February 16, 2022
Written by Erica Larsen.
Two of the most common conditions veterinarians encounter in horses today—pituitary pars intermedia dysfunction (or PPID) and equine metabolic syndrome (EMS)—can also be two of the most confounding for owners. They’re alike but different. They have some of the same potential sequelae. And treatment and management protocols can include some of the same aspects but have very different goals.While EMS and PPID’s clinical signs often overlap, said Liz MacDonald, BVMS, MS, Dipl. ACVIM, a clinical instructor in equine medicine at the Virginia-Maryland College of Veterinary Medicine’s Marion duPont Scott Equine Medical Center, in Leesburg, Virginia, they’re two distinct diseases. She reviewed the many differences—and a few key similarities—between these two endocrine disorders during a Jan. 11 lecture presented by the Virginia-Maryland College of Veterinary Medicine.
The similarities between EMS and PPID are relatively straightforward: “Both diseases can have profound effects on the health and welfare of horses, and both warrant diagnosis and ongoing treatment,” MacDonald said.
Both conditions, she said, predispose a horse to developing potentially life-threatening laminitis, along with other conditions. Additionally, early clinical signs for both conditions can be easy to overlook while severe signs are more obvious.
And, unfortunately, neither condition has a cure.
The dissimilarities between the diseases can be complex. Here’s what we know about each:
PPID—Most commonly identified in older horses (it affects approximately 20% of horses 15 and older and is uncommon in horses under 10 years, MacDonald said), PPID is a hormonal disease caused by an enlargement in the pituitary gland’s pars intermedia (which plays a key role in regulating hormones). Horses with PPID have an increased hormone output, which causes metabolic issues, she explained. PPID has no breed or sex predilection; there’s no way to stop it from developing; and researchers haven’t identified any genetic links, she said.
Early PPID signs—including delayed shedding, a slightly longer-than-normal hair coat, lethargy, and loss of muscle tone, particularly on the topline—are easy to overlook because many are also associated with the normal aging process, MacDonald said.
If a horse is overweight, diminishing muscle tone can cause the fat to take on a lumpy appearance, she added.
Clinical signs of more advanced disease are easier to identify, she said, and can include a generalized long hair coat, a potbellied appearance, decreased performance, abnormal sweating patterns, and excessive water intake and urination. Aside from laminitis, common issues associated with PPID include tendon and ligament laxity and/or inflammation, degenerative suspensory ligament disease, recurrent infection, infertility, dental disease, and chronic skin conditions.
MacDonald said two tests can definitively diagnose PPID: a thyrotropin-releasing hormone (TRH) stimulation test and a baseline ACTH (adrenocorticotropin, a hormone that’s elevated in horses with PPID) level test. The former (a dynamic test that gauges ACTH level response to a stimulus, making it more sensitive) is best to use in horses with few or early clinical signs, she said, while the latter (a less sensitive measurement of ACTH levels in a single blood sample) is usually adequate to confirm PPID in a horse with more advanced clinical signs. She also recommended assessing whether horses have concurrent insulin dysregulation (ID, abnormal blood insulin levels), because this information can be crucial when designing a management plan.
Veterinarians use Prascend (the FDA-approved pergolide formulation) to treat PPID. “Clinical signs won’t necessarily resolve completely,” MacDonald said, “but we should see some improvement in them and the insulin dysregulation and hope we don’t see new ones developing.”
If clinical signs don’t improve and/or new ones develop or ACTH levels continue to rise, horses might benefit from a higher dose. She encouraged owners to work with their veterinarians to ensure horses are receiving appropriate doses.
In addition to medical treatment, PPID horses typically require some sort of dietary management. What type, however, depends largely on each individual horse, she added:
“There are lots of different factors when it comes to dietary management,” MacDonald said. “Discuss with your veterinarian or an equine nutritionist what’s best for your individual horse.”
Finally, she noted, because PPID horses can have a diminished immune system, it’s crucial to keep up on regular preventive health care, such as vaccines and targeted deworming. She also noted that regular hoof care is key, as farriers can sometimes pick up early signs of laminitis and, thus, allow earlier treatment and management changes.
EMS—This condition is associated with ID, increased fat deposition on the body, and a reduced ability to lose weight, she said. Risk factors for its development include obesity, a cresty neck, random or abnormal fat deposits on the body, and concurrent diseases, including PPID. Additionally, unlike PPID, researchers have identified that some breeds appear to have an increased genetic risk of developing EMS: Ponies, Spanish breeds, gaited breeds, Morgans, Miniature Horses, Warmbloods, and donkeys all appear to be more at risk for developing EMS than other types of horses, she said.
While EMS can’t necessarily be prevented, she said, addressing risk factors can reduce the likelihood of serious complications, such as laminitis.
MacDonald said a horse’s body condition score (BCS) can be a useful tool in considering whether EMS could be at play. On the 1 to 9 scoring system, a BCS of 4 to 6 is generally considered “ideal,” while anything over 6 is considered overweight or obese. Additionally, even if the horse doesn’t appear overweight, a cresty neck and abnormal fat deposits in specific areas (behind the wither and/or shoulder, over the ribs, and along the back and tailhead) could suggest EMS is at play.
Veterinarians have several options for diagnosing EMS:
MacDonald recommended assessing horses’ insulin status to ensure proper dietary management.
Medical therapies aren’t commonly used to treat EMS horses (though some cases benefit from medication, so she encouraged owners to discuss this with their veterinarians). Management largely depends on diet and exercise.
Because most EMS horses are overweight and many have ID, she said dietary management generally focuses on reducing weight and keeping insulin levels as steady and low as possible. She recommended:
Additionally, MacDonald said, exercise can help accelerate weight loss and, at certain intensities, improve insulin sensitivity.
Of course, she noted, “this can be challenging when a horse develops laminitis. Then, all we have to rely on is diet since we can’t exercise them during active laminitic episodes.”
In sum, while PPID and EMS are very different conditions, they both increase the risk of a horse developing laminitis. And when they occur simultaneously in the same horse, the laminitis risk elevates even further, MacDonald said.
“They’re lifelong issues that you have to manage,” she said. “Unfortunately, you don’t treat it once and it’s resolved.”
But the good news?
“With a definitive diagnosis and proper management, the impact on a horse’s quality of life should be minimal,” MacDonald said.