Immune Mediated Myositis (IMM)

Equine immune-mediated myositis (IMM) is a disease occurring in Quarter horses and QH-related breeds that typically 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 “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 reining and cow types (Gianino G, et al. J Vet Intern Med 2019) 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 has identified a genetic variant that confers susceptibility to IMM (Finno CJ, et al. Skeletal Muscle 2018).

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