A new muscle atrophy scoring system (MASS) for horses was published in 2022 in the United States. The collaborators included researchers from the Gluck Equine Research Center, the University of Kentucky, the Waltham Petcare Science Institute, and Lincoln Memorial University.¹

Muscle atrophy or “wastage” might compromise welfare and performance, but assessing muscle mass can be challenging. This original work showed the muscle atrophy scoring system had the potential to provide a simpler, more objective, and more reliable method of practically evaluating and monitoring muscle mass in horses.

It was therefore suggested that this muscle atrophy scoring system could be useful in monitoring muscle mass in horses with various equine medical conditions. In particular, it was thought to be of value in older horses. It also is valuable for those with various medical conditions. Those could include pituitary pars intermedia dysfunction (PPID, also known as Cushing’s disease). That is because the condition might be associated with a loss of muscle mass.

Research on Muscle Atrophy

A study titled “Muscle Atrophy Scores in a Population of Aged Horses and Ponies With and Without PPID” was reported on at the 5^th Global Equine Endocrine Symposium (GEES). The meeting was organised by Boehringer Ingelheim and held in January 2023. The presenter was Pat Harris, MA, PhD, DECVCN, VetMB, MRCVS. She is head of the Equine Studies Group at Waltham Petcare Science Institute in the United Kingdom and Director of Science for Mars Horsecare. 

Harris described in detail a study undertaken by the University of Melbourne in collaboration with Waltham, Boehringer Ingelheim, and Queensland University. This study took place in Australia. It evaluated the utility of the new muscle atrophy scoring system in a group of 31 equids. There were (8 ponies and 13 horses included, all of which were older than 15 years. The researchers established a PPID-positive diagnosis in 12 of the 31 animals based on clinical signs, baseline ACTH, and a TRH stimulation test. 

One of the reasons for this research was to evaluate the MASS scoring system in ponies. The MASS scoring system had previously been researched in horses.

Specifics of the Study

All animals were assessed for body condition score (BCS: 1-9), cresty neck score (CNS: 0-5), MASS scores (1-4 for various areas of the body, using lean or adipose charts as appropriate). 

For the MASS scores, the researchers assessed the neck, back, and hindquarters separately using one of two scales (A or B, depending on the individual’s body condition) and scored each from 1-4. A score of 1 indicated no muscle wastage. A score of 4 indicated severe muscle wastage. Then they added the scores together. (Editor’s note: Researchers also assessed the belly, although they now do not recommend using the belly for practical reasons.)

Seven ponies and five horses were diagnosed with PPID. Ten of the 12 PPID cases showed some evidence of muscle atrophy. Fifteen of 19 non-PPID animals showed no signs of muscle atrophy, with three other animals showing mild signs of atrophy.

Study Results

Overall, the PPID animals had significantly more muscle atrophy than similarly aged non-PPID animals. The majority of the non-PPID animals showed no signs of muscle atrophy. Seven of the PPID animals had a MASS score of 7 or higher.

Only one non-PPID animal had such a value (a 32-year-old mare with marked atrophy and chronic weight loss). These differences in MASS existed despite there being no overall difference in body condition score or cresty neck score between the two groups. 

Study Conclusions

In her conclusions, Harris stated: “The results confirm that muscle atrophy is a common feature of PPID and that the scoring system developed in horses is also applicable to ponies, including Shetland/Miniature ponies.

“The majority of non-PPID animals (of a similar age to the PPID group) showed little or no evidence of muscle atrophy, indicating that old age per se is not automatically associated with significant muscle loss in healthy animals,” Harris stressed.

“As the PPID animals had similar BCS and CNS to the non-PPID cohort, there appears to be no loss of adiposity due to this condition (assuming they are otherwise healthy),” she noted. “Rather, PPID appears to be associated with a specific loss of muscle tissue. These findings warrant further investigation and may have important implications for optimizing the nutrition of horses and ponies with PPID.”  

Use of MASS

Harris emphasized the MASS could be an important muscle atrophy monitoring tool for owners and veterinarians. 

Veterinarians examining older horses and ponies should pay specific attention to monitoring muscle atrophy. This is especially true in animals with, or suspected of having, PPID.

Editor’s note: You can access the full Proceedings of the 5th GEES Symposium here. Search for the presentation “Muscle Atrophy Scores in a Population of Aged Horses and Ponies With and Without PPID.”

Resource

1. Development and Evaluation of a Muscle Atrophy Scoring System (MASS) for Horses. Alisa C. Herbst, Mackenzie G. Johnson, Hayley Gammons, Stephanie E. Reedy, Kristine L. Urschel, Patricia A. Harris, and Amanda A. Adams. 2023. Journal of Equine Veterinary Science.