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Pituitary pars intermedia dysfunction (PPID, also known as Cushing’s syndrome) can cause an array of problems. These include a slow-to-shed coat, muscle weakness,  compromised immunity  and chronic laminitis. Now, researchers have added one more PPID-related complication to the list: increased risk of bone fractures.

PPID occurs when an  enlarged and overactive  pituitary gland leads to excessive levels of adrenocorticotropic hormone (ACTH).  “PPID isn’t identical across species, but ACTH does play a role. So there are similarities,” says Harold Schott, DVM, PhD, of Michigan State University. “We were thinking about the effects of Cushing’s disease in dogs and people, where a decrease in bone mineral density and fractures have been documented. So we thought it would be wise to look into that for horses.” 

The study

The Michigan State University study was based on five horses with unmanaged PPID and 10 healthy horses to serve as controls. After taking blood samples to measure ACTH levels, the researchers examined each horse’s lumbar vertebrae and cannon bones for signs of fractures or weakness.

They found that the vertebrae of horses with PPID had significantly lower bone-mineral density compared to the vertebrae  of the healthy horses. The cannon bones did not show this difference, however. This, says Schott, may be due to the weight-bearing function of those bones.

“In veterinary school we learn about ‘Wolff’s Law,’ which explains how bone responds to strain, becoming stronger,” says Schott. “The bones of the vertebral column don’t bear weight like supporting limbs do, so this could account for the changes we are seeing in the vertebrae of PPID horses, but not the cannon bones.”

A reduction in bone density increases the risk of fracture. “You can get fractures in these vertebrae just from movement,” says Schott.  “A horse may lie down to  roll, putting stress on the  vertebrae. If the bone is  even slightly fragile, this  could be enough.” Although none of the study horses had vertebral fractures,  Schott says there have been reported cases of fractures of other bones in older horses with advanced PPID.

What still needs to be learned

Because the study horses were not receiving treatment for PPID, it’s unknown whether medication might help prevent loss of bone density. “We’d like to study that, as well as the role nutrition may play,” says Schott. “We’ve proposed a study to look at calcium metabolism in PPID horses and how that might play a role in management of the condition.”

In the meantime, Schott says, these findings underscore the fact that PPID is a systemic disease. “My overall goal is to get people to not just think of PPID as a long, fuzzy haircoat problem but as an endocrine disorder with multiple potential problems.  PPID affects the entire body, so we need to look beyond their shaggy coats to help them.”   

Reference: “Lumbar vertebral bone density is decreased  in horses with pituitary pars  intermedia dysfunction,”  Equine Veterinary Journal,  December 2023