With its trademark symptom of a long, often curly coat, Cushing’s disease is a common hormonal disorder among older horses – but with the right approach they can go on to enjoy a long and happy life.
Cushing’s disease is the most common endocrine (or hormonal) disorder to affect the equine brigade’s older generation. But despite the fact there’s no cure, and the condition can have major welfare implications – including recurring bouts of laminitis – there’s hope for the many horses and ponies who are affected.
With the right care they can go on to lead a full and happy life. Here we dismiss the myths, explain the latest research and answer your questions about the disease.
What causes Cushing’s?
Cushing’s disease is the result of a dysfunction in the middle part of the pituitary gland – a small gland located near the base of the horse’s brain that’s responsible for regulating many of his hormonal systems.
In horses with Cushing’s they see age-related degeneration of the neurons (nerves) and this, in turn, leads to the reduced production of a chemical called dopamine, which is important in controlling hormone secretions from the pituitary gland.
With this reduction in dopamine the normal inhibition of the pituitary gland is lost, resulting in the excessive secretion of several hormones – and it’s these hormones that result in the clinical signs.
How common is it?
Recent studies show that around 15% of horses over 15 years of age, and up to 40% of horses over 25, are affected. However, there is limited information about the prevalence of the condition in the general equine population.
The average age at diagnosis tends to be about 19 to 20, though it’s not unheard of, but much less common, in horses under the age of 15. Studies have shown that for every year of age above 15, the risk of Cushing’s disease increases by 21%.
There is no difference in risk between mares, geldings and stallions and all breeds can be affected.
While it was previously considered to be more common in ponies, recent studies have shown that horses are just as likely to suffer from the disease – the apparent incidence in ponies probably reflects the fact that ponies live longer.
What are the symptoms?
The abnormally high levels of hormones from the pituitary gland have a wide range of effects on the body, and symptoms can progress so slowly that early or mild cases can be difficult to differentiate from normal signs of ageing.
The main symptoms include:
- A long (sometimes curly) coat, which doesn’t shed normally in the spring. However, this common symptom doesn’t appear in all cases, and often subtle coat changes, such as retaining a few long hairs after shedding their winter coat, will be the only abnormality shown.
- Repeat episodes of laminitis
- Lethargy or reduced ability to exercise
- Loss of muscle
- Excessive sweating
- Fat redistribution, which can often lead to a pot-bellied appearance. Classically, owners describe fat pads above the eyes, but it is more common to see deposits below the eyes – similar to bags under the eyes, or a combination of both.
- Increased thirst and urination.
- Increased susceptibility to infections, such as conjunctivitis and mud fever. It’s important to remember that Cushing’s disease is a slowly progressive disease and so affected horses will not always show the full range of clinical signs.
What’s the link between Cushing’s and Laminitis?
Older horses appear to have an increased risk of laminitis and it’s likely much of this increased risk can be attributed to Cushing’s disease. Testing for Cushing’s should be considered in any horse or pony over 15 who’s affected by a bout of laminitis for the first time, is having recurrent episodes of laminitis, or an episode of laminitis that fails to respond as expected to appropriate treatment.
While Cushing’s disease causes many hormone changes, laminitis associated with the disease is due to increased cortisol and insulin resistance. It can be difficult to treat the laminitis without managing the Cushing’s disease.
There’s a growing body of opinion that suggests over 80% of horses and ponies with laminitis may have the problem because of an underlying endocrine disease like Cushing’s or Equine Metabolic Syndrome, so it’s critical to get to the diagnosis early to aid treatment and limit recurrence of the laminitis.
How is it diagnosed?
In some cases, the clinical signs alone can be sufficient to be confident of making a diagnosis. In earlier cases, or where there is an uncertainty, diagnosis involves a blood test.
Unfortunately, diagnosis can’t be based on routine blood samples. There are a range of tests available, and your vet will be able to advise on the most appropriate test in each case.
Sometimes, to be confident of a diagnosis, blood tests may need to be repeated at different intervals to take into account seasonal changes in hormone levels. These can also be useful in monitoring the effects of treatment, and trial treatment can be useful in supporting the diagnosis.
How can Cushing’s be treated?
There is only one licensed veterinary treatment available for Cushing’s disease, and this contains a medicine called pergolide. The aim of treatment with pergolide is to re-establish the inhibitory effects of dopamine on the affected pituitary gland, which helps to bring hormone concentrations in the blood stream back to normal levels again.
Most horses will show improvement of clinical signs within the first few weeks of treatment with the drug. Pergolide has very few side effects, however occasionally horses may lose their appetite when they first start treatment. Treatment is usually life-long and tablets are given one daily, though thankfully the cost has reduced considerably over the past 10 years.
Other treatments may include management of laminitis where applicable, and in some cases horses may need additional medication to treat insulin resistance, although this often improves with Cushing’s disease treatment alone. Your vet will be able to advise you on this.
How can the condition be managed?
Ongoing management of the condition should include:
- Regular veterinary check ups.
- Regular vaccination against tetanus and equine flu.
- Regular dental checks and teeth rasping (horse’s affected by Cushing’s may have secondary complications of age-associated dental disease).
- Regular farrier visits.
- An appropriate de-worming programme – this should ideally incorporate faecal egg counts and a blood sample for tapeworm.
- Monitoring your horse’s water intake.
- Identifying and treating any minor infections early.
- Clipping any excessive coat, particularly during the summer months.
- Providing adequate shelter from the weather and rugging up where necessary.
- Regular grooming which can improve the coat and help early detection of wounds and skin infections.
- Reducing stress as much as possible.
- Providing an appropriate diet to maintain a good body condition and prevent obesity. For more information on dietary recommendations, please speak to your veterinary surgeon or an equine nutritionist. An anti-oxidant supplement may also be beneficial.
What’s the prognosis for horses with Cushing’s?
It is certainly not the case that all horses with Cushing’s will develop all the clinical signs. Laminitis is the biggest clinical problem and unfortunately the most common reason why horses with Cushing’s have to be euthanised.
While treatment doesn’t actually stop the disease progressing, it helps to prevent the negative effects of excess hormone levels. Many affected horses will live happily for years following diagnosis and treatment and can often continue their usual level of exercise.
With the appropriate care some horses cope well even without medical treatment, although in these cases continuing exposure to these hormones will increase their risk of serious problems such as laminitis.