Q. I read on your site that two or three Quarter Horse stallions might be traced to the syndrome of polysaccharide storage myopathy (PSSM). I have a performance Quarter Horse filly with PSSM, and I feel it is important to know who these sires are so I can be an informed and responsible breeder. The filly is great, but in picking out a stallion for her I would not want to double up on the genetics for this syndrome.



A. At this time the genetics of PSSM aren't completely understood. We know that numerous mares with PSSM have produced foals with it, indicating that it is inherited. We also know from the pedigrees of affected horses that the sires and dams of PSSM horses share common bloodlines. Thus, it would appear that this might be a recessive trait where a copy must be inherited from both sire and the dam for the foal to have PSSM.

If PSSM is a recessive gene as we suspect, and you bred to an unaffected stallion, all offspring from your affected mare would be carriers of the trait. If you inadvertently bred to a stallion which is a carrier (no test exists yet to determine this), there is a 50% chance the foal will have PSSM. We therefore do not recommend breeding PSSM mares. Hopefully research will provide an accurate test for carriers and affected horses in the future.

About the Author

Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR

Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, is the Mary Anne McPhail Dressage Chair in Equine Sports Medicine and a Professor of Large Animal Clinical Sciences at Michigan State University. She is a leading researcher on the subject of tying-up and the genetic basis for equine neuromuscular disorders.

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