if it has any effects in terms of performance, behaviour and so forth? And also, can young horses get it or is it just a disease in older horses?
Tina, Coromandel
Vet Dave van Zwanenberg replies:
Tina, many thanks for asking about Cushing’s, as it is a disease I believe is very much under-diagnosed in New Zealand horses, and with treatment can greatly improve the horse’s quality of life.
Just to confuse all horse owners, equine Cushing’s has been renamed as PPID. This is because it is a very different disease model to the human Cushing’s, and therefore should not have the same name. PPID is typically a disease of older horses, with recent studies estimating its prevalence at around one in four horses over 15 and one in three horses over 20. However, recent data from the UK has shown that it can start in horses as young as 8 or 9, though these are much less common.
In a normal horse, the brain signals to a small gland situated just underneath it, called the pituitary, to release a hormone called ACTH. This hormone travels through the blood to reach the adrenals which are located just next to the kidneys, where its job is to signal the adrenals to produce steroid.
This steroid, which also travels via the bloodstream, has many effects throughout the body as nearly all the cells in the body are steroid-responsive in some shape or form.
The amount of steroid in the blood is monitored by the brain, which then modulates the amount of signal it sends to the pituitary, creating a nice feedback loop.
In PPID, this feedback loop is dysfunctional, with the monitoring part of the brain less able to direct the part of the brain which talks to the pituitary gland. This leads to an over-production of ACTH by the pituitary gland, leading to a subsequent over-production of steroid by the adrenal glands. It also causes an increase in size of the pituitary.
The excess steroid and increased size of the pituitary gland lead to all of the clinical signs seen in PPID. The severity of clinical signs seen depends on the degree of dysfunction of the feedback loop; thus PPID is best seen as a disease with clinical signs of varying shades of grey rather than a black or white disease process.
Unfortunately, the dysfunction of the feedback loop in the brain cannot be fixed; however, what can be done is amplify whatever feedback signal is still getting through to have more clout in reducing ACTH production by the pituitary. This is done by daily administration of a drug called pergolide (tradename: Prascend). In simple terms, the drug is making the brain to listen harder to the feedback signal it is getting.
Our inability to repair the dysfunction in the feedback loop means that, as you correctly noted, that the disease process is incurable – but very much manageable with medication.
The excess steroid production promotes the breakdown of muscle as an energy source. This leads to a loss of muscle mass which has a direct effect on performance. Early signs of PPID in sporthorses can be a loss of top-line, or simply poor performance.
Horses with PPID are often more lethargic and quiet than their non-Cushing’s counterparts. Although horses becoming less active is an expected sign of ageing, there is often also underlying PPID as well, making the depression more marked. With treatment, these horses become much perkier and are often described as behaving how they were five years prior.
