Equine Cushing’s Disease is a disease that is commonly discussed within the equine community, and it has been reported to affect between 20-33% of all horses by the age of 20. Although commonly referred to as Equine Cushing’s Disease, the more correct term is pituitary pars intermedia dysfunction (PPID). To explain what actually happens in horses with PPID, the pituitary gland, which resides just beneath the hypothalamus in the brain, drives the production of hormones in the body. PPID results from abnormal, but benign, growth of a specific region of the pituitary gland called the pars intermedia. Growth of this part of the pituitary gland occurs secondary to the degeneration of the hypothalamus. The hypothalamus links the endocrine and nervous systems, playing a critical role in controlling thirst, hunger, sleep and mood.
As the pars intermedia grows, it produces excessive amounts of various hormones, most importantly adrenocorticotropin hormone (ACTH). ACTH is renowned for its ability to affect cortisol levels, with cortisol, being commonly referred to as the stress hormone. The effect of cortisol on the body can be profound, as it can be involved in many biological processes, including maintenance of blood sugar levels when a horse is fasting, generation of energy during exercise, activation of the “fight or flight” response and important roles in the immune system. When the body has persistently excessive levels of cortisol, the effects can include muscle wasting, increased susceptibility to infection, as well as a higher risk of insulin resistance and laminitis.
Recognising PPID
As mentioned, all horses are at risk of developing PPID, so being aware of the signs (which are often subtle) is important for your horse’s ongoing health. Commonly reported clinical signs include; excessive hair growth or lack of seasonal shedding, recurrent laminitis, muscle-wasting, development of a pendulous abdomen, recurrent infections, abnormal sweat patterns, excessive thirst and urination, and behavioural changes, primarily; dullness or depression. Most of these clinical signs are a direct result of excessive cortisol levels circulating within the body.
Diagnosing PPID
PPID is often diagnosed based on clinical signs. However, it is strongly recommended to confirm suspicions with laboratory diagnosis before commencing medical treatment. Several tests are available for diagnosing PPID in horses, but in Australia currently, one of the more commonly recommended tests is called the endogenous ACTH test. This test only requires one blood sample to be taken at any time of the day. What is important to note however, is that the sensitivity and specificity of this test increases during Autumn, making the test more accurate during these months.
Treatment of PPID
Once a laboratory diagnosis has been made, commencement of treatment with Pergolide Mesylate, which usually comes in either a liquid or a tablet form, is recommended. Pergolide Mesylate is an ergot-derived dopamine agonist. Treatment with this product leads to a down-regulation in the production of POMC derived peptides within the brain, and ultimately, through a chain of hormonal interaction, reduces the levels of ACTH and cortisol within the body. It is recommended to re-test horses 3 months after the introduction of Pergolide Mesylate to ensure clinical effectiveness through decreased levels of endogenous ACTH. This re-testing also allows adjustments in dosage to be made if required. Annual blood tests are also recommended to ensure adequate dosing is maintained.
Ongoing management of horses with PPID.
Aside from medical interventions, appropriate management of horses with PPID is imperative for their ongoing health. Appropriate nutrition is vital, as well as ensuring they receive regular dental care and hoof care. It is also important to ensure appropriate body condition is maintained as overweight horses with PPID may be at a higher risk of insulin resistance and developing equine metabolic disease.
Introducing ‘Bud’!
‘Bud’ is our case of the month, as he was recently diagnosed with PPID, based on laboratory confirmation as well as clinical signs. It was first noticed by Bud’s owner that he seemed dull in his demeanour. Over time he was also noticed to develop a pot-belly appearance, and was beginning to develop dryness to his coat in patches, and abnormal malting patterns.
Bud received regular de-worming treatments, and hoof care, and recently received his routine dental examination. Despite this, he was still thought to be abnormal in his demeanour and not looking as well as he should! It was at this point that it was discussed whether testing for PPID should be explored.
A blood test was taken, and sent for evaluation of endogenous ACTH. A normal cut-off level for non-Autumn months is considered to be around 29.7 pg/ml (although this varies between laboratories) so Bud’s circulating levels of ACTH were around 4 times higher than normal! ‘Bud’ has now been prescribed Pergolide to commence medical treatment, and we will re-test his ACTH levels in 3 months’ time to assess the effectiveness of this treatment!