EQUINE PPID (CUSHING’S DISEASE)

Pituitary pars intermedia dysfunction (PPID), commonly known as Cushing’s disease, is one of the most common endocrine disorders in horses and ponies. It most commonly affects older horses (with the average horse age being approximately 20 years of age), however it has been reported in horses as young as seven years old. Overall, the majority (over 85%) of horses diagnosed with PPID are older than 15 years of age. PPID is caused by an enlargement of a portion of the pituitary gland (an important gland that is located at the base of the horse’s brain).

Pathophysiology

The pituitary gland is a small pea-sized gland located at the base of the brain. The gland is crucially important in controlling a range of functions including metabolism, reproduction, growth and lactation. In horses and ponies suffering from PPID, age-related degeneration means that an imbalance occurs in the hormones secreted from the pituitary gland, which in turn imbalances the secretion of other hormones produced in the body. It is the consequences of these hormonal changes which result in the clinical signs of PPID.

Clinical signs

Horses with PPID typically are older horses that present with pasture-associated or seasonal laminitis, hair coat abnormalities (such as delayed, incomplete, curling of, excessive growth, or lack of hair coat shedding upon the warmer months of the year), and generalized muscle wasting/weight loss. Affected horses often have a history of impaired or delayed healing of wounds, which is a result of immune suppression associated with the disease. They are prone to recurring abscesses, tooth infections, sinusitis, and higher intestinal parasite loads than normal horses. Upon progression of the disease, horses may also have muscle atrophy along their back and develop a ‘potbellied’ appearance.

Diagnosis

1. History – Chronic laminitis, recurring infections

2. Clinical signs

3. Physical exam

4. Blood Test : Plasma adrenocorticotropic hormone (ACTH) concentration – Considered to be the best commercially available screening test for PPID. This test is most accurate in March/April for horses in Australia.

PPID Vs Equine Metabolic Syndrome (EMS)

PPID and EMS are both very common endocrine disorders, affecting up to 30% of certain breeds of horses and ponies. However, they are very distinct diseases which require different approaches with respect to treatment. PPID usually affects older horses in contrast to EMS which most often affects younger-to-middle-aged animals.

PPID and EMS share several similarities. Both are endocrine disorders. Both are associated with horses that have a history of acute or chronic laminitis, especially seasonal or pasture-associated. The onset of laminitis in both disorders has been related to underlying insulin dysregulation. In some instances, EMS and PPID can occur concurrently in horses. Hormonal testing is required to properly diagnosis both EMS and PPID.

Management

1. Pergolide mesylate : The same medication used in humans to treat people with Parkinson’s disease. Administered once daily, orally in pill or liquid form to aid in reducing symptoms.

2. Diet Modification : Switching to a low soluble carbohydrate, high protein feed.

3. Regular body clipping if required

4. Frequent fecal worm egg counts to develop a tailored strategic deworming program.

5. Restrict access to lush pasture and feed an appropriate senior horse feed.

Complications

1. Chronic Laminitis: Horses with PPID have an increased risk of developing laminitis, especially during the spring when turned out to pasture with lush grass.

2. Insulin resistance: Horses with PPID may also have insulin resistance, meaning that glucose is not absorbed from the bloodstream into the body’s cells as it should, resulting in high levels of glucose in the blood.
High parasite load: Horses with PPID are shown to have higher intestinal parasite loads than normal horses.

3. Neurological signs: As the pituitary gland continues to grow, it can often exert pressure to other portions of the horse’s brain, which can on rare occasion cause the onset of neurological signs.