We have had IMM listed in our list of Genetic Issues under "Other Genetic Issues" for a while now but thought since the recent release of a readily available "For Purchase" test has brought a lot more notice to this disease we would publish an article on it.
Have you ever had a horse that lost a drastic amount of weight in a very short time? Was it just after you vaccinated it or it came down with Strangles? Good chance the horse had a mutated gene which caused that issue.
IMM stands for Immune-mediated myositis - it is a dominant disease and you can have a genetic test done on your horse to see if it carries a copy of the mutated gene. Even though it is dominant (meaning it only needs one copy to show symptoms) many horses that have this mutation never show any signs. That is because it seems to need a trigger. Right now known triggers are Strangles and certain vaccines.
An IMM horse having an attack will rapidly lose muscle along the backbone - to the pont where you can see the skeleton. This happens within 2 days to 2 weeks, It is treatable with Corticosteroids - and should be treated ASAP. Once treated, horses can usually come back to their original weight within a few months.
So far, the IMM MYH1-E321G variant has only been found in Quarter Horses, but it seem to be fairly widespread in the cutting and reining lines.
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.