The disease once referred to as Cushings, is now officially called Equine Pituitary Pars Intermedia Dysfuction. It’s quite a mouthful, so it’s shortened to PPID.
The disease is one of the most common disorders of the endochine (gland) system in middle-aged and geriatric horses; it’s progressive and leads to the degeneration and death of neurons in the brain.
It has been reported in horses as young as seven years old, but it is traditionally a disorder of older horses. The prevalence of PPID increases with each year of age in horses. The average age of affected horses is 19 years, and it may affect up to 30 percent of horses over the age of 20. Recent studies in Australia have reported that approximately one in five horses over 15 has PPID.
PPID results from overactivity of part of the pituitary gland, which is located at the base of the horse’s brain. The pituitary gland receives signals from nerves that originate in another area of the brain, the hypothalamus. These signals trigger the pituitary to release proteins and hormones that travel through the blood acting as signals for various pathways and systems within the body. When the nerves are damaged, as happens with PPID, the pituitary enlarges and produces excessive amounts of hormones. As the nerves continue to degenerate, the disease progresses.
What does a horse with PPID look like?
Signs of PPID are diverse and vary in severity according to disease stage. Rather than being a black and white disease, PPID comes in varying shades of grey.
There are, however, two major categories of disease stage with PPID: early and advanced. Clinical signs associated with the disease can differ, and not all signs are present in any one individual horse; sadly it seems that not all horses have read the text book when it comes to PPID.
Usually, a diagnosis is made based on a combination of clinical signs and diagnostic testing.
With early stages of PPID, horses may show no symptoms at all, or display vague and non-specific signs including decreased athletic performance, loss of muscle tone and mass, change in demeanour, lethargy, delayed hair coat shedding, change in body conformation, regional fat deposition, and laminitis.
With horses with early PPID, diagnostic testing provides great assistance in identifying the disorder.
The clinical signs in advanced PPID tend to be quite evident, though again, only some of these signs may be present. These include lethargy, laminitis, increased amount of hair, late or unusual shedding of the winter haircoat, lowered immunity leading to recurrent infections, reduced reproductive fertility, skeletal muscle atrophy often seen as a loss in top line, regional fat deposition especially behind the eyes, excessive drinking and urination, and a potbellied appearance.
The stress hormone
One of the hormones produced as a result of the pituitary gland pathway is cortisol, a steroid hormone. Cortisol, sometimes referred to as the “stress hormone,” travels through the blood and acts on many different organ systems.
In a normal horse, the brain will monitor the level of cortisol and other hormones in the blood. Through a feedback mechanism, it then adjusts the level of production of those hormones by altering the amount of signal released by the pituitary gland.
But in a horse with PPID, the feedback mechanism in the brain no longer works correctly. Despite appropriate or even excessive amounts of cortisol and other hormones in the blood, the pituitary gland keeps signalling the body to produce cortisol. This results in an over-production of cortisol, as well as other hormones. The excessive cortisol results in many of the clinical signs seen with PPID.
Diagnosing PPID
In most cases, the recommended blood test is a resting adrenocorticotropic hormone (ACTH) level. ACTH is one of the hormones released from the pituitary gland. A high level of resting ACTH suggests a diagnosis of PPID and the loss of the feedback loop, indicating that the pituitary is overstimulating the production pathway for cortisol and other hormones.
There is no wrong time to test a horse for PPID; however, the best time to test is autumn (March, April, and May in New Zealand). This is because there is a seasonal variation of ACTH concentrations.
Research indicates that in the autumn all horses have an increased production of ACTH. However, in horses with PPID, this increase is more marked than in normal horses. This allows for greater distinction in test results between normal horses and those with PPID in the autumn.
PPID comes with many varying symptoms and degrees of severity, so there is crossover in the reference ranges between horses with and without PPID.
In autumn, this degree of crossover is much less, meaning the test is more sensitive and specific and will differentiate more clearly between the horses with early PPID and those horses without PPID.
The good news for horse owners who think that they may have a horse with PPID is that over March, April, and May of this year, the drug company Boehringer Ingelheim is offering free ACTH blood testing.
Simply contact your veterinarian to arrange for a test, and she or he will liaise with Boehringer Ingelheim and the laboratory to organise the blood test.
How is PPID treated?
There are two main threads to the management of a horse with PPID.
The first is preventative health care. Because horses with PPID tend to be older horses, it is important to address the routine conditions associated with aging. In addtion, horses with PPID can have a lowered immunity. For this reason, optimal management includes routine dentistry, parasite control, and farrier care. These techniques and feeding a high-quality diet are key things that owners can do to lessen the effects of the disease process.
The second component of management of horses with PPID is medical therapy.
The medication of choice for treatment of PPID is pergolide (Note that current research indicates that chasteberry is not an effective treatment for PPID).
Pergolide works by restoring the feedback mechanism for the pituitary gland. It is available as a tablet that is given once daily. It can be dissolved in water and fed mixed in with molasses or a small volume of hard feed.
Pergolide is excellent at reducing the clinical signs of PPID. However, if the medication is stopped, the loss of the normal feedback mechanism in the brain which leads to PPID still remains, and clinical signs will recur.
Response to pergolide therapy typically takes six to eight weeks and sometimes up to 12 weeks to become clinically apparent. Repeat blood testing one to two months after initiating therapy is recommended to assess the response to treatment and also to determine if the dose of pergolide needs to be altered.
