Researchers at Michigan State University (MSU) recently found that senior horses with pituitary pars intermedia dysfunction (PPID, formerly known as equine Cushing’s disease) experience a reduction in spinal bone density, potentially increasing fracture risk. While fractures in PPID horses are rare, the study highlights the need for further investigation into this hormonal disorder common in aged equids.

Pituitary pars intermedia dysfunction (PPID, formerly known as equine Cushing’s disease), is a complex hormonal health disorder that frequently affects senior (16 and older) horses and ponies. It is caused by an overactive pituitary gland, which is located at the base of the brain and responsible for producing certain hormones.

Horses with PPID often display signs such as a longer coat and/or difficulty shedding in warmer months, drinking and urinating more than usual, increased sweating, and losing weight. They are also at a higher risk of developing the painful and potentially life-threatening condition laminitis.

“Fractures, although rarer among the clinical signs of horses with PPID, have also been documented, but we know little about their prevalence and pathogenesis,” Schott, DVM, PhD, Dipl. ACVIM, professor of large animal clinical sciences at MSU’s College of Veterinary Medicine, in East Lansing.

“In our study, we wanted to determine if the lumbar vertebral (spine) bone density was decreased in aged horses with PPID compared to aged control horses without PPID,” said Schott. “We know that this can happen because loss of vertebral bone density occurs in people and dogs with hyperadrenocorticism,” which is a serious illness caused by overproduction of cortisol by the adrenal glands.

Studying Bone Density in PPID Horses

To discover any potential differences between the horse groups, the researchers measured the bone density of the fourth lumbar (L4) vertebral body in horses with and without PPID and compared the results. They compared five aged horses with PPID (over 21-years-old) to a control group of six aged horses without PPID (over 20-years-old) and four young (3-8-years-old) horses with PPID.

“We recorded that the bone mineral density in the fourth lumbar vertebrae was decreased by about 25% in aged horses with PPID as compared to the aged control horses without PPID and young horses in our study,” Schott said. In contrast, when the researchers compared the bone mineral density of the cannon bones—which, of course, are weight-bearing—in the aged PPID horse group with those numbers in the aged non-PPID horse group and the young horse group, they saw no significant difference (decrease) in bone density.

“Loss of bone mineral density appears to be a novel problem in horses with PPID,” said Schott. “Although the number of horses involved in our study was small, we documented that bone mineral density of non-weight-bearing bones, such as the spine, can be decreased in PPID horses.”

Take-Home Message

Schott said horses with untreated PPID might be at an increased risk of bone fracture, which could be life threatening depending on the fracture location. However, based on current knowledge, such fractures appear to be uncommon.

The researchers did not determine why horses with PPID lose bone mineral density, but they emphasized more studies are needed to provide adequate management advice for horse owners.

The study, “Lumbar vertebral bone density is decreased in horses with pituitary pars intermedia dysfunction” appeared in Equine Veterinary Journal in December 2023.