Pituitary pars intermedia dysfunction (PPID, or equine Cushing’s disease) and equine metabolic syndrome (EMS) are by far the most common endocrine disorders in horses. In fact, recent study results suggest that up to 30% of horses of some breeds suffer from one of these conditions during their lifetimes.
While these potentially career- and life-ending diseases are distinctly different in the damage that they cause, they do share some similarities; laminitis, for instance, is associated with both. This can make distinguishing them from one another and selecting appropriate treatment challenging.
Kelsey Hart, DVM, PhD, Dipl. ACVIM, assistant professor of large animal internal medicine in the University of Georgia’s Department of Large Animal Medicine, reviewed current recommendations for PPID and EMS testing at the 2015 American Association of Equine Practitioners’ Convention, held Dec. 5-9 in Las Vegas.
First, a quick rundown of how each disease works:
- PPID occurs most commonly in horses older than 12. Various factors related to the aging process inhibit function of the horse’s pars intermedia, located in the pituitary gland. This results in an overproduction of hormones, such as adrenocorticotropic hormone (ACTH) and cortisol, that leads to the common clinical signs of PPID: abnormal hair coat and failure to shed, abnormal sweating, loss of muscle mass, and increased water intake and urination, among others.
- EMS, on the other hand, can develop in horses of any age. Researchers believe adipose (fat) tissue abnormalities alter the metabolism of affected horses, leading to characteristic signs that include regional fat deposits, insulin resistance, and, as mentioned, laminitis.
“Differentiating between the two diseases is still an art,” said Hart. Practitioners must use clinical signs, screening tests, and dynamic testing (which involves comparing samples at a specific time interval) to make an accurate diagnosis.
“The classic presentation of PPID is an older (late teens and upward) animal that presents with pasture-associated or seasonal laminitis, generalized muscle wasting/weight loss, and hair coat abnormalities,” she explained. “The classic presentation for EMS is also an animal that presents with pasture-associated laminitis, but in contrast this is a younger to middle-aged animal with a normal hair coat that has always been an easy keeper and is clearly obese with obvious fat deposits in the crest of the neck and tailhead.”
Classic Clinical Signs
|Laminitis, often seasonal (spring/autumn) and/or pasture-associated||Laminitis, often seasonal (spring/autumn) and/or pasture-associated. Onset may be gradual, with abnormal growth rings on hooves or chronic laminitic changes noted on radiographs but no clinical signs.|
|Middle-aged and older, often older than 12||Any age|
|Predisposed breeds/types: Ponies, Morgans||Predisposed breeds/types: Ponies, Morgans, Paso Finos, Arabians, Saddlebreds, Quarter Horses, Tennessee Walking Horses|
|Abnormal hair coat: early or late shedding/hair growth, retention of guard hairs, failure to shed out completely or at all, a long curly coat||Normal hair coat and shedding pattern|
|Muscle wasting/weight loss, ranging from difficulty maintaining condition to topline muscle loss to severe emaciation||Normal to increased body condition, often has been an “easy keeper” or obese since a young age|
|Abnormal fat distribution: Regional adiposity, or fat deposits in the crest of the neck, tailhead, prepuce, and flanks. Can make muscle wasting/weight loss harder to detect.||Abnormal fat distribution: Regional adiposity, or fat deposits in the crest of the neck, tailhead, prepuce, and flanks. Can occur with or without generalized obesity.|
|Decreased immunity/delayed healing||No apparent changes in immunity or healing|
|Polyuria/polydipsia (excessive urination/drinking)||No changes in urination or water consumption|
She cautioned, however, that these signs are really only helpful in advanced stages of disease. Further, the two conditions can appear concurrently in some horses. Thus, she recommended veterinarians run hormonal tests to accurately differentiate between the two.
Veterinarians start by measuring resting hormone concentrations to screen for PPID, she said. They can perform this simple test in the field—but again, it’s most useful for advanced disease.
To do this, the veterinarian takes a blood sample to determine the horse’s baseline ACTH concentration. Keep in mind, however, that season affects both healthy and Cushing’s horses’ ACTH concentrations, with levels naturally increasing during fall months.
“We used to say don’t test in the fall because you can get false positives,” said Hart. “But it’s possible to test in the fall with seasonally correct ranges for the fall (ACTH) spike.”
Testing for EMS can be a bit more complicated. While the fasted insulin concentration test (in which the veterinarian measures insulin concentrations from a blood sample after the horse has been fasted overnight) is the most straightforward, Hart cautioned that a number of other factors, such as stress, diet, and pregnancy, can affect a horse’s insulin regulation. So veterinarians should consider these factors when interpreting test results.
For PPID, the overnight dexamethasone suppression test (ODST, where the veterinarian administers the steroid dexamethasone, which in healthy horses will cause a decrease in ACTH) has long been the gold standard, but Hart suggested that the thyrotropin-releasing hormone stimulation (TRH stim, where TRH administration causes an exaggerated ACTH response in PPID horses) test might be a more useful method for early disease diagnosis. For these reasons, she suggested not testing from July through November and retesting horses with borderline results.
Hart said she thinks the simplest and most useful dynamic field test for EMS is the oral sugar test, which assesses the horse’s insulin response to dietary carbohydrates—in this case, corn syrup. To be effective, this test requires normal gastric function, so make sure the horse isn’t colicky before testing, she said. Also avoid running this test if your horse is currently experiencing a bout of laminitis.
In summary, there is no ideal, one-size-fits-all testing method to differentiate between these two diseases, said Hart.
While scientists are constantly refining this area of research, “It is unlikely that a single diagnostic testing approach will ever be applicable to or useful in all presentations of these diseases,” she said. “It is important to remember that all of the tests described can have false positives and false negatives,” particularly in the early stages of both PPID and EMS.
Work with your veterinarian to outline a diagnostic and treatment plan that’s best for your individual horse.