From an Immune-Mediated Myopathies study dated 2006, University of Minnesota:

Immune-Mediated Polymyositis

Immune-mediated polymyositis (IMM) has recently been reported in horses. In the largest retrospective study, 31 of 1350 horses that received a muscle biopsy for evaluation of neuromuscular disease were diagnosed with IMM.

The breed and age of all horses identified to date include 32 horses with Quarter Horse bloodlines and four other breeds including two ponies, one Icelandic horse, and one Thoroughbred.

A bimodal age distribution seems to occur in affected horses with all horses identified to date being either < 8 yr of age or >16 yr of age.

In approximately one-third of horses with IMM, exposure to S. equi or a respiratory disease seems to be a triggering factor.