IMM stands for Immune-mediated myositis - it is a dominant disease Genetic Testing is Available
IMM is characterized by rapid-onset muscle wasting often brough on by a recent infection - often seen with stangles, flu vaccines or other infections. Horses with IMM will often rapidly lose enough weight to see the spine and hip bones within a very short time (2 days to 2 weeks). The desease is dominant but not all horses with tthe defective gene will develop IMM - it usually needs an infection or vaccinatio to trigger it - also IMM symptoms have shown in horses without the defective gene so obviously there are more genes to be discovered yet.
So far, the IMM MYH1-E321G variant has only been found in Quarter Horses - now that the test is available - a lot of horses are coming up with this defect. Many horses never show any signs unless they are triggered by a vaccination or they get Strangles.
IF you own a horse that fits the description above (rapid muscle loss after an infection or vaccination, especially over the back and hind end muscles) seek help from a veterinarian - IMM horses do respond to treatment with corticosteroids. BRIDGEquine would also be interested in your horses pedigree so please contact us.