A long, curly coat that fails to shed normally is the classical clinical sign of Cushing’s disease in horses, which is more correctly known as pituitary pars intermedia dysfunction (PPID).
In PPID, the pituitary gland, which is a located at the base of the brain and produces hormones in response to brain signals, suffers damage to the mechanisms that control the levels of these hormones. This leads to an excessive production of specific hormones, which enter the horse’s circulation and affect the whole body.
All breeds and types may be affected, but ponies appear to be at greatest risk. It is usually seen in horses and ponies over the age of 15 years. Mares and geldings are equally affected.
PPID typically develops slowly over a period of several years. It is now thought that many horses and ponies have PPID to some degree, even if they do not show the most obvious signs.
While there is no cure, treatment may extend or improve the horse’s quality of life, but medication is expensive and once started, it is best if it is maintained.
Signs of PPID
The signs of Cushing’s/PPID are often vague and variable, and may easily be confused with other diseases or simply put down to “old age”.
Weight loss and lethargy are commonly seen, despite a normal or occasionally increased appetite. Loss of muscle mass, particularly over the saddle area and rump, is caused by protein breakdown.Some horses and ponies may develop the appearance of a “pot belly” due to weakening and stretching of the abdominal muscles. Fat may be deposited along the crest of the neck, above the tail and above and behind the eyes.Sweating is a common sign, particularly in areas where the coat is long.Affected horses may drink and urinate more than usual, although this can be very hard to assess, especially in field-kept animals.Affected horses may appear more docile and tolerant of pain than others.
Recurrent infections (such as dental, respiratory and skin disease) are common due to suppression of the immune system. In addition, wound healing may be delayed and simple things such as mouth ulcers can become an issue.
Sometimes, there are other potential clues, like the coat that looks as if it has been permed or repeatedly suffering from foot abscesses.
What causes Cushing’s disease/PPID?
Cushing’s/PPID is caused by progressive nerve degeneration in an area of the brain called the hypothalamus, leading to a reduction in secretion of the neurotransmitter dopamine. Dopamine is important in controlling the secretions of a part of the pituitary gland called the pars intermedia, which in turn is responsible for controlling the secretion of hormones including ACTH (adrenocorticotropic hormone) and cortisol.
When the pars intermedia is not exposed to enough dopamine from the hypothalamus, abnormally high levels of various hormones are produced which leads to the clinical signs associated with the disease.
How is it diagnosed?
Your vet is likely to make an initial diagnosis based on the horse’s history and clinical signs, which can then be confirmed by testing for hormone levels in blood.
There have been lengthy periods when the blood test is free to horse owners in the UK and Republic of Ireland so speak to your vet to see if this offer is running if you’re planning to have your horse tested. The cost of the test is subsidised by the company that makes the medication licensed for treating horses for this condition.
Management of Cushing’s disease/PPID
The most effective way to control the signs of Cushing’s (PPID) is with medication. In addition, supportive management such as clipping, dentistry and weight control will help ensure you’re providing the best quality of life possible for your horse and minimising the risk of complications, particularly laminitis.
Horses with thick coats will sweat, becoming uncomfortable and distressed in warm weather. They will benefit from regular clipping all year round.
Dental care is very important to help prevent infections and maximise the absorption of nutrients from the diet. Dental problems can lead to painful chewing and quidding, which will reduce appetite resulting in further weight loss.
Careful assessment of diet is necessary and giving a soft pelleted feed designed for older or veteran horses may be helpful. Most crucially, it is important to minimise the risk of laminitis: professional advice from a nutritionist may be needed to ensure a balanced diet.
Regular use of a weigh tape is useful to monitor weight, and the diet should be altered as necessary to maintain body condition.
Routine foot care is also important to help prevent recurrent abscesses and laminitis.
Vaccinations, regular worming and treatment for ectoparasites (lice, etc) are more important than usual and should be kept up to date. Due to suppression of the immune system, prompt treatment of any infection is recommended.
Treatment of PPID
Although a number of different medicines have been assessed as treatments for Cushing’s (PPID), only one is currently a licensed treatment in the UK. This treatment contains pergolide, which acts to compensate for the neurons that are no longer producing the dopamine.
In a small number of cases, a vet may prescribe other medications including:
- Bromocriptine (replaces dopamine production)
- Trilostane (inhibits cortisol production)
- Cyproheptadine (inhibits serotonin production)
An improvement in a horse’s demeanour and attitude may be seen within one to two weeks of commencing treatment; however, changes to the hair coat can take up to six months to show a full improvement.
Treatment does not halt the progression of the disease, but it can lessen the clinical signs and improve the animal’s quality of life.
Many affected horses and ponies are able to live comfortably with good management for at least a couple of years. But ultimately, recurrent bouts of laminitis or infection or other complications of old age, may necessitate euthanasia.
https://onlinelibrary.wiley.com/doi/pdf/10.1111/eve.12903 – Does pergolide therapy prevent laminitis in horses diagnosed with pituitary pars intermedia dysfunction? February 2018