Vets Discuss Developments in Endocrine and Metabolic Conditions in Horses

Key takeaways included treating horses as individuals and adjusting medications and management strategies as needed.
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Vets Discuss Developments in Endocrine and Metabolic Conditions in Horses
Panelists encouranged veterinarians and horse owners to monitor EMS horses for early signs of PPID. Uncontrolled PPID could exacerbate insulin dysregulation (abnormal blood insulin levels) in horses that are already insulin resistant, increasing their laminitis risk. | Photo: iStock
Endocrine diseases such as pituitary pars intermedia dysfunction (PPID) and equine metabolic syndrome (EMS) can be challenging to manage. Veterinarians discussed stubborn and unusual cases during a table topic session at the 2019 Annual American Association of Equine Practitioners Convention, held Dec. 7-11 in Denver.

PPID

Attendees focused primarily on PPID and first looked at affected horses whose bloodwork refuses to drop to a normal range despite treatment with pergolide (Prascend). Session moderators Hal Schott, DVM, PhD, Dipl. ACVIM, of Michigan State University, and Nick Frank, DVM, PhD, Dipl. ACVIM, of Tufts University, agreed that in these cases the key is to focus on clinical signs (e.g., abnormal hair coat, failure to shed, abnormal sweating, loss of muscle mass, increased water intake and urination, etc.).

Clinical improvement is of greater importance than perfect bloodwork, they said. Sometimes, however, veterinarians see the opposite, where blood adrenocorticotropic hormone (ACTH, the hormone that gets overproduced in PPID horses) values are normal yet clinical signs still exist. This might be due to the pituitary gland releasing hormones other than ACTH, they said, and veterinarians might need to increase Prascend doses in these cases to try to improve symptoms.

Clinical signs that particularly warrant a trial with higher doses of Prascend include a known history of laminitis and signs of immunosuppression, such as sinus infections and sole abscesses. Schott said he increases Prascend doses in 1 milligram increments to a total of 3 milligrams, while Frank said he increases them in 0.5 milligram increments to 3 milligrams, then adds the antihistamine cyproheptadine or increases the pergolide dose further to 5 milligrams. This can become cost-prohibitive for some clients, in which case veterinarians can try cheaper, although possibly less-effective, drugs such as cyproheptadine.

Veterinarians might also reach for cyproheptadine for horses that don’t handle Prascend well. Session attendees discussed the difficulty of getting some horses to take Prascend and administration techniques they have found helpful. These included starting with half a tablet every other day, rotating feeds, and using novel feeds. The discussion then shifted to the need for individualized medicine, acknowledging that each PPID horse is different and needs individualized management.

Other key PPID-related takeaways from this session included:

  • Horses that develop PPID at younger ages are more challenging to diagnose. Disease prevalence in young horses is lower, so vet’s confidence in the diagnostic tests is also lower. Therefore, looking for clear clinical signs as part of the diagnosis is very important in younger horses.
  • When using the thyrotropin-releasing hormone (TRH) stimulation test as a diagnostic, don’t give horses access to grain the night before or morning of the test. Researchers currently don’t recommend using this test between July and November, because results vary greatly, reducing confidence in positive test results. Veterinarians can use this test, however, during this period to identify a definitive negative.
  • Long-term studies of horses on Prascend show that most horses over 10 years of age receiving Prascend will need a dose increase at some point after being on the medication.
  • Veterinarians might need to increase medication doses in the fall to account for the seasonal ACTH increases, coming down again in December with a low in April.

EMS

While much of the session focused on PPID, attendees also discussed managing EMS horses. Veterinarians and horse owners need to monitor these horses for early PPID signs, such as:

  • Beginning-stage muscle mass loss;
  • Weight loss or decreased ability to gain weight;
  • Lethargy and decreased performance;
  • Slowed shedding; or
  • Maintaining winter hairs in regions such as the backs of legs or elbows.

This monitoring is important because uncontrolled PPID could exacerbate insulin dysregulation (abnormal blood insulin levels) in horses that are already insulin resistant, increasing their laminitis risk.

Diet and management are still the main focuses of EMS treatment. However, Frank said he will use the thyroid medication levothyroxine to accelerate weight loss in obese horses and metformin in horses with persistently increased insulin concentrations, as it may blunt glucose and insulin response in some cases. While metformin is an effective drug in human medicine, its oral bioavailability (absorption rate) in horses is low, said Frank.

Ideally, he added, veterinarians should administer metformin 30 minutes before feeding, but this is generally impractical, so most put it in the horse’s feed. If metformin is going to be effective, the results are almost immediate, but Frank recommends trying the drug for about 14 days before ruling it out. If giving it to a horse with active laminitis, he recommends not stopping treatment until the laminitis is under control, in the event it is helping.

A new drug class called sodium-glucose cotransporter 2 (SGLT2) inhibitors are promising for treating insulin dysregulation, he added. Some of the SGLT2 inhibitors developed for use in humans with type-2 diabetes are being administered to severely affected horses with active laminitis.

Dietary management is still crucial for horses with EMS and insulin dysregulation. Again, the key is individualized medicine. Some horses can handle higher levels of nonstructural carbohydrates (NSC) in their diets than others. Similarly, some might tolerate pasture access while others will never be able to be on pasture and will require much tighter control of their dietary NSC. Both moderators agreed that they would not risk feeding a diet with an NSC greater than 12% to a horse with a high resting insulin concentration of more than 30 microunits per milliliter. They recommended testing a horse’s glucose and insulin responses to the meal he’s being fed as a way of determining whether that diet is appropriate for that individual horse.

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Written by:

Clair Thunes, PhD, is an equine nutritionist who owns Clarity Equine Nutrition, based in Gilbert, Arizona. She works as a consultant with owners/trainers and veterinarians across the United States and globally to take the guesswork out of feeding horses and provides services to select companies. As a nutritionist she works with all equids, from WEG competitors to Miniature donkeys and everything in between. Born in England, she earned her undergraduate degree at Edinburgh University, in Scotland, and her master’s and doctorate in nutrition at the University of California, Davis. Growing up, she competed in a wide array of disciplines and was an active member of the U.K. Pony Club. Today, she serves as the district commissioner for the Salt River Pony Club.

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