Common Misunderstandings About Equine Endocrine Issues
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Test your knowledge about the tricky world of PPID, EMS, ID, and more
There’s EMS. There’s ACTH. Then there’s TRH. OST. ID. And of course there’s PPID.
Oh, the acronyms of equine endocrine disorders. Who doesn’t get lost looking at all of them? That’s just the start, because the risk factors, diagnostics, and management approaches for these conditions can be pretty confusing, as well. Add in the fact that scientific knowledge about equine metabolic syndrome (EMS), pituitary pars intermedia dysfunction (PPID, formerly Cushing’s disease), and insulin dysregulation (ID) is advancing constantly, and suddenly you might feel like your own pituitary gland is going to explode due to the stress hormones from trying to figure all this out.
To help tease through the mess of letters and conditions, we’ve developed this—admittedly tricky—true-false quiz about some of the most common misconceptions concerning equine endocrine disorders. Test your endocrine IQ with the most up-to-date scientific knowledge provided by our sources: Dianne McFarlane, DVM, PhD, Dipl. ACVIM, professor and chair of the University of Florida’s Department of Large Animal Clinical Sciences, in Gainesville; and Cathy McGowan, BVSc, MACVSc, PhD, DEIM, Dipl. ECEIM, FHEA, FRCVS, professor of equine internal medicine at the University of Liverpool’s Department of Equine Clinical Science and director of veterinary postgraduate education, in the U.K.
Good luck! (Or should that be GL?)
1. Regardless of the endocrine disorder, ID means laminitis risk.
True.
Horses are considered to have ID when they produce too much insulin—a condition known as hyperinsulinemia. This leads to a diagnosis of EMS, which is often also characterized by obesity, according to Equine Endocrinology Group (EEG) recommendations and guidelines.
About 30% of horses with PPID also have ID, meaning they have both PPID and EMS, the EEG reports. That means if your horse has been diagnosed with PPID, it’s very important to test for ID, too.
Because ID can cause laminitis, McGowan says it is essential to run insulin tests on all horses with an endocrine diagnosis, whether PPID or EMS, to manage their laminitis risk.
2. PPID is an “older horse” disease.
True.
PPID is a slowly progressive, degenerative disease of dopamine-producing neurons in the hypothalamus, similar to Parkinson’s disease in humans, McFarlane says. It’s associated with long hair coats; muscle loss, especially along the topline; lethargy; reduced performance; abnormal sweating; recurrent infections; and suspensory ligament breakdown.
Although the mechanism of neuronal loss is unclear, it might be caused by age-related oxidative stress and accumulation of proteins that are misfolded (i.e., something went awry when the proteins translated into their native three-dimensional structure to become biologically functional), as occurs with Parkinson’s, says McFarlane. In any case, they’re certainly related to aging.
“We’re not expecting all horses to be 30 years old before they get PPID,” she says. “But most are going to be at least in their later teens. It’s very unusual to see a 10-year-old horse with PPID.”
In the rare case of a young horse having PPID, it’s considered more likely to be a spontaneous tumor, McGowan explains. By contrast, 20% of horses 15 years and over have PPID because of processes that accompany aging. “This is an aging-related disease; that’s no myth at all,” she says.
3. EMS is a fat horse disease.
Mostly true.
Certainly, obesity is the biggest risk factor for EMS, hallmarks for which are ID and, hence, laminitis, McFarlane says. Therefore, obesity is a common clinical sign of EMS.
But that doesn’t mean all obese horses have EMS, McGowan explains, as some horses are simply “healthy obese.” Likewise, it doesn’t mean all EMS horses are obese.
Even so, obesity is such a high risk factor that even overweight Thoroughbreds—a low-risk breed for EMS—can develop the syndrome. “Certainly, obese horses are more prone to EMS, so you can’t take obesity out of the equation,” she says.
Lean horses with EMS “are the hardest cases” to treat, McFarlane adds. “You have to be sure you’re getting enough calories (in them) while being very, very careful because you don’t want to get that insulin spike.”
4. Management affects EMS and PPID risks.
True … and False.
It’s true for EMS. While EMS might have a genetic component, how we manage horses—especially the way we feed them—can also place them at risk of developing the disease, says McFarlane. Even if they do develop EMS, good management can help control and even prevent its most devastating effect, laminitis, adds McGowan.
That’s especially true of “thrifty” horses that tend to hold on to weight even through the harsher winter months, McFarlane adds. Owners should carefully consider these horses’ rations with their nutritionists. “Too much sugar in the blood happens either because you’re feeding them too much sugar or the tissues are not taking up sugar,” she says. “So, diet is key.”
The same is not true for PPID: Researchers don’t currently have any evidence that management is related to risk, she explains. “We just don’t know enough to say whether there are things we do in caring for our older horses that might contribute to PPID risk,” McFarlane says.
5. You can see PPID by looking at the brain.
True.
The “PPI” in PPID is for pituitary pars intermedia—a section of the pituitary gland in the brain that enlarges in PPID-affected horses. Enlargement of the PPI is hard proof a horse has PPID, says McFarlane. Veterinarians can image the pituitary gland of a live horse using magnetic resonance imaging (MRI) and computed tomography (CT).
However, “these imaging techniques are expensive and require general anesthesia and, therefore, are not practical methods to diagnose PPID in your old horse anyway,” McFarlane says. Positron emission tomography (PET) scans in humans can show the amount of functional dopaminergic neurons for diagnosing Parkinson’s disease, but horses’ heads are too big to fit into current PET scanners.
6. An accurate diagnosis of insulin dysregulation requires fasting and an OST.
False.
The current expert recommendation for diagnosing ID is an oral sugar test (OST), in which horses receive a small dose of corn syrup or other sugar on an empty stomach about an hour or an hour and a half before the veterinarian draws a blood sample for insulin testing. But many horses don’t need such a complex and expensive test, because their blood insulin levels would already test positive without six hours of fasting and the glucose boost, says McGowan.
“The nice thing about insulin is if it’s highly abnormal, then you’re comfortable that it’s abnormal,” she says. “It’s a misconception that basal insulin is not good enough to be a diagnostic test.”
If the basal test comes back negative but the clinical signs suggest EMS, an OST is warranted, she explains. Vets might also perform an insulin tolerance test, in which they collect blood before and 30 minutes after injecting insulin, without the horse fasting.
Recently, researchers have suggested it’s not always necessary to fast a horse before performing an OST. However, EEG experts have decided against making recommendations to skip fasting at this time, McGowan says.
7. Reliable ACTH testing requires TRH stimulation.
False.
An important tool in diagnosing PPID is a blood test for concentrations of adrenocorticotropic hormone (ACTH), because the enlarged, dysfunctional pituitary gland produces too much of it, our sources say. Veterinarians usually diagnose PPID by drawing blood samples in the field at any time to run a basal ACTH test.
Similar to ID testing, though, veterinarians can diagnose PPID by using a test that measures hormone levels in response to a stimulant. For PPID, the stimulant is thyrotropin-releasing hormone (TRH). Veterinarians inject TRH into the bloodstream and test ACTH concentrations exactly 10 or 30 minutes later.
Also like ID testing, this complex stimulation test isn’t always necessary, says McGowan. Basal tests usually provide the readings needed to confirm what veterinarians have already seen in a clinical exam, she says.
Even so, if the basal results are borderline or they don’t match clinical signs, or if veterinarians are trying to catch PPID in its earliest stages, a TRH stimulation test might be a good option, she says.
Timing is critical, however, McFarlane adds. Recent research shows that when collecting the sample 10 minutes after TRH administration, even a minute can make the difference between a positive or negative result following TRH stimulation (TheHorse.com/1113886).
8. Clinical signs of PPID are more important than ACTH readings.
True.
To assess the likelihood a horse has PPID, experts have developed diagnostic thresholds of ACTH concentrations.
Contrary to popular belief, however, no single threshold point says whether a horse has or doesn’t have PPID, says McGowan.
“One of the biggest myths about endocrine disorders is this idea of a single cutoff value for PPID,” she says. “People get hung up on it. But it’s just simply not true.”
An accurate PPID diagnosis therefore depends first and foremost on a thorough clinical exam. If the veterinarian suspects PPID based on that exam, then—and only then—should he or she perform an ACTH test, she says.
The clinical exam is so critical, in fact, it should guide the way a veterinarian interprets the test results, McGowan adds. A 2020 study on 75,892 horses showed the importance of recognizing the wide variations in normal versus abnormal ACTH concentrations; since then, the EEG has recommended viewing ACTH depending on the horse’s clinical signs.
“The study found that when confirming a case of PPID—in a horse with clear clinical signs—you had a 95% chance of confirming it with a lower threshold,” she explains. “And when ruling out PPID—in a horse with early or subtle signs—you had a 95% chance of correctly ruling it out with a higher threshold. And then there was the space in between—the equivocal zone—where veterinarians have to use their judgment and the clinical signs of the horse to decide if it is a rule-in or a rule-out.
“So, basically, if the horse has got clear clinical signs, use the lower value, but if the horse has got subtle signs, use the higher,” says McGowan.
That’s important because it keeps the risk of false positives very low—meaning horses are less likely to get overdiagnosed, she says.
McFarlane notes she’s frequently seen horses misdiagnosed with PPID and treated for years, without having the disease, just because of a test result. Fortunately, the treatment for PPID—pergolide—seems harmless, even at high doses. “But it’s less harmless for the owner’s pocketbook,” she adds, in these incorrectly diagnosed cases.
Nevertheless, tests can be particularly helpful when clinical signs are unclear, McFarlane says. Indeed, the long, shaggy coats and wasting muscles in late-stage PPID horses might be easy to spot, “all the way from the other side of the pasture,” she says. But it’s easy to confuse subtle signs with general signs of aging or other conditions that need treatment. “There are a lot of signs that aren’t really specific to PPID, so that can make the disease a bit difficult to diagnose,” she says.
If the horse does have another health condition causing the signs—and gets misdiagnosed with PPID—that means delayed treatment for the true condition, she says. So while testing plays an important role in diagnosis, it’s vital to consider results in the context of the veterinarian’s physical assessment. “The tests are very good to excellent, but are they more important than a clinical exam? Absolutely not,” McGowan says.
9. You can’t rely on ACTH tests in the fall.
False.
ACTH values naturally increase in the fall, our sources say, and that increase seems to be particularly exaggerated when performing TRH stimulation. So while seasons can affect test results, the important thing is to use laboratories that take season into consideration. “There are several well-reputed laboratories that do that,” McGowan says.
Still, it’s important to consider what “season” truly means, adds McFarlane, who first discovered season’s effect on ACTH in 2004 while doing research in Canada. In theory, ACTH might start rising just after the summer solstice in late June, when the days begin to shorten, she says. Assuredly, by August the pituitary gland is already gearing up for the upcoming winter with its rising ACTH concentrations.
Plus, researchers have shown that “normal” ACTH can vary across geographic regions, she says. To complicate matters more, the extent of those variations can depend on the breed. “I’m definitely a lot less enthusiastic about making a positive diagnosis on borderline animals in the fall,” she says. “There are a lot of subtleties, and that means there’s a real art and science to the diagnosis.”
Regardless, when well-informed laboratories perform the tests and veterinarians interpret the results according to the clinical exam, diagnoses can be accurate even in autumn, our sources say.
10. Breed matters in both EMS and PPID diagnoses.
True.
Without a doubt, easy-keeper breeds are at greater risk of developing EMS, says McFarlane. The EEG lists these as all pony breeds and Miniatures, all Spanish breeds, all five-gaited breeds (including Morgans), and Warmbloods. Donkeys might also be an at-risk species, but research is pending.
As for PPID, breed doesn’t seem to be a risk factor, says the EEG.
That said, “thrifty” breeds are at greater risk of being misdiagnosed with PPID—especially in the fall. They naturally have heavier winter coats and also higher ACTH spikes in the fall after TRH stimulation. “The amount of increase that they can have in their seasonal response is really phenomenal,” McFarlane says. “That’s led to a lot of confusion on diagnosis for a long time. It would have been great to have understood those breed-specific differences earlier on.”
Take-Home Message
When it comes to understanding endocrine disorders in horses, it’s easy to get lost in not only the terms and acronyms but also the risk factors, testing, diagnoses, and latest scientific findings. If you scored at least 5 out of 10 correct on this quiz, though, you’re well on your way to comprehending these complex issues. And if you got 8 or more right, you deserve a helmet salute for your savvy equine endocrine knowledge. Regardless, we hope the expert-guided information will help you lead your horses toward healthier, more hormone-balanced lives.
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