Immune Mediated Myositis (IMM)

Equine immune-mediated myositis (IMM) is a disease occurring only in Quarter horses which causes rapid and severe symmetrical wasting of the topline muscles, often following exposure to or vaccination against respiratory infection by Streptococcus equi, the organism responsible for equine distemper (“Strangles”). The loss of muscle mass is primarily attributable to inflammatory destruction of fast-twitch muscle fibers, with a diagnosis of IMM being based on biopsy of the atrophied muscle and identification of invasion of muscle fibers and local blood vessels by T-lymphocytes and macrophages, two types of white blood cells involved in adaptive immune responses and the clean-up of cellular debris. Full muscle mass can be regained within several weeks to months. However, approximately 40% of horses affected by IMM will experience at least one recurrence of an atrophic episode, with the extent of muscle loss and resultant decrease in quality of life being severe enough in some cases to warrant euthanasia. Among Quarter horses, IMM appears to be especially prevalent in cutting and reining types and has a strong familial relationship, suggesting that predisposition to this disease has a genetic basis. In collaboration with Dr. Stephanie Valberg at Michigan State University, the Finno Lab is currently investigating a genetic variant hypothesized to confer susceptibility to IMM.

Horses affected by IMM rapidly lose up to 30% of muscle mass from the epaxial and gluteal (topline) muscle groups and occasionally the semimembranosus and semitendinosus (rump) muscles via an inflammatory process. Histologic evaluation of the atrophied muscles reveals T-lymphocytic and macrophagic infiltration of both local blood vessels (vasculitis) and muscle fibers (myositis), with predominantly fast-twitch muscle fibers affected. As in many diseases involving muscle damage, serum creatine kinase (CK) and aspartate transaminase (AST) concentrations are elevated. Horses with IMM may also exhibit nonspecific clinical signs such as inappetence, malaise, and generalized weakness. Both sexes are equally affected and most cases are under 4 or over 17 years of age when they first present with muscle atrophy.

Are you concerned that your horse may have IMM? Contact our lab for more information on how you can aid in equine genetic research.