Horses with IMM experience rapid, widespread muscle atrophy of the gluteal muscles, along with the epaxial muscles that run along either side of the spine. | Photo: iStock

Immune-mediated myositis (IMM) is a sometimes-fatal muscle disease that occurs predominantly in Quarter Horses and related breeds. Veterinarians know very little about the condition and affected horses’ likelihood of survival, so Lazslo Hunyadi, DVM, MS, PhD, Dipl. ACVIM, a practitioner with Equine Sports Medicine and Surgery, in Weatherford, Texas, conducted a retrospective study to find out. He presented his findings at the 2017 American Association of Equine Practitioners convention, held Nov. 17-21 in San Antonio, Texas.

Horses with IMM experience rapid, widespread muscle atrophy (wasting) of the gluteal muscles, along with the epaxial muscles that run along either side of the spine. They also demonstrate generalized muscle stiffness and lethargy.

Hunyadi partnered with veterinarians from the University of California, Davis (UC Davis), and looked at medical records from that hospital of horses diagnosed with IMM from 1991 to 2014. They found 68 horses with a definitive diagnosis of IMM via muscle biopsy. Of these, 76% were Quarter Horses, 14% were Thoroughbreds, and 8% were other breeds such as warmbloods and Arabians. The Quarter Horses tended to be younger, with the average age less than 4 years old, while the average among other breeds was more than 10. The average hospital stay was 10 days