neuromuscular disease in horses

Historically, veterinarians have reported finding parasites in the skeletal muscle of some horses with neuromuscular disease. One they’ve always written off as an incidental finding or not actually related to the neuromuscular disorder, is the protozoan parasite Sarcocystis fayeri.

Monica Aleman, MVZ, PhD, Dipl. ACVIM, an associate professor of medicine and epidemiology at the University of California, Davis (UC Davis), School of Veterinary Medicine, believes, however, that there’s more to it. She studied S. fayeri’s prevalence in horses with neuromuscular disease and presented her results at the 2018 American Association of Equine Practitioners Convention, held Dec. 1-5 in San Francisco, California.

“At UC Davis we’ve found large numbers of S. fayeri in the skeletal muscle of diseased horses,” she said. “There are also anecdotal reports of horses with lameness and stiffness (but no characteristic signs of incoordination or muscle wasting) responding to treatment with antiprotozoal drugs.”

She said confirming S. fayeri in horses’ skeletal muscle is also important from a public health standpoint because it’s toxic to humans consuming raw horse meat.

In her study Aleman evaluated skeletal muscle samples from 392 equids treated for suspected neuromuscular disease at UC Davis from 2000 to 2014. She found that:

  • 50 study horses had encysted S. fayeri in their skeletal muscle;
  • 35 of those 50 had concurrent neuromuscular disease;
  • 51 (83.6%) of 61 muscle samples from those 35 affected horses contained encysted parasites; and
  • 10 horses with S. fayeri were treated with antiprotozoals (Marquis), and their clinical signs resolved within 15 days. “This doesn’t necessarily prove S. fayeri causes disease but is worth considering and performing further investigation,” said Aleman.

She said these findings indicate that S. fayeri infection is significantly more prevalent in young adult horses (younger than 7) with neuromuscular disease than in healthy horses.

“Consider muscle biopsy in horses with neuromuscular disease of undetermined cause or the presence of clinical signs such as stiffness, muscle atrophy (wasting), dysphagia (difficulty swallowing), weakness, gait abnormalities, myositis (tying-up), or myalgia (muscle pain),” she said.

In summary, said Aleman, neuromuscular disease might be associated with S. fayeri infection in muscle because its presence in large numbers could affect muscle function.