Endocrinology

Novel, Repeatable Score Can ID Fibrotic Changes in Obese Horses

By Paul Basilio

Obesity is an increasingly troubling issue in equids, just as it is in humans. Excess body fat can restrict a horse’s functionality as an athlete or as breeding stock, and it can set them up for an increased risk for health and welfare concerns, such as hyperinsulinemia-associated laminitis.

The term “unhealthy adipose tissue” has been used to describe the changes associated with this pathologic type of adipose tissue. In human medicine, major pathologic changes in adipose tissue, such as adipocyte hypertrophy and fibrosis of the extracellular matrix, have been associated with glucose and lipid dysmetabolism.

Adipocyte hypertrophy has been shown in theintra-abdominal adipose tissue of horses with metabolic dysregulation, but data are lacking regarding fibrosis.“Fibrosis has been identified in the nuchal ligament deposit of metabolically dysregulated horses and a histologic grading scale has been developed for breed-related changes in Iberian horses,” said Emma Stapley, DVM, during a presentation here at the 2024 ACVIM Forum in Minneapolis, Minn., “but it’s not exactly what we’re looking for in terms of a metabolic marker.”

Putting the ‘e’ in FAT
Dr. Stapley and colleagues set out to establish a scoring system based on the human fibrosis of adipose tissue (FAT) system. In addition to their equine FAT (eFAT) score, the team applied an automated adipocyte measuring tool to the nuchal ligament deposit to try to provide equivalent measures to what they could generate manually. They also aimed to describe the ultrasonographic variations in the nuchal adipose tissue. Their hypothesis was that the eFAT score and average adipocyte area would correlate with ultrasound image intensity.

“Finally, we wanted to compare our eFAT score, adipocyte area, and ultrasound image intensity with historic weight gain and current BCS in our horses,” said Dr. Stapley, who is a resident at the University of Wisconsin-Madison. “We hypothesized that fibrosis score, adipose area, and image intensity would all be positively correlated with recent weight gain and BCS.”

The prospective pilot study involved 10 university-owned mares who were co-housed on free-choice grass hay. The mares were weighed every month for the 2 years leading up to collection of adipose tissue specimens, and they were subdivided into obese and non-obese horses for analysis. The obese horses had an average BCS of 7.5, and the non-obese horses had an average BCS of 5. Horses that gained weight during the study period tended to gain 10% of their body weight.

The results
Unfortunately, the automated adipocyte measuring tool significantly underestimated adipocyte size, so it was excluded from further analysis.

On ultrasound, results showed that thinner crests were more hyperechoic, and the standard deviation of ultrasound image intensity was negatively correlated with BCS.

“Similarly, ultrasound image intensity was significantly higher in non-obese versus obese horses,” Dr. Stapley explained. “The heterogeneity of the image was negatively correlated with the standard deviation of adipocyte diameter. Essentially, more heterogeneously sized adipocytes generated more hypoechoic images.”

The team found that weight gain was correlated negatively with adipocyte area, and adipocyte area was also significantly greater in horses with static weight gain versus the horses that were actively gaining weight.

“It’s counterintuitive,” she noted. “We know that there are 2 main measures by which animals accumulate adipose tissue. They can develop it by hypertrophy of their current adipocytes—which is consistently associated with dysmetabolism across species—or they can create new adipocytes, which is associated with a healthier phenotype of adipose tissue. It’s entirely possible that our horses were depositing increasing amounts of fat via adipogenesis, not hypertrophy.”

All told, the eFAT score proved to be quite repeatable with appropriate training.

“It is useful to have a repeatable way to describe fibrotic changes, especially in a fat deposit that is amenable to non-terminal and repeated sampling,” Dr. Stapley concluded. “Previous work has shown relatively good histologic consistency along the length of that deposit, as well.” MeV