Equine Cushing's Disease, more correctly known as Pituitary Pars Intermedia Dysfunction (PPID), is a disease that causes several hormones (including cortisol) to circulate in the blood at abnormal levels. PPID affects more than 20% of horses over 15 years old. Worryingly, the signs of PPID can appear to be part of the normal ageing process, making the disease both complex and difficult to diagnose.
Cause
PPID affects the middle lobe of the pituitary gland (called the Pars Intermedia), located in the middle of the base of the brain. Because the pituitary gland is the control centre of the body’s hormonal system, dysfunction in this gland can cause multiple hormonal imbalances.
As the disease takes hold, it causes the middle lobe of the pituitary gland to increase in size, while the outer two lobes compress and shrink. Hormone production from the enlarged middle lobe increases, while production decreases in the shrunken outer lobes as they begin to lose function.
Although the causes of this disease are not well understood it is more common in some types of horse, including ponies and Morgan horses, indicating that there may be a genetic component.
Signs
Horses around 20 years old are most commonly diagnosed with PPID, although some significantly younger horses develop the condition. Unfortunately, signs of PPID vary greatly depending on the age of the horse and at which stage the disease is diagnosed. However, the most common sign across the board is the horse’s coat becoming long and curly, as it does not undergo its normal shedding process.
Other symptoms may include:
- Excessive drinking
- Excessive urination
- Sweating
- Laminitis
- Lack of energy
- Loss of muscle mass
- Repeated infections, such as hoof abscesses, tooth rot and sinusitis
- The appearance of ‘bulging eyes’ due to body fat redistribution
High blood glucose levels could be another potential sign of PPID. Because insulin is produced in the pancreas and it's production is moderated by a feedback system involving the pituitary gland, horses with PPID can also become insulin resistant; this is the horse equivalent of becoming a diabetic.
Diagnosis
As we mentioned above, the most useful diagnostic sign of PPID is the presence of long curly hair in an aged horse. In fact, this sign is more effective at predicting PPID than any other clinical test.
However, clinical tests do exist for horses with more subtle signs of PPID. These test results can change depending on the season of taking the blood sample. Other single tests for hormone levels including adrenocorticotropic hormone (ACTH), thyrotropin-releasing hormone (TRH) and other hormones produced by the pituitary can be used but are unlikely to be diagnostic alone.
Treatment
Due to its complexity, treatment of this condition rarely results in a cure or remission, something that understandably frustrates many horse owners. With this in mind, when it comes to treating PPID, the key aim should be to improve the quality of the horse’s life.
Pergolide is the most commonly used treatment in cases of PPID and doses can be adjusted based on the horse’s improvement as measured by ongoing clinical tests. Because of the wide and complex effects of this disease, routine preventive health measures such as worming, dental checks and coat management need to be carefully adhered to. For horses that are insulin resistant, decreasing body fat and adhering to a low-starch diet can help to improve their health outcomes.
As usual, early recognition and diagnosis of this condition is essential to its control and the long-term treatment of affected horses.
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