What is Equine Cushing’s Disease?
Equine Cushing’s Disease was first discovered in the 1930s and although commonly still called this, it was renamed as Pituitary Pars Intermedia Dysfunction (PPID) in the 1990’s as it then emerged that the disease in horses is different from Cushing’s disease in humans and dogs, as it effects several hormone levels in equines not just one. PPID is therefore an endocrine system disease which affects the pituitary gland which is situated within brain, below the hypothalamus. PPID is usually common in older horses and the most recognised sign is the formation of a long, wavy coat with ‘looking older and skinnier’, which people can mistake for old age. Other symptoms include sweating easily, being lethargic, drinking and urinating excessively and horses can run high temperatures.
The pituitary gland usually controls the production and release of hormones, blood pressure, electrolytes and many metabolic, growth and reproductive functions and it has an important link to the nervous system. The pituitary gland is regulated by large dopamine levels secreted by the brain’s nerve cells in a fully functioning system. Unfortunately in a horse with PPID, for a reason yet unknown, nerve cells have a decreased anti-oxidation capacity which means they are more likely to perish resulting in a reduction of dopamine levels (which regulate the pituitary gland). Without regulation, the pituitary gland secretes excessive amounts of the following hormones:
- adrenocorticotropic hormone (ACTH)
- pro-opiomelanocortinderived peptides (POMC)
- melanocytestimulating hormone (MSH)
- corticotropin-like intermediate lobe peptide (CLIP)
- beta-endorphin (ß-END)
The pituitary also causes increased levels of the steroid cortisol which depress the immune system meaning a greater risk from other infections and a delay in any healing.
The increase in the above hormones leads to abnormal glucose and fat metabolism so horses may start to appear thin but have fat deposits over the neck crest, tail head, shoulders and mammary glands/or prepuce. Insulin issues caused by abnormal metabolism presents a greater risk of laminitis. In many cases horses with PPID can have repeated mild laminitis episodes which cause accumulative damage over time. As the disease progresses, the quickened protein metabolism caused by excess hormones causes muscle breakdown on the topline and haunches, and progresses to other areas. Abdominal muscles can become too weak to hold the intestines creating a ‘pot belly’ appearances and there is often intermittent front leg stiffness.
The malfunctioning gland itself, grows in size and leads to non-cancerous (and sometimes cancerous) cells forming a tumour in the section of the gland known as the ‘pars intermedia’ – hence the name Pituitary Pars Intermedia Dysfunction. The tumour can then press against the brain and in advanced stages can lead to neurological issues. There is still some debate which comes first, the tumour or hypertrophy; the latter is where the tissues enlarge due to the extra work caused by the malfunction of the gland.
What is the prognosis for horses with PPID?
PPID is a progressive disease which can not be cured at present, it can only be managed with treatment. The most commonly used drug on the market to date is Pergolide mesylate, a dopamine therapy, which acts on the receptors of the pars intermedia to suppress tissue enlargement or tumour growth and it stimulates dopamine release, which in turn regulates the pituitary gland.
Homeopathic vet Mark Elliot carried out research into two homeopathic remedies, ACTH 30c and Quercus robur 30c, which were prepared in a ‘standardised’ manner (i.e. usually homeopathic remedies treat the individual not the disease so are prepared individually). The overall success rate was 80% which concludes that homeopathy lends itself to the treatment of Cushing’s Disease.
If you recall the brain nerve cells in PPID horses have a low anti-oxidant capability so perish easily (causing the decrease in dopamine) so therefore anti-oxidant supplements such as vitamin C and E may play as useful part in the diet. Chaste berry (Agnus castus) is a possible dopamine stimulating herb and there are tests ongoing at present as to its efficiency. Magnesium supplementation is often suggested to support this disease.
It can be difficult to keep the weight on PPID horses, so a beet or soy pulp may help, but cases are very individual so there is no hard and fast rule as there is always a laminitis risk. Therefore a low-carbohydrate/high fat diet is often recommended, grass hays are a safer option than legumes such as clover and alfalfa as they contain less carbohydrates.
Other management measures include parasite control, dental care, farrier care, and possibly body clipping if long curly hair is a symptom.
With improved diagnostic and treatment nowadays many horses are living longer better quality lives.
James Casey, DVM, MS. Cushing’s Disease in Horses. Available: http://www.equinehorsevet.com/pdf/Cushing’s%20disease.pdf
Janice Posnikoff, DVM. 2005. Orange County Equine Veterinary Services in Southern California. Available: http://www.horsechannel.com/horse-health/
Mark Elliot. 2001. Cushing’s Disease: a new approach to therapy in equine and canine patients. British Homeopathic Journal 90, 33-36.
Michigan State University College of Veterinary Medicine. 2017. Equine Endocrinology Fact Sheet. Available: https://animalhealth.msu.edu/ClientEducation/MKTG.ED.008.pdf
Wise Geek. What is PPID. Available: http://www.wisegeek.com/what-is-equine-pituitary-pars-intermedia-dysfunction-ppid.htm