“Cushing’s Disease” is a name commonly given to a disease of horses called Pituitary Pars Intermedia Dysfunction (PPID). It is the most common endocrine disease seen in older horses. It’s thought to affect around 20% of horses over the age of 15 and 2.9% of the general population. It is rare, but not unheard of in horses under the age of 10. However, it is worth noting that this may be an underestimate as not all horses will be tested for the disease. 

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What causes it?

It is a degenerative condition that develops over time leading to an overproduction of hormones (especially ACTH) by the pituitary gland. The only consistent risk factor for the development of PPID is increasing age. In other words, this is not a disease we can prevent as we cannot prevent ageing. Mares and geldings are affected in equal proportion. 

What are the clinical signs?

The most common clinical signs associated with PPID are hypertrichosis (excessive hair coat), hair colour changes, usually darker hair turning copper in colour, bulging of the tissue around the eyes, laminitis, muscle wastage, particularly loss of topline and the development of a pot belly, excessive drinking and excessive sweating. Lethargy, depression and weight loss are also sometimes seen along with an increased susceptibility to infections. 

In the UK, many horses will be diagnosed with PPID following an episode of laminitis, which often prompts diagnostic testing. One can have a high index of suspicion that a horse may have PPID, based on clinical presentation; but it is usually confirmed by means of a blood test to measure the levels of ACTH (Adrenocorticotropic hormone). This test is often enough to reach a diagnosis. But in rare cases, the test may need to be repeated or another test, with a hormone known as TRH may be needed. 

Should we treat cases of PPID?

It is well understood that horses with PPID are at an increased risk of developing laminitis. Laminitis has been demonstrated in 24-82% of horses with PPID and can be life limiting or life threatening in some cases. Even mild cases will affect the welfare of the horse and can be costly in terms of vet’s fees. 

Horses with PPID are also susceptible to Equine Metabolic Syndrome (EMS) and it should be mentioned that these diseases can coexist. So cases of laminitis may need to be tested for both conditions. 

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PPID is not a curable disease, but in many cases, it can be managed

From a medical point of view, cases are usually treated with a dopamine receptor activator (agonist) known as pergolide. This drug can slow down the enlargement of the pituitary gland. Some horses will respond quite well to this treatment, as evidenced by lower levels of circulating ACTH and a reduction in the severity of clinical signs. Horses on treatment will need to be monitored to ensure effective dosage; which is why it is useful to confirm the diagnosis with a blood test, even if clinical signs are very obvious. If we know the pre-treatment levels of ACTH, we can monitor the response more easily. The level of ACTH does not predict the severity of the disease, but it can be used to check the response. In general vets recommend retesting 6 weeks after starting treatment and every 6-12 months afterwards, unless the condition worsens. 

As we know these animals are at risk of laminitis, we also need to address their management in terms of dietary restriction and pasture access. To assess the risk of laminitis, we often consider the clinical signs, history and insulin levels as well as response to medication.

In addition to dietary management, we should also ensure that we monitor other clinical signs

For example, if the horse has hypertrichosis, they should be clipped and groomed to ensure comfort and good skin health. These horses can be more susceptible to infections. So a close eye should be kept on their overall health including routine vaccinations, faecal worm egg counts, tapeworm testing and dental care to reduce the risk of problems developing. 

What if the horse has no clinical signs associated with PPID? 

Owners often ask if we should treat asymptomatic cases who seem otherwise fit and well. ACTH levels are very accurate in identifying cases of PPID, ie, if the horse has a high ACTH, it is very likely that the horse has PPID. However, we should look at the overall health of the horse. If they have PPID, it is likely that they will develop clinical signs over time and when that happens is difficult to predict. Given the high incidence of laminitis in cases of PPID, one could argue that we should be treating all cases. 

It is likely (although yet not fully proven) that early treatment with pergolide may slow the progression of the disease. But, the decision to treat is best made following advice from your veterinary surgeon who knows your horse and their management. If the horse has experienced even a single episode of laminitis, then medical treatment is advisable in addition to management to reduce the risk of further incidents.

The downside of treatment is mainly financial

Some horses will show a transient anorexia when on pergolide; but this often resolves if the drug is stopped and restarted at a lower dose before increasing the dose slowly. Long term treatment is generally well tolerated without serious side effects.

If you are concerned that your horse may have PPID or would benefit from testing, please speak to your veterinary surgeon who will be best placed to advise you on your individual situation. 

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