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