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.

When looking at common clinical signs, 80% of horses had rapid, diffuse muscle atrophy, 74% had a stiff gait, 44% had a fever of around 103°F, and fewer than 10% were recumbent (down). Sixty percent of horses had leukocytosis (a high white blood cell count, which can indicate infection), and 97% had increased creatine kinase and aspartate transaminase levels (enzymes that rise in response to muscle insult). Prior to showing signs of IMM, 38% of horses had undergone flu/rhino vaccination and/or suffered from respiratory infection.

Hunyadi said all horses that received medical therapy (e.g., corticosteroids and antibiotics, as needed) immediately upon admission to UC Davis (87%) survived to discharge. Of the horses that didn’t receive immediate medical treatment, 1% died and 12% were euthanized. “Fever and additional illness (e.g., kidney failure) were associated with decreased survival,” he said.

All surviving horses regained their muscle mass in three to six months, but Hunyadi noted a 32% recurrence rate within six months.

In conclusion, said Hunyadi, “Horses with IMM can have high survival rates, especially if treated early. We did not identify a definitive etiology (cause), but IMM was associated with a recent history of respiratory infection or vaccination, and Quarter Horses and related breeds were overrepresented.”

For the best outcome, he said, veterinarians must recognize signs of disease early, initiate treatment with corticosteroids and antimicrobials immediately, treat any concurrent illness, and monitor kidney variables (measurements that can indicate function), knowing that recurrence is possible in these horses.