Laminitis, PPID, and Hyperinsulinemia: What's the Link?
Equine endocrinopathies (endocrine system disorders) can be a bit of a puzzle. Some are generally easy to identify—late-stage pituitary pars intermedia dysfunction (PPID), for example—while others are more challenging, such as insulin dysregulation (including hyperinsulinemia, which refers to high levels of insulin in the blood). Oh, and affected horses sometimes have more than one endocrinopathy going on at the same time, and the diseases can even be associated with each other.

Knowing whether horses with PPID and concurrent insulin dysregulation are at the highest risk of developing laminitis would underscore the importance of screening all PPID horses for insulin dysregulation. To find out if this is the case, Lisa Tadros, DVM, PhD, Dipl. ACVIM, an assistant professor of endocrinology at the Michigan State University College of Veterinary Medicine, and colleagues recently set out to determine whether the degree of hyperinsulinemia correlates with laminitis severity in horses when they’re first diagnosed with PPID. She presented the team’s findings at the 2016 American College of Veterinary Internal Medicine Forum, held June 8-11 in Denver, Colorado.

The team hypothesized that there would be a correlation between the magnitude of hyperinsulinemia and laminitis severity, and that some owners would be unaware of mild and chronic laminitic changes in their horses.

The team employed 38 client-owned horses with PPID and collected data on the animals’ resting blood serum insulin concentrations, owner-reported laminitis history, and radiographic evidence of laminitis. They graded laminitis severity based on the degree of coffin bone rotation and grouped horses into five categories; they considered horses with no prior history, no morphologic hoof changes, and no radiographic evidence of laminitis to be nonlaminitic. They also separated horses into three groups based on their degree of hyperinsulinemia: normal (termed normoinsulinemic), mild, or severe.

Key findings included:

  • Owners reported a history of laminitis in 37% of horses; however, the researchers classified 76% of horses as laminitic;
  • Horses with an owner-reported history of laminitis were more likely to be hyperinsulinemic than normoinsulinemic;
  • Likewise, owners were more likely to recognize laminitis as the severity of hyperinsulinemia increased; and
  • Resting insulin concentrations were higher in horses with moderate to severe radiographic evidence of laminitis than in those with mild to no evidence of laminitis.

Ultimately, the team’s findings supported their hypothesis.

“Although radiographic abnormalities were present in most animals at the time of PPID diagnosis, chronic laminitis remained unrecognized by many owners, particularly in horses that were normoinsulinemic or only mildly hyperinsulinemic,” Tadros said.

Because early disease recognition can, in some cases, improve the chances of a better outcome, owners should be vigilant for signs that something’s not normal.

“With regard to PPID specifically, horses with advanced disease can have the classic appearance of a teddy bear, with a long hair coat that fails to shed,” Tadros told The Horse. “However, animals with early PPID can look physically normal.”

Some subtle signs she encouraged owners to watch for include:

  • Hair coat abnormalities (“The earliest signs of hair coat abnormalities include shedding later than other horses in the herd and failure to completely shed the long guard hairs along the chin, neck, sides, and backs of the legs,” she said);
  • Unexplained laminitis in a middle-aged or older horse;
  • Loss of muscle mass, usually most obviously over the topline;
  • Weight loss;
  • The development of regional fat deposits over the neck crest, tailhead, or other areas of the body (this can be seen even in horses that are otherwise very thin, she added);
  • Decreased athletic performance;
  • A change in attitude, such as lethargy;
  • Recurrent infections (such as sinusitis, dental disease, skin infections, hoof abscesses, urinary tract infections, respiratory infections, and a heavy intestinal worm burden);
  • A pendulous abdomen;
  • Abnormal sweating;
  • Persistent lactation in mares;
  • Infertility; and
  • Drinking and urinating excessively.

Likewise, owners should watch for signs of mild chronic laminitis that could be easily missed.

“Mild laminitis can go unrecognized, particularly in horses that are not exercised regularly,” Tadros explained. “These animals might be foot-sore for a few days, but this may not be obvious in a horse that is simply walking around in a pasture. Subtle abnormalities to watch for during a laminitis flare-up include a stiff or stilted gait, excessive shifting or rocking of weight back and forth between legs, or a reluctance to lift the legs when asked.”

She said these signs are usually most obvious in the forelimbs, but can affect the hind limbs, as well.

Owners can also watch for subtle signs that a horse has experienced laminitis in the past, even if the animal isn’t currently in the midst of a flare-up, she said.

“These can include recurrent hoof abscesses, soreness after foot trimming, a tendency to get sole bruises on harder ground, and the presence of divergent growth rings or concavity of the dorsal hoof wall,” she said. “Often, farriers will notice these changes and call them to an owner’s attention.”

Tadros also stressed the importance of identifying laminitis early on a structural level.

“Every laminitis episode, even a mild one, causes some degree of damage to the laminar tissues,” she said. “The structure of laminar tissue is very complex. The architecture is never fully restored afterwards; healing always includes the formation of some abnormal scar tissue. Laminar damage and scar tissue formation leads to loss of structural integrity. Because the laminae are responsible for anchoring the coffin bone to the overlying hoof capsule and must, therefore, withstand high mechanical forces, loss of tissue strength can eventually result in rotation and/or sinking of the coffin bone within the hoof capsule.

“A severe laminitis episode can result in marked laminar tissue damage and weakening,” she continued. “However, even a horse with mild chronic laminitis can eventually develop severe laminar changes. The cumulative damage caused by repeated mild laminitis episodes—which is a common scenario in horses with endocrinopathies—eventually leads to loss of laminar structural integrity. For this reason, it is essential to identify horses that are at a high risk of developing laminitis, such as those with endocrinopathies, before damage to the laminar tissue occurs. Interventions can then be implemented to lower the laminitis risk. Likewise, the earlier a horse with endocrinopathic laminitis is identified and treated—by improving the metabolic state and providing therapeutic farrier care—the better the chances are of preventing severe cumulative laminar damage and maintaining soundness.”

Tadros said any signs of laminitis warrant a veterinarian’s attention.

“A veterinarian should be involved in the care of any horse with laminitis or endocrine disease,” she stressed. “While owners can certainly do a lot on their own with regard to proper routine management, horses with endocrine diseases require periodic blood tests and often need medications (such as pergolide to manage PPID). Thorough examination of the hooves, sometimes including radiographs, is necessary to manage horses with chronic laminitis; this should be a team effort involving the owner, farrier, and veterinarian.”

Tadros said her team’s research is ongoing: “Some of the questions we are currently exploring include 1) whether horses with PPID and severe insulin dysregulation are those that are also concurrently affected by equine metabolic syndrome (the PPID-affected horses with more mild insulin dysregulation or normal insulin and glucose metabolism may have PPID only) and 2) whether those PPID-affected animals with the more severe insulin dysregulation are also those at the highest risk of developing laminitis,” which she said seems to be the case.

The study, “Association between hyperinsulinemia and laminitis severity at the time of pituitary pars intermedia dysfunction diagnosis,” was carried out by Tadros and coauthors Jennifer Fowlie DVM, MS, Dipl. ACVS; Kent Refsal, DVM, PhD; Judith Marteniuk, DVM, MS; and Harold Schott II, DVM, PhD, Dipl. ACVIM.