The Crossroads of PPID, Insulin Resistance, and Laminitis
For years, veterinarians have warned owners of Cushingoid horses about the predisposition they carry for laminitis. However, recent research suggests it’s not that black and white.

Dianne McFarlane, DVM, PhD, Dipl. ACVIM, is a professor at the Oklahoma State University College of Veterinary Medicine, in Stillwater, who studies geriatric horses. At the 2021 American College of Veterinary Internal Medicine Forum, held virtually and in Orlando, Florida, she presented a synopsis on years of laminitis research indicating equine metabolic syndrome (EMS) might be the culprit behind the painful hoof disease, rather than pars pituitary intermedia dysfunction (PPID, formerly known as equine Cushing’s disease).

Hoof Anatomy and Understanding Laminitis

An understanding of hoof anatomy is crucial to discerning laminitis. The hoof houses the coffin bone, which is suspended by highly vascular and well-innervated structures called laminae. When a horse has laminitis (which typically occurs in the front feet), the laminae become inflamed, which can allow the coffin bone to rotate if severe. The inflammation results in the clinical signs associated with this disease, including lameness, heat, a shortened stride, and a rocking or leaning-back stance (in which the horse relieves pressure on the affected front feet by shifting weight to its hind limbs).

The three main types of laminitis include:

  1. Sepsis-induced laminitis is secondary to a systemic illness, such as Potomac horse fever or salmonella.
  2. Supporting-limb laminitis occurs in response to extreme overloading of the limb, after the opposite limb becomes non-weight-bearing. The constant pressure can lead to failure of the opposing hoof’s laminae.
  3. Endocrinopathic laminitis is associated with grain overload or metabolic diseases and known to account for up to 90% of laminitis cases, per the United States Department of Agriculture.

Surveys of owners from the United Kingdom and Australia reveal that 9-15% of horses have undergone some degree of laminitis. However, an equine veterinarian survey showed 45% of owners seeking veterinary care didn’t realize laminitis was the cause of their horses’ lameness. This goes to show that owners might not recognize the clinical relevance of the progressive condition.

Veterinarians know horses diagnosed with PPID can develop laminitis, McFarlane said. But not all horses with PPID become laminitic. Research revealed those that develop laminitis likely have concurrent hyperinsulinemia, or elevated insulin, circulating in their system.

Obesity is the hallmark of EMS-afflicted horses, often accompanied by large fat deposits over the neck and tailhead. These horses cannot regulate their insulin levels, resulting in chronic hyperinsulinemia.

The question remains: Does PPID cause EMS? Likely not, McFarlane said. PPID results in the degeneration of the hypothalamus and pituitary gland, resulting in excessive pituitary intermedia hormone ACTH (adrenocorticotropic hormone). Scientists have theorized that pituitary intermedia hormones disrupt the clearance of insulin in the horse’s liver, but this warrants more research.

Studies evaluating the use of Prascend, the FDA-approved brand of pergolide used to treat PPID, have also looked at its effect on hyperinsulinemia. Horses diagnosed with both PPID and EMS demonstrated improved ACTH and insulin levels after Prascend treatment. “The data suggest that perhaps giving Prascend improved insulin regulation, but it seems more due to the comorbidity of PPID,” said McFarlane.

If it’s not because of PPID, why is laminitis so common? “This is a condition that we really created by intensively and incorrectly managing horses that are genetically suitable for thrifty conditions,” said McFarlane. We’ve achieved this mostly through overfeeding, feeding incorrect diets, episodic feeding, and limited exercise, she said. These practices result in more insulin dysregulation, which can lead to clinical laminitis.

If you are concerned about your horse’s metabolic status, contact your veterinarian. He or she can help you formulate a management and diet plan, recommend blood testing, and perform hoof radiographs to evaluate for rotation.